| CAUTION | If ECM replacement is instructed in following testing, always ensure ECM connectors and ground circuits are okay. If either are suspect, repair and repeat testing to confirm ECM malfunction. |
Note. Heated oxygen sensor No. 1 refers to oxygen sensor closest to engine block. Heated oxygen sensor No. 2 refers to oxygen sensor farthest from engine block.
Note. For component location, see appropriate illustration in THEORY & OPERATION article.
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 driven rich beyond a certain value. Possible causes are
- Exhaust system leak.
- Improper fuel pressure.
- Fuel injector restricted.
- Defective heated oxygen sensor No. 1.
- Defective Mass Airflow (MAF) meter.
- Defective Engine Coolant Temperature (ECT) sensor.
- Vehicle is ran out of fuel.
DTC P0172 is set when fuel trim is driven lean beyond a certain value. Possible causes are
- Exhaust system leak.
- Improper fuel pressure.
- Fuel injector blocked or leaking.
- Defective heated oxygen sensor No. 1.
- Defective Mass Airflow (MAF) meter.
- Defective Engine Coolant Temperature (ECT) sensor.
- Vehicle is ran out of fuel.
Diagnostic Aids
Using scan tool, read FREEZE FRAME data. Freeze frame data records engine conditions when malfunction is detected.