Engine Controls - Tests W/codes - Tbi: Diagnosis Chevrolet Sportvan G20

SELF-DIAGNOSTIC SYSTEM

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. When malfunction is detected and light is turned on, a corresponding trouble code will be stored in ECM memory. To retrieve stored codes, see RETRIEVING CODES (NON-SCAN) in this article. 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. Appropriate diagnostic circuit check MUST be performed before using trouble code charts. See BASIC TESTING article.

2) If trouble codes were displayed (other than Code 12), determine whether codes are hard or intermittent. Hard codes cause SERVICE ENGINE SOON light to illuminate continuously with engine running. See HARD OR INTERMITTENT TROUBLE CODE DETERMINATION in this article. For diagnosing hard codes, proceed to appropriate trouble code chart in this article. For diagnosing intermittent codes, proceed to INTERMITTENTS in TESTS W/O CODES article. Exceptions are Code 13, 15, 24, 44 and 45 charts, which can help diagnose intermittent codes.

3) If trouble codes were not displayed and a driveability problem exists, refer to SYMPTOMS in TESTS W/O CODES article. From there you will be sent to the appropriate area in SYSTEMS & COMPONENT TESTING article.

4) After repairs are made, clear trouble codes and perform SYSTEM PERFORMANCE CHECK or FIELD SERVICE MODE check in BASIC TESTING article.

Scheme 50

1. Turn ignition on with engine off. SERVICE ENGINE SOON light should glow. Locate Assembly Line Data Link (ALDL) connector, attached to ECM wiring harness. Most ALDL connectors are located under dash on driver's side of vehicle. For exact location of ALDL, see appropriate COMPONENT LOCATIONS illustration in SYSTEM/COMP TESTS article. Turn ignition on. Insert jumper wire from terminal "B" (diagnostic test terminal) to terminal "A" (ground) of ALDL connector. (Scheme 50) NOTE: Inserting jumper wire into test and ground terminals of ALDL connector with engine running will cause fuel-injected vehicles to enter field service mode and codes will not flash. See FIELD SERVICE MODE in BASIC TESTING article.
2. SERVICE ENGINE SOON light should flash codes. Each code is flashed 3 times. If codes DO NOT flash, perform DIAGNOSTIC CIRCUIT CHECK in BASIC TESTING article. To exit diagnostic mode, turn ignition off and remove jumper wire from ALDL connector.

READING TROUBLE CODES

The ECM stores component failure information under a related trouble code which can be recalled for diagnosis and repair. Read trouble codes by counting SERVICE ENGINE SOON light flashes or with diagnostic "Scan" tester connected to the ALDL connector. The tester is faster, more accurate and capable of reading information which would require testing individual ECM and sensor/solenoid connector terminals with a digital voltmeter. See SCAN DATA TABLES and SCAN TESTER USAGE in this article.

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, SERVICE ENGINE SOON light flashes can be read by grounding ALDL diagnostic 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 ALDL 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.

ECM TROUBLE CODE DEFINITION

Note. Trouble code charts should only be used if SERVICE ENGINE SOON light is illuminated (indicating a current problem exists). Exceptions are Codes 13, 15, 24, 44 and 45 charts, which may be used to help diagnose intermittent codes. Anytime Codes 51, 52, 54 or 55 are displayed with another code, start with 50-series code first and proceed to low profile numbered codes.

HARD OR INTERMITTENT TROUBLE CODE DETERMINATION

During any diagnostic procedure, determine if codes are hard or intermittent failure codes. 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.
2. Apply parking brake and place transmission in Neutral or Park. Block drive wheels and start engine. SERVICE ENGINE SOON light should go out. Run warm engine at specified curb idle for 2 minutes and note SERVICE ENGINE SOON light.
3. If SERVICE ENGINE SOON light 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 SERVICE ENGINE SOON light does not come on, all stored trouble codes were intermittent failures. Exceptions are noted under DIAGNOSTIC PROCEDURE in this article.

CLEARING TROUBLE CODES

Turn ignition switch to ON position and ground diagnostic terminal lead at ALDL connector. Turn ignition switch to OFF position and remove ECM fuse from fuse block for 10 seconds. Replace fuse. Remove diagnostic terminal ground lead.

DIAGNOSTIC AIDS

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

SPECIAL TOOLS (DIAGNOSTIC)

Note. A special "Scan" tester, plugged into the ALDL, 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 table in this article.

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, dwell meter, 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.

DIAGNOSTIC TESTS

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

Chart A-5A, Schematic, Fuel Pump Relay Ckt "R & V" Series. Scheme 51

Flow Chart A-5A, Fuel Pump Relay Circuit "R & V" Series. Scheme 52

Verify a clean, tight connection for ground circuit No. 413. An open circuit at sensor signal terminal or ground terminal will result in a Code 13.

Code 13: Open Oxygen Sensor Circuit. Scheme 53

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

Code 14: Coolant Sensor Signal Voltage Low. Scheme 54

After engine starts, temperature should rise steadily to about 194°F (90°C) and stabilize when thermostat opens. If the engine is allowed to cool overnight, the 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: Coolant Sensor Signal Voltage High. Scheme 55

A "Scan" tester displays throttle position in volts. Closed throttle voltage should be about .43 volt (2.8L) or .45-.95 volt (on all others). TPS voltage should increase at a steady rate to 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: Circuit Diagram. Scheme 56

Code 21: Flow Chart TPS Signal Voltage High. Scheme 57

A "Scan" tester displays throttle position in volts. Closed throttle voltage should be about .43 volt (2.8L) or .45-.95 volt (on all others). TPS voltage should increase at a steady rate to 4.5 volts as throttle angle increases. Code 22 will also set if TPS signal or ground circuits are open or grounded.

Code 22: Flow Chart TPS Signal Voltage Low. Scheme 58

If the engine is allowed to cool overnight, the coolant and MAT sensors should read close to each other, when measured with a "Scan" tester. When Code 15 is set, the ECM will turn on the cooling fan. A Code 23 will also result if signal and ground circuits become open.

Code 23: Mat Sensor Temp Low (2.5L). Scheme 59

A faulty or misadjusted Park/Neutral switch may set a false Code 24. Use "Scan" tester and check for proper signal in Drive, while wiggling shifter. "Scan" tester MPH reading should closely match speedometer when vehicle is moving.

Code 24: Vehicle Speed Sensor (Except 3.1L). Scheme 60

A faulty or misadjusted Park/Neutral switch may set a false Code 24. Use Scan tester and check for proper signal in Drive, while wiggling shifter. Scan tester MPH reading should closely match dash while vehicle is in motion.

Code 24: Circuit Diagram & Flow Chart VSS Sensor (3.1L). Scheme 61

If the engine is allowed to cool overnight, the 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: MAT Sensor Temp High (2.5L). Scheme 62

The Code 32 chart is an EGR system functional check. If the EGR system works properly and a Code 32 has set, check for other causes for an increased open throttle block learn value.

1. Check for a blocked/restricted intake and EGR passages.
2. Check vacuum lines.
3. Check MAP sensor function. A MAP sensor may shift enough in calibration to change air/fuel mixture without setting a MAP related code.

Code 32: EGR System Error Circuit Diagram. Scheme 63

Code 32: EGR System Error Flow Chart (2.5L, 5.0L, 4.3L Except S & T, & 5.7L Under 8500 GVWR). Scheme 64

The Code 32 chart is a functional check of the EGR system. If the EGR system works properly and a Code 32 has set, check for other causes for an increased open throttle block learn value.

1. Check for a blocked/restricted intake and EGR passages.
2. Check vacuum lines.
3. Check MAP sensor function. A MAP sensor may shift enough in calibration to change air/fuel mixture without setting a MAP related code.

Code 32: EGR System Error (2.8L, 4.3L, S & T, & 5.7L Over 8500 GVWR). Scheme 65

With the ignition switch in the "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 the accuracy of the suspected sensor. Readings should be the same within +/- .4 volt. 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: MAP Sensor Signal Voltage High Circuit Diagram. Scheme 66

Code 33: MAP Sensor Signal Voltage High Flow Chart. Scheme 67

With the ignition switch in the ON position and engine off, manifold pressure is equal to atmospheric pressure and the signal voltage will be high. Comparing BARO readings with a known good vehicle using the same sensor is a good way to check the accuracy of the suspected sensor. Readings should be the same within +/- .4 volt. 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 stored Code 22 is stored, check MAP signal circuit for short to ground.

Code 34: MAP Sensor Signal Voltage Low Flow Chart. Scheme 68

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. 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.

The "Scan" tester cannot help diagnose a Code 42 problem. See INTERMITTENTS in H - TESTING W/O CODES article.

Code 42: EST With HEI Ignition Circuit Diagram. Scheme 69

Code 42: EST With HEI Ignition Flow Chart (All But R/V & G/P Models). Scheme 70

Code 42: EST With HEI Ignition Flow Chart (R/V & G/P Models). Scheme 71

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

Code 43: ESC Control Signal Voltage Low (Except 2.5L). Scheme 72

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

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