Note. Bank No. 1 refers to bank that includes cylinder No. 1. Bank No. 2 refers to bank without cylinder No. 1. Sensor No. 1 refers to sensor closest to engine block.
| CAUTION | If ECM replacement is instructed in following testing, always ensure ECM harness connector and ground circuit are okay. If either are suspect, repair and repeat testing to confirm ECM malfunction. If ECM is replaced, ECM must be programmed with proper ignition key code for engine immobilizer system. For programming procedures, see COMPUTER RELEARN PROCEDURES article in GENERAL INFORMATION. |
Circuit Description
Fuel trim refers to feedback compensation value compared against basic injection time. Fuel trim includes short-term and long-term fuel trim. If total of short term fuel trim value and long term value is within 25 percent, system is functioning properly. DTC P0171 is set when fuel trim is rich beyond a certain value. Possible causes are
- Exhaust system leak.
- Air intake hose loose.
- Fuel line pressure.
- Injector blockage.
- Faulty Mass Airflow (MAF) sensor.
- Faulty Engine Coolant Temperature (ECT) sensor.
- Faulty A/F sensor.
DTC P0172 is set when fuel trim is lean beyond a certain value. Possible causes are
- Exhaust system leak.
- Fuel line pressure.
- Injector blockage or leak.
- Faulty Mass Airflow (MAF) sensor.
- Faulty Engine Coolant Temperature (ECT) sensor.
- Faulty A/F sensor.
Diagnostic Aids
Short-term and long-term fuel trim value should be within plus or minus 35 percent of each other at 176°F (80°C). ECM controls voltage of AFR+, AFL+, AFR- and AFL- terminals at ECM to a fixed voltage (3.3 volts at AFR+ or AFL+ terminal; 3.0 volts at AFR- or AFL- terminal). It is impossible to confirm A/F sensor output voltage without using a scan tool. OBD-II scan tools will display one fifth A/F sensor output voltage as Lexus scan tool will. Using scan tool, read freeze frame data. Freeze frame records engine conditions when malfunction is detected.
During fuel enrichment, output voltage of A/F sensors maybe less than 0.56 volt (2.8 volts on Lexus scan tool). During fuel cut, output voltage of A/F sensors maybe more than 0.76 volt (3.8 volts on Lexus scan tool). If output voltage of A/F sensor remains at 0.66 volt (3.3 volts on Lexus scan tool) during all conditions, A/F sensor circuit may be open. If output voltage of A/F sensor remains at 0.76 volt (3.8 volts on Lexus scan tool) or more during all conditions, A/F sensor circuit may be shorted. If output voltage of A/F sensor remains at 0.56 volt (2.8 volts on Lexus scan tool) or less during all conditions, A/F sensor circuit may be shorted.