Safety Warning
DIY auto repair can cause serious injury, fire, or vehicle damage. These guides are for informational purposes only. Always follow OEM torque specs, wear PPE, and consult a certified mechanic if you are unsure. You are solely responsible for your safety.
Short Term Fuel Trim
The short term fuel trim values change rapidly in response to the HO2S signal voltages. These changes "fine tune" the engine fueling. The ideal fuel trim values are around zero percent. A positive fuel trim value indicates that the PCM is adding fuel to compensate for a lean condition. A negative fuel trim value indicates that the PCM is reducing the amount of fuel to compensate for a rich condition. When the PCM determines that the short term fuel trim is out of the operating range, the following DTCs will set:
- DTC P0171: FUEL TRIM SYSTEM LEAN
- DTC P0172: FUEL TRIM SYSTEM RICH
NO RELATED
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When to See a Mechanic
Stop DIY work and contact a certified mechanic immediately if any of the following apply:
- • You smell fuel, burning insulation, or see smoke.
- • Brakes feel soft, pull hard to one side, or make grinding noises.
- • The engine overheats, stalls repeatedly, or misfires under load.
- • You are missing required tools, torque specs, or safe lifting equipment.
- • You are not confident in the next step or safety outcome.