Engine Controls - Tests W/codes - 4.3L: Diagnosis Chevrolet Pickup K2500

SELF-DIAGNOSTIC SYSTEM

All vehicle are equipped with either an Electronic Control Module (ECM), Powertrain Control Module (PCM) or Vehicle Control Module (VCM). Unless specifically stated, references to ECM also apply to PCM or VCM equipped vehicles.

The ECM is equipped with a self-diagnostic system, which detects system failures or abnormalities. When a malfunction occurs, ECM will illuminate the SERVICE ENGINE SOON light located on instrument panel. This light is also referred to as the Malfunction Indicator Light (MIL). When malfunction is detected and MIL is turned on, a corresponding trouble code will be stored in ECM memory. To retrieve stored codes, see READING TROUBLE CODES or RETRIEVING CODES (NON-SCAN) . Malfunctions are recorded as HARD FAILURES or as INTERMITTENT FAILURES.

DIAGNOSTIC PROCEDURE

Diagnosis of the computerized engine control system should be performed in the following order

1. Ensure all engine systems not related to the computer are operating properly. DO NOT proceed with testing unless all other problems have been repaired. Perform diagnostic circuit check before using trouble code charts. See appropriate BASIC TESTING article in this section below. F - BASIC TESTING (for Astro & Safari) F - BASIC TESTING (for "S" & "T" Series) F - BASIC TESTING - GASOLINE (for "C" & "K" Series) F - BASIC TESTING - GASOLINE (For "G" & "P" Series)
2. If trouble codes were displayed (other than Code 12), determine whether codes are hard or intermittent. Hard codes cause Malfunction Indicator Light (MIL) to illuminate continuously with engine running. See «HARD OR INTERMITTENT TROUBLE CODE DETERMINATION»(ref-19732-S17325261882001110500000) . For diagnosing hard codes, proceed to appropriate trouble code chart. For diagnosing intermittent codes, proceed to INTERMITTENTS in appropriate TESTS W/O CODES article in this section below. Exceptions are Code 13, 15, 24, 44 and 45 charts, which can help diagnose intermittent codes. H - TESTS W/O CODES (for Astro & Safari) H - TESTS W/O CODES (for "S" & "T" Series) H - TESTS W/O CODES - GASOLINE (for "C" & "K" Series) H - TESTS W/O CODES - GASOLINE (For "G" & "P" Series)
3. If trouble codes were not displayed and a driveability problem exists, refer to SYMPTOMS in appropriate TESTS W/O CODES article in this section above. From there you will be sent to the appropriate area in appropriate I - SYS/COMP TESTS article in this section below. I - SYSTEM/COMPONENT TESTS (for Astro & Safari) I - SYSTEM/COMPONENT TESTS (for "S" & "T" Series) I - SYSTEM/COMPONENT TESTS - GASOLINE (for "C" & "K" Series) I - SYSTEM/COMPONENT TESTS - GASOLINE (For "G" & "P" Series)
4. After repairs are made, clear trouble codes and perform FIELD SERVICE MODE CHECK in appropriate BASIC TESTING article in this section below. F - BASIC TESTING (for Astro & Safari) F - BASIC TESTING (for "S" & "T" Series) F - BASIC TESTING - GASOLINE (for "C" & "K" Series) F - BASIC TESTING - GASOLINE (For "G" & "P" Series)

READING TROUBLE CODES

Note. Trouble codes retrieved from ECM/PCM/VCM may be related to either engine or transmission. For engine-related codes, use this article. For transmission-related codes, see appropriate G - ELECTRONIC TRANSMISSION TESTS W/ CODES article in the ENGINE PERFORMANCE section below. To identify whether codes relate to transmission or engine, see TROUBLE CODE IDENTIFICATION table.

1. G - TESTS W/CODES - ELECTRONIC TRANSMISSION (for Astro & Safari)
2. G - TESTS W/CODES - ELECTRONIC TRANSMISSION (for "S" & "T" Series)
3. G - TESTS W/CODES - ELECTRONIC TRANSMISSION (for "C" & "K" Series)
4. AUTO TRANS DIAGNOSIS - 4L60-E & 4L80-E (For "G" & "P" Series)

The ECM stores component failure information under a related trouble code which can be recalled for diagnosis and repair. Read trouble codes by counting Malfunction Indicator Light (MIL) flashes or with diagnostic scan tester connected to the Data Link Connector (DLC). The tester is faster, and capable of reading information which would require testing individual ECM and sensor/solenoid connector terminals with a digital voltmeter. See SCAN TESTER DATA table and SCAN TESTER USAGE .

Note. When using a scan tester, there is a time delay between serial data updates. For instantaneous response, a digital voltmeter must be used.

If scan tester is not available, MIL flashes can be read by grounding DLC terminal with ignition on and engine off. For example, FLASH, FLASH, pause, FLASH, longer pause, indicates Code 21. The first series of flashes are the first digit of trouble code. The second series of flashes are the second digit of trouble code. Trouble codes are displayed starting with the lowest code. Each code is displayed 3 times and will continue as long as DLC is grounded.

Note. Trouble codes will be recorded at various operating times. Some codes require sensor or switch operation for 5 seconds and others may require longer under certain conditions. Some codes may not set in a service bay operational mode.

TROUBLE CODE ID (EXCEPT 4.3L M/T "S" & "T" SERIES PICKUP)

1. G - TESTS W/CODES - ELECTRONIC TRANSMISSION (for Astro & Safari)
2. G - TESTS W/CODES - ELECTRONIC TRANSMISSION (for "S" & "T" Series)
3. G - TESTS W/CODES - ELECTRONIC TRANSMISSION (for "C" & "K" Series)
4. AUTO TRANS DIAGNOSIS - 4L60-E & 4L80-E (For "G" & "P" Series) TROUBLE CODE ID (4.3L "S" & "T" SERIES PICKUP M/T) Code Probable Cause P0107 MAP Signal Voltage Low (4.3L) P0108 MAP Signal Voltage High (4.3L) P0122 TP Sensor Circuit Signal Voltage Low P0123 TP Sensor Circuit Signal Voltage High P0131 HO2S Circuit Low Signal Voltage P0132 HO2S Circuit High Signal Voltage P0139 HO2S Circuit Open (4.3L) P0171 Fuel Trim Lean (Lean Exhaust) P0172 Fuel Trim Rich (Rich Exhaust) P0320 Ignition Control Error (4.3L) P0321 Spark Reference Circuit P0327 Knock Sensor Circuit Grounded (4.3L) P0328 Knock Sensor Circuit Open (4.3L) P0342 Cam Signal Circuit P0400 EGR Error P0500 VSS Error (4.3L) P1222 Fuel Pump Voltage Low P1530 A/C Head Pressure Switch Circuit P1531 Low A/C Refrigerant Charge P1558 Cruise System Problem P1561 Cruise Vent Solenoid Circuit P1562 Cruise Vacuum Solenoid Circuit P1565 Cruise Servo Position Sensor Circuit P1567 Cruise Switches Circuit P1568 Cruise System Problem P1571 (1) Traction Control Torque Out Of Range P1573 (1) Traction Control Fault (Loss Of ABS/TCS Serial Data) P1599 Cruise Power Management P1623 PROM Error P1632 System Voltage High (4.3L) (1) Transmission code. For automatic transmission code diagnosis, see appropriate G - ELECTRONIC TRANSMISSION TESTS W/ CODES article in this section. See bullet list following this table.
5. G - TESTS W/CODES - ELECTRONIC TRANSMISSION (for Astro & Safari)
6. G - TESTS W/CODES - ELECTRONIC TRANSMISSION (for "S" & "T" Series)
7. G - TESTS W/CODES - ELECTRONIC TRANSMISSION (for "C" & "K" Series)

Note. Trouble code charts should only be used if Malfunction Indicator Light (MIL) is illuminated (indicating a current problem exists). Exceptions are Code 13, 15, 24, 44 and 45 charts, which may be used to help diagnose intermittent codes. Anytime Code 51, 52 or 55 is displayed with another code, start with 50-series code first and proceed to lower numbered codes.

HARD OR INTERMITTENT TROUBLE CODE DETERMINATION

During any diagnostic procedure, determine if codes are due to hard or intermittent failure. Diagnostic charts will not usually help diagnose intermittent codes. To determine hard codes and intermittent codes, proceed as follows

1. MANUALLY enter diagnostic mode. Read and record all stored trouble codes. Exit diagnostic mode and clear trouble codes. See «CLEARING TROUBLE CODES»(ref-19732-S08897769172001110500000) .
2. Apply parking brake and place transmission in Neutral or Park. Block drive wheels and start engine. MIL should go out. Run warm engine at specified curb idle for 2 minutes and note MIL.
3. If MIL comes on, manually enter diagnostic mode. Read and record trouble codes. This reveals hard failure codes. Codes 13, 15, 24, 44, 45 and 55 may require a road test to reset hard failure after trouble codes were cleared.
4. If MIL does not come on, all stored trouble codes were intermittent failures. Exceptions are noted under DIAGNOSTIC PROCEDURE.

CLEARING TROUBLE CODES

Turn ignition switch to ON position and ground diagnostic test terminal "B" at DLC. see scheme 1 Turn ignition switch to OFF position and remove ECM fuse from fuse block for 10 seconds. Replace fuse. Remove diagnostic terminal ground lead. Codes may also be cleared using the General Motors Tech 1 scan tester.

Diagnostic Aids

Diagnostic aids (located in many trouble code charts) are provided as additional tips to help with diagnosis when inspected circuit is okay.

SPECIAL TOOLS (DIAGNOSTIC)

Note. A special scan tester, plugged into the DLC, can read trouble codes, check system voltages on the serial data line and save a great deal of time. For additional information, see tester owner's manual. Also, see SCAN TESTER USAGE and SCAN TESTER DATA .

The computerized engine control system is most easily diagnosed using a scan tester. However, other tools may aid in diagnosing problems if a scan tester is unavailable. These tools are a tachometer, test light, ohmmeter, digital voltmeter with 10-megohm input impedance (minimum), vacuum pump, vacuum gauge, fuel injector test lights and 6 jumper wires 6" long (one wire with female connectors at both ends, one wire with male connector at both ends and 4 wires with male and female connectors at opposite ends). A test light, rather than a voltmeter, must be used when indicated by a diagnostic chart.

TROUBLE CODE CHARTS - 4.3L (EXC M/T "S" & "T" SERIES PICKUP)

Note. The following diagnostic mini-schematics are supplied courtesy of General Motors Corp.

Verify a clean, tight connection for the sensor ground. An open circuit at sensor signal terminal or ground terminal will result in a Code 13.

CODE 13 TERMINAL & CIRCUIT WIRING IDENTIFICATION

Code 13 Schematic (4.3L) Open Oxygen Sensor Circuit. Scheme 3

Code 13 Flow Chart (4.3L) Open Oxygen Sensor Circuit. Scheme 4

After engine is started, temperature should rise steadily to about 194°F (90°C), then stabilize when thermostat opens. If engine is allowed to cool overnight, coolant temperature sensor and MAT sensor (if equipped) should read close to each other, when measured with a scan tester.

CODE 14 TERMINAL & CIRCUIT WIRING IDENTIFICATION

TEMPERATURE-TO-RESISTANCE VALUES (1) (2)

Code 14 Schematic (4.3L) Coolant Sensor Signal Voltage Low. Scheme 5

Code 14 Flow Chart (4.3L) Coolant Sensor Signal Voltage Low. Scheme 6

After engine starts, temperature should rise steadily to about 194°F (90°C) and stabilize when thermostat opens. If engine is allowed to cool overnight, coolant temperature sensor and MAT sensor (if equipped) should read close to each other when measured with a scan tester. Code 15 will also set if sensor signal or ground circuit is open.

CODE 15 TERMINAL & CIRCUIT WIRING IDENTIFICATION

TEMPERATURE-TO-RESISTANCE VALUES (1) (2)

Code 15 Flow Chart (4.3L) Coolant Sensor Signal Voltage High. Scheme 7

Check for poor connections or damaged harness.

Code 16 Schematic ("G" Series) VSS Buffer Fault. Scheme 8

Code 16 Schematic ("L" & "M" Series) VSS Buffer Fault. Scheme 9

Code 16 Schematic ("S" & "T" Series A/T & Util. Vehicles M/T) VSS Buffer Fault. Scheme 10

Code 16 Flow Chart VSS Buffer Fault. Scheme 11

A scan tester displays throttle position in volts. Closed throttle voltage should be less than 1.25 volts. TPS voltage should increase at a steady rate to about 4.5 volts as throttle angle increases. Code 21 will also result if ground circuit is open or TPS signal circuit is shorted to voltage.

CODE 21 TERMINAL & CIRCUIT WIRING IDENTIFICATION

Code 21 Schematic (4.3L) TPC Signal Voltage High. Scheme 12

Code 21 Flow Chart (4.3L) TPC Signal Voltage High. Scheme 13

A scan tester displays throttle position in volts. Closed throttle voltage should be less than 1.0 volt. TPS voltage should increase at a steady rate to about 4.5 volts as throttle angle increases. Code 22 will also set if TPS signal or ground circuits are open or grounded.

CODE 22 TERMINAL & CIRCUIT WIRING IDENTIFICATION

Code 22 Flow Chart (4.3L) TPS Signal Voltage Low. Scheme 14

If engine is allowed to cool overnight, coolant and MAT sensors should read close to each other, when measured with a scan tester. A Code 23 will also result if signal and ground circuits become open.

CODE 23 TERMINAL & CIRCUIT WIRING IDENTIFICATION

TEMPERATURE-TO-RESISTANCE VALUES (1) (2)

Code 23 Schematic (4.3L) Intake Air Temperature Low. Scheme 15

Code 23 Flow Chart (4.3L) Intake Air Temperature Low. Scheme 16

Code 24 will set when no vehicle speed is detected at vehicle start off. Code 72 will set when VSS signal is present and is lost. Check all connections, especially those at transmission pass-through connector. If code is intermittent, see INTERMITTENTS in appropriate TESTS W/O CODES article in this section below. While Code 24 is set, scan tester will display an RPM derived from input speed. If input speed sensor is not operational at start-up, this can cause VSS to read zero.

1. H - TESTS W/O CODES (for Astro & Safari)
2. H - TESTS W/O CODES (for "S" & "T" Series)
3. H - TESTS W/O CODES - GASOLINE (for "C" & "K" Series)
4. H - TESTS W/O CODES - GASOLINE (For "G" & "P" Series)

Code 24, Schematic ("P" Series M/T) Vehicle Speed Sensor. Scheme 17

Code 24, Schematic ("S" & "T" Series Utility Vehicles M/T) Vehicle Speed Sensor. Scheme 18

Code 24, Flow Chart Vehicle Speed Sensor. Scheme 19

Scan tester reading should closely match speedometer reading with drive wheels turning. If vehicle is equipped with an automatic transmission, check park/neutral switch adjustment. If no problem is found while using flow chart, see INTERMITTENTS in appropriate TESTS W/O CODES article in this section below.

1. H - TESTS W/O CODES (for Astro & Safari)
2. H - TESTS W/O CODES (for "S" & "T" Series)
3. H - TESTS W/O CODES - GASOLINE (for "C" & "K" Series)
4. H - TESTS W/O CODES - GASOLINE (For "G" & "P" Series)

Code 24 Schematic (4.3L "T" Series Pickup M/T Vehicle Speed Sensor. Scheme 20

Code 24 Flow Chart Vehicle Speed Sensor. Scheme 21

Code 24 will set when no vehicle speed is detected at vehicle start off. Code 72 will set when VSS signal is present and is lost. Check all connections, especially those at transmission pass-through connector. If code is intermittent, see INTERMITTENTS in appropriate TESTS W/O CODES article in this section below. While Code 24 is set, scan tester will display an RPM derived from input speed. If input speed sensor is not operational at start-up, this can cause VSS to read zero.

1. H - TESTS W/O CODES (for Astro & Safari)
2. H - TESTS W/O CODES (for "S" & "T" Series)
3. H - TESTS W/O CODES - GASOLINE (for "C" & "K" Series)
4. H - TESTS W/O CODES - GASOLINE (For "G" & "P" Series)

Code 24 Schematic ("G" Series & "S" & "T" Series A/T) Vehicle Speed Sensor Transmission Output Signal. Scheme 22

Code 24 Schematic ("L" & "M" Series) Vehicle Speed Sensor Transmission Output Signal. Scheme 23

Code 24 Schematic ("P" Series M/T) Vehicle Speed Sensor Transmission Output Signal. Scheme 24

Code 24 Flow Chart Vehicle Speed Sensor Transmission Output Signal. Scheme 25

If engine is allowed to cool overnight, coolant temperature sensor and MAT sensor should read close to each other, when measured with a scan tester. A Code 25 will also result if sensor signal circuit is shorted to ground.

CODE 25 TERMINAL & CIRCUIT WIRING IDENTIFICATION

TEMPERATURE-TO-RESISTANCE VALUES (1) (2)

Code 25 Flow Chart (4.3L) Intake Air Temperature High. Scheme 26

Before replacing ECM, use an ohmmeter and check EGR valve resistances. Resistance between terminals "A" and "E" should be 9.5-10.5 ohms. Resistance between terminals "B" and "D" should be greater than 3000 ohms. Resistance between terminals "B" and "C" should start at 700 ohms and increase to about 4000 ohms as pintle is slowly moved inward.

Code 32 Schematic ("C" & "K" Series A/T) EGR Circuit Error With Linear EGR. Scheme 27

Code 32 Schematic ("P" Series Except 7.4L & "S" & "T" Series 4.3L W/ Calif. Emissions) EGR Circuit Error With Linear EGR. Scheme 28

Code 32 Schematic ("G" Series) EGR Circuit Error With Linear EGR. Scheme 29

Code 32 Schematic ("L" & "M" Series & "S" & "T" Series Pickup 4.3L (VIN W) Exc. W/ Calif. Emissions) EGR Circuit Error With Linear EGR. Scheme 30

Code 32 Schematic ("S" & "T" Series 4.3L (VIN Z) Exc. Calif. Emissions) EGR Circuit Error With Linear EGR. Scheme 31

Code 32 Flow Chart EGR Circuit Error With Linear EGR. Scheme 32

Prior to replacing ECM, check resistance of all ECM-controlled solenoids and relays. Replace any with a resistance value less than 20 ohms.

Code 32 Schematic ("P" Series 4.3L M/T) EGR Circuit Error. Scheme 33

Code 32 Flow Chart ("P" Series 4.3L M/T) EGR Circuit Error. Scheme 34

Prior to replacing PCM, check resistance of all PCM-controlled solenoids and relays. Replace any with a resistance value less than 20 ohms.

Code 32 Schematic ("C" & "K" Series 4.3L) EGR Ckt. Error Using Manifold Vac., 2-Terminal Sol. & Back Press. EGR. Scheme 35

Code 32 Schematic ("G" Series A/T) EGR Ckt. Error Using Manifold Vac., 2-Terminal Sol. & Back Press. EGR. Scheme 36

Code 32 Schematic ("P" Series 4.3L) EGR Ckt. Error Using Manifold Vac., 2-Terminal Sol. & Back Press. EGR. Scheme 37

Code 32 Flow Chart EGR Ckt. Error Using Manifold Vac., 2-Terminal Sol. & Back Press. EGR. Scheme 38

Prior to replacing PCM, check resistance of all PCM-controlled solenoids and relays. Replace any with a resistance value less than 20 ohms. EVRV solenoid cannot be checked with a DVOM due to solid state circuitry.

Code 32 Schematic ("C" & "K" Series) EGR Circuit Error Models Using Ported Vacuum & EVR Solenoid. Scheme 39

Code 32 Schematic ("G" Series) EGR Circuit Error Models Using Ported Vacuum & EVR Solenoid. Scheme 40

Code 32 Schematic ("S" & "T" Series 4.3L Pickup A/T & 4.3L Utility M/T) EGR Circuit Error Models Using Ported Vacuum & EVR Solenoid. Scheme 41

Code 32 Flow Chart EGR Circuit Error Models Using Ported Vacuum & EVR Solenoid. Scheme 42

With ignition switch in ON position and engine off, manifold pressure is equal to atmospheric pressure and signal voltage is high. Comparing BARO readings from a known good vehicle using the same sensor is a good way to check accuracy of suspected sensor. Readings should be within .4 volt of each other. Code 33 will also result if ground circuit is open or MAP signal circuit is shorted to voltage or to 5-volt reference circuit.

CODE 33 TERMINAL & CIRCUIT WIRING IDENTIFICATION

Code 33 (4.3L) MAP Sensor Signal Voltage High. Scheme 43

Code 33 Flow Chart (All Models) MAP Sensor Signal Voltage High. Scheme 44

With ignition switch in ON position and engine off, manifold pressure is equal to atmospheric pressure and signal voltage will be high. Comparing BARO readings with a known good vehicle using the same sensor is a good way to check accuracy of suspected sensor. Readings should be within .4 volt of each other. A Code 34 will also result if 5-volt reference and MAP signal circuits are open or shorted to ground. If 5-volt reference circuit is not shorted to ground and a Code 22 is stored, check MAP signal circuit for short to ground.

CODE 34 TERMINAL & CIRCUIT WIRING IDENTIFICATION

Code 34 Flow Chart (All Models) MAP Sensor Signal Voltage Low. Scheme 45

A slow, unstable idle may be caused by a system problem that cannot be overcome by IAC. Scan counts will be greater than 60 if too low, and zero counts if too high. If idle is too high, stop engine. With ignition on, ground ALDL test terminal "B". Wait 45 seconds for IAC to seat, then disconnect IAC. Start engine. If idle speed is greater than 800 RPM, inspect vehicle for vacuum leaks.

If BARO is low, a 96 second reset will result. If Code 36 is set, Code 35 will usually also be set. Repair actuator circuit first. If IAC counts are zero, idle speed control actuator will disengage and raise idle speed.

Code 36 Schematic ("C" & "K" Series) Idle Speed Actuator Fault. Scheme 46

Code 36 Schematic ("P" Series) Idle Speed Actuator Fault. Scheme 47

Code 36 Flow Chart Idle Speed Actuator Fault. Scheme 48

The scan tester cannot help diagnose a Code 42 problem. See INTERMITTENTS in TESTS W/O CODES article in this section below.

1. H - TESTS W/O CODES (for Astro & Safari)
2. H - TESTS W/O CODES (for "S" & "T" Series)
3. H - TESTS W/O CODES - GASOLINE (for "C" & "K" Series)
4. H - TESTS W/O CODES - GASOLINE (For "G" & "P" Series)

Code 42 Schematic ("L" & "M" Series 4.3L VIN Z) Ignition Control. Scheme 49

Code 42 Schematic ("P" Series A/T) Ignition Control. Scheme 50

Code 42 Schematic ("G" Series) Ignition Control. Scheme 51

Code 42 Schematic ("P" Series M/T) Ignition Control. Scheme 52

Code 42 Schematic ("L" & "M" Series 4.3L VIN W) Ignition Control. Scheme 53

Code 42 Schem.("S" & "T" Series 4.3L A/T& Util. Vehicles M/T) Ignition Control. Scheme 54

Code 42 Schematic ("T" Series 4.3L Pickup M/T) Ignition Control. Scheme 55

Code 42 Flow Chart Ignition Control. Scheme 56

Control module applies and monitors a 5-volt DC signal to knock sensors. Internal knock sensor circuitry pulls this DC signal down to about 2.5 volts. When knock sensor detects detonation, it generates an AC signal which rides back on DC signal to VCM. Knock signal intensity is dependent upon knock signal level.

Code 43 Schematic ("S" & "T" Series) Knock Sensor (Dual Sensors). Scheme 57

Code 43 Flow Chart ("S" & "T" Series) Knock Sensor. Scheme 58

ECM/PCM applies and monitors a 5-volt DC signal to knock sensor. Internal knock sensor circuitry pulls this DC signal down to about 2.5 volts. When knock sensor detects detonation, it generates an AC signal which rides back on DC signal to ECM/PCM. Knock signal intensity is dependent upon knock signal level.

Code 43 Schematic ("C", "G", "K" & "P" Series A/T) Knock Sensor W/O Spark Module (Single Sensor). Scheme 59

Code 43 Flow Chart ("C", "G", "K" & "P" Series A/T) Knock Sensor W/O Spark Module (Single Sensor). Scheme 60

Code 43 can be caused by a faulty knock sensor connection at ESC module or ECM. Also, check controller-to-ECM signal line for an open or short to ground.

Code 43 Schematic ("C" & "K" Series) Knock Sensor W/ Spark Control Module. Scheme 61

Code 43 Schematic ("P" Series) Knock Sensor W/ Spark Control Module. Scheme 62

Code 43 Flow Chart ("C", "K" & "P" Series) Knock Sensor W/ Spark Control Module. Scheme 63

Using scan tester, observe Block Learn Memory (BLM) value at different RPMs. If Code 44 conditions exist, block learn value will be around 150-172.

Code 45, rich exhaust, is most likely caused by one of the following

An intermittent may be caused by a poor connection, rubbed through wire insulation or a wire broken inside the insulation. Check for

1. Intermittents: An intermittent or open in Gray or Light Green wire circuits will set Code P0107.
2. Sensor Accuracy: With ignition on and engine off, pressure signal is equal to atmospheric pressure with signal high. This information is used by VCM as an indication of altitude. Comparison of this reading with a known good vehicle with same sensor is a way to check accuracy of suspect sensor. Reading should be the same within 0.4 volt.
3. Disconnect sensor from bracket and twist sensor by hand to check for intermittents. Output changes greater than 0.1 volt indicate a bad sensor or connection.

Code P0107 Schematic (4.3L "S" & "T" Series Pickup M/T) MAP Sensor Circuit. Scheme 64

An intermittent may be caused by a poor connection, rubbed through wire insulation or a wire broken inside the insulation. Check for

1. Intermittents: An intermittent or open in wire circuits will set Code P0108.
2. Sensor Accuracy: With ignition on and engine off, pressure signal is equal to atmospheric pressure with signal high. This information is used by VCM as an indication of altitude. Comparison of this reading with a known good vehicle with same sensor is a way to check accuracy of suspect sensor. Reading should be the same within 0.4 volt.
3. Disconnect sensor from bracket and twist sensor by hand to check for intermittents. Output changes greater than 0.1 volt indicate a bad sensor or connection.

Tech 1 displays engine temperature in degrees celsius (°C) and fahrenheit (°F). After engine is started, temperature should rise steadily to 194°F (90°C), then stabilize when thermostat opens. An intermittent may be caused by a poor connection, rubbed through wire insulation or a wire broken inside the insulation. Check for

1. Poor Connection Or Damaged Harness: Inspect PCM harness connectors for backed out terminals, improper mating, broken locks, improperly formed or damaged terminals, poor terminal to wire connection, and damaged harness.
2. Intermittent Test: Using Tech 1, monitor engine coolant temperature while moving related connectors and wiring harness. If failure is induced, display will change. This may assist in isolating the location of the malfunction.
3. Shifted Sensor: See TEMPERATURE-TO-RESISTANCE VALUES table to test ECT sensor at various temperature levels to evaluate the possibility of a shifted (mis-scaled) sensor which may result in driveability complaints.
4. A faulty connection, or an open in sensor circuits will cause Code P0117 to set.

TEMPERATURE-TO-RESISTANCE VALUES (1)

Code P0117 Schematic (4.3L "S" And "T" Series Pickup M/T) Coolant Temperature Sensor. Scheme 65

Code P0117 Flow Chart Coolant Temperature Sensor. Scheme 66

Tech 1 displays engine temperature in degrees celsius (°C) and fahrenheit (°F). After engine is started, temperature should rise steadily to 194°F (90°C), then stabilize when thermostat opens. An intermittent may be caused by a poor connection, rubbed through wire insulation or a wire broken inside the insulation. Check the following for

1. A short to ground in circuit No. 410 (Yellow wire).
2. Intermittent Test: Using Tech 1, monitor engine coolant temperature while moving related connectors and wiring harness. If failure is induced, display will change. This may assist in isolating the location of the malfunction.
3. Shifted Sensor: See TEMPERATURE-TO-RESISTANCE VALUES table to test ECT sensor at various temperature levels to evaluate the possibility of a shifted (mis-scaled) sensor which may result in driveability complaints.

TEMPERATURE-TO-RESISTANCE VALUES (1)

Code P0118 Flow Chart Coolant Temperature Sensor. Scheme 67

Tech 1 displays throttle position in volts. Voltage should increase steadily as throttle is moved toward WOT. An open or short to ground in reference and signal circuits will cause Code P0122 to set. Check for

1. Poor Connection Or Damaged Harness: Inspect PCM harness connectors for backed out terminals, improper mating, broken locks, improperly formed or damaged terminals, poor terminal to wire connection, and damaged harness.
2. Intermittent Test: Monitor TP sensor voltage display on Tech 1 while moving related connectors and wiring harness. If failure is induced, display will change. This may assist in isolating the location of the malfunction.
3. TP Sensor Scaling: Observe TP sensor voltage display while depressing accelerator pedal with engine stopped and ignition on. Display should vary from closed throttle TP sensor voltage when throttle is closed (.20-.74 volt) to greater than 4 volts when throttle is held at WOT position.

Code P0122 Flow Chart Throttle Position Sensor. Scheme 68

Tech 1 displays throttle position in volts. With closed throttle, ignition on, or at idle, voltage should be .20-.74 volt. If voltage is not as specified, replace TP sensor to set. Check for

1. Poor Connection Or Damaged Harness: Inspect PCM harness connectors for backed out terminals, improper mating, broken locks, improperly formed or damaged terminals, poor terminal to wire connection, and damaged harness.
2. Intermittent Test: Monitor TP sensor voltage display on Tech 1 while moving related connectors and wiring harness. If failure is induced, display will change. This may assist in isolating the location of the malfunction.
3. TP Sensor Scaling: Observe TP sensor voltage display while depressing accelerator pedal with engine stopped and ignition on. Display should vary from closed throttle TP sensor voltage when throttle is closed (.20-.74 volt) to greater than 4 volts when throttle is held at WOT position.

Code P0123 Schematic (4.3L "S" & "T" Series Pickup M/T) Throttle Position Sensor. Scheme 69

Code P0123 Flow Chart Throttle Position Sensor. Scheme 70

Code P0123 Flow Chart. Scheme 71

If oxygen sensor heater is not operating properly, system may go into open loop operation after extended idle. Tech-1 voltage varies between 0.1-1.0 volt in closed loop. Code P0139 sets in 1 minute if voltage is fixed between 0.35-0.55 volt. System will go into open loop in about 15 seconds.

Oxygen sample in sensor is needed for proper operation. Sample is directed through sensor wiring. All sensor wiring and connectors should be examined for breaks or contamination which may prevent reference oxygen from reaching sensor.

Code P0139 Schematic (4.3L "S" & "T" Series Pickup M/T) Heated O2 Sensor Open Circuit. Scheme 72

Code P0139 Flow Chart (4.3L "S" & "T" Series Pickup M/T) Heated O2 Sensor Open Circuit. Scheme 73

Using Tech-1, observe long term fuel trim values at different RPM and airflow conditions. If conditions for Code P0171 exist, long term fuel trim values will be around 150. Check for

1. Oxygen Sensor Wiring: Reference oxygen is supplied through sensor wiring. Check sensor wiring for breaks or contamination tha may prevent reference oxygen from reaching sensor.
2. Intermittent Ground: Check for an intermittent ground in wire between sensor and connector.
3. Fuel Contamination: Water near in-tank fuel pump can be delivered to injectors, causing a lean exhaust and setting Code P0171.
4. Fuel Pressure: If fuel pressure is too low, system will be lean. Monitor fuel pressure while driving at various speeds and loads to confirm.
5. Exhaust Leaks: If an exhaust leak exists, engine may pull outside air into exhaust and past sensor. Vacuum or crankcase leaks can cause a lean condition.
6. Sensor Ground Circuit: If Tan wire circuit is open, voltage at signal terminal may be about 0.45 volt. This may also cause a Code P0139 to be set.
7. Oxygen Sensor: If all checks are okay, heated oxygen sensor is faulty.

Code P0171 Flow Chart. Scheme 74

Using Tech-1, observe long term fuel trim vaules at different RPM and airflow conditions. If conditions for Code P0171 exist, long term fuel trim values will be around 115. Check for

1. Oxygen Sensor Wiring: Reference oxygen is supplied through sensor wiring. Check sensor wiring for breaks or contamination that may prevent reference oxygen from reaching sensor.
2. Fuel Pressure: System will go rich is pressure is too high. VCM will compensate for some increase, but code will set if pressure gets too high.
3. Fuel Contamination: Check for oil contaminated fuel.
4. HEI Shielding: An open ground in ignition system reference low circuit may result in electrical interference. VCM looks at this at reference pulses. Pulses result in a higher than actual engine speed signal. VCM then delivers too much fuel, causing system to go rich. Tachometer will also show higher than actual engine speed, which can help in diagnosing problem.
5. MAP Sensor: If an output causes VCM to sense high manifold pressure (low vacuum), system will go rich. Disconnecting MAP sensor will allow VCM to set a fixed MAP value. Substitute a different MAP sensor if rich condition is gone when sensor is disconnected.
6. Pressure Regulator: Check for leaking fuel in pressure regulator diaphragm by checking for liquid fuel.
7. Throttle Position Sensor: An intermittent TP sensor output will cause system to go rich due to false indication of engine accelerating.
8. Engine Coolant Temperature Sensor: Check for shifted sensor that could cause a rich exhaust, but set Code P0117.

Tech-1 tool does not have ability to help diagnose any Code P0320 problems. Check for intermittents. An intermittent may be caused by a poor connection, rubbed through wire insulation, or a broken wire inside insulation. Check for backed out connector terminals or broken down insulation.

Code P0320 Schematic (4.3L "S" & "T" Series Pickup M/T) Ignition Control Error. Scheme 75

Code P0320 Flow Chart (4.3L "S" & "T" Series Pickup M/T) Ignition Control Error. Scheme 76

Check Dark Blue wire circuit for a potential ground. Check knock sensor(s) connections. A worn connector may not fit properly because of underhood temperatures. Check for proper PROM installation.

Code P0327 Schematic (4.3L "S" & "T" Series Pickup M/T) Knock Sensor Grounded. Scheme 77

Code P0327 Flow Chart (4.3L "S" & "T" Series Pickup M/T) Knock Sensor Grounded. Scheme 78

Check Dark Blue wire circuit for a potential open. Check knock sensor(s) connections. A worn connector may not fit properly because of underhood temperatures. Check for proper PROM installation.

Code P0328 Flow Chart (4.3L "S" & "T" Series Pickup M/T) Knock Sensor Open. Scheme 79

Before replacing VCM, check resistance of each VCM controlled relay and solenoid coil, except EVRV. EVRV is solid state. Replace any solenoid when resistance measures less than 2 ohms. An intermittent may be caused by a broken wire inside insulation. Check for

1. Poor Connection Or Damaged Harness: Inspect PCM harness connectors for backed out EGR control circuit terminal, improper mating, broken locks, improperly formed or damaged terminals, poor terminal to wire connection, and damaged harness.

Code P0400 Schematic (4.3L "S" & "T" Series Pickup M/T) (EGR) Error. Scheme 80

Code P0400 Flow Chart (4.3L "S" & "T" Series Pickup M/T) (EGR) Error. Scheme 81

Check that proper calibration is in the VCM for vehicle speedometer.

Code P0500 Schematic (4.3L "S" & "T" Series Pickup M/T) VSS Circuit. Scheme 82

Code P0500 Flow Chart (4.3L "S" & "T" Series Pickup M/T) VSS Circuit. Scheme 83

Fuel pump should run for 2 seconds and then turn off if VCM senses no reference pulses. Fuel pump relay is located in convenience center.

Code P1222 Schematic (4.3L "S" & "T" Series Pickup M/T) Fuel Pump Voltage Low. Scheme 84