Applicable Systems
OBD-II Universal
Symptoms
The long-term fuel trim has reached -25%, indicating the engine is running rich. The ECM is reducing fuel delivery to compensate for a rich condition.
Possible Causes
- Fuel pressure regulator leaking, delivering excess fuel
- Fuel injector stuck open, over-fueling one cylinder
- Mass airflow (MAF) sensor over-reporting airflow, causing excess fuel
- Oxygen sensor giving a false lean signal, causing the ECM to add fuel
Troubleshooting Steps
- Step 1: Read the fuel trim values (short-term and long-term) with the OBD-II scanner
STFT and LTFT above +10% indicate lean, below -10% indicate rich - Step 2: If LTFT is negative (rich), check for fuel system leaks
Inspect the fuel pressure regulator vacuum line for fuel presence - Step 3: Test the MAF sensor by reading grams per second at idle and comparing to specification
Clean or replace the MAF if the reading is abnormally high - Step 4: If the oxygen sensor is suspect, read the O2 sensor voltage with the scanner
A stuck-lean O2 sensor (low voltage) causes rich operation
How to Reset
Reset: After repair, clear the fuel trim adaptation by disconnecting the battery for 10 minutes or using the scanner reset function.
Common Replacement Parts
- MAF sensor cleaner: Spray, non-residue (Do not use carburetor cleaner on MAF sensors)
- Oxygen sensor: Air/fuel ratio or wideband type (Match to vehicle and bank position)
- Fuel pressure gauge: 0-7 bar, mechanical or electronic (For fuel system testing)
Frequently Asked Questions
- Q: What is fuel trim?
The ECM's adjustment to fuel delivery based on oxygen sensor feedback. Positive trim = adding fuel (lean condition). Negative trim = subtracting fuel (rich condition). Range: +/-25%.
- Q: What is the MAF sensor?
Mass Airflow sensor measures the amount of air entering the engine. The ECM uses this to calculate fuel delivery. A dirty or faulty MAF causes incorrect fuel mixture.
- Q: How do I clean a MAF sensor?
Remove the sensor and spray MAF cleaner through the housing. Do not touch the sensing element. Allow to dry fully before reinstallation.
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Source: SAE J2012 Standard
Category: OBD-II
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