| 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. |
Note. Bank No. 1 refers to bank which includes cylinder No. 1. Bank No. 2 refers to bank without cylinder No. 1. Heated oxygen sensor No. 1 refers to sensor closest to engine block.
Circuit Description
Fuel trim refers to feedback compensation value compared against basic injection time. Fuel trim includes short-term and long-term fuel trim.
DTC P0171 is set when fuel trim is rich beyond a certain value. Possible causes are
- Air induction system malfunction.
- Improper fuel pressure.
- Fuel injector restricted.
- Defective heated oxygen sensor (bank No. 1 or 2, sensor No. 1).
- Defective Mass Airflow (MAF) meter.
- Defective Engine Coolant Temperature (ECT) sensor.
- Exhaust system leak.
DTC P0172 is set when fuel trim is lean beyond a certain value. Possible causes are
- Improper fuel pressure.
- Fuel injector restricted or leaking.
- Defective heated oxygen sensor (bank No. 1 or 2, sensor No. 1).
- Defective Mass Airflow (MAF) meter.
- Defective Engine Coolant Temperature (ECT) sensor.
- Exhaust system leak.
Diagnostic Aids
Using scan tool, read freeze frame data. Freeze frame records engine conditions when malfunction is detected.