P0170 serious

Fuel Trim Malfunction (Bank 1)

The P0170 code means the vehicle's onboard diagnostic system has detected: Fuel Trim Malfunction (Bank 1). This is a serious severity code.

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Keep driving?: Yes, but fix soon
DIY difficulty: medium
Estimated cost: $0-$500 depending on cause

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Common Symptoms

Check Engine Light on
Rough idle
Hesitation
Black smoke or lean stumble
Poor fuel economy

Probable Causes (Ranked by Likelihood)

Vacuum leak causing lean condition Very High
Dirty or failed MAF sensor High
Fuel delivery problem (pump, injectors, regulator) High
Failed upstream O2 sensor causing incorrect fuel trim Moderate
Exhaust leak near O2 sensor affecting readings Low

Step-by-Step Diagnostic Procedure

Check short-term and long-term fuel trim values (STFT and LTFT) on Bank 1. Values beyond plus or minus 25% trigger this code.
If LTFT is very positive (+25%), the engine is running lean: check for vacuum leaks and MAF sensor issues.
If LTFT is very negative (-25%), the engine is running rich: check for leaking injectors or high fuel pressure.
Perform a smoke test for vacuum leaks.
Test MAF sensor, fuel pressure, and injector pulse width.

Frequently Asked Questions

What is a fuel trim?

Fuel trim is the ECU correction applied to the base fuel map to keep the air-fuel ratio at stoichiometric (14.7:1). Positive trim means the ECU is adding fuel (lean); negative trim means it is removing fuel (rich). Values beyond plus or minus 25% trigger P0170.