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.
- Keep driving?
- Limited driving only
- 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
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.
What actually makes the computer set a fuel trim code?
The OBD II regulation requires the system to flag the fuel delivery system when it can no longer hold emissions at or below 1.5 times the applicable FTP standards, and — on vehicles with adaptive feedback control — when that control has used up all the adjustment the manufacturer allows (Title 13 CCR 1968.2, section (e)(6.2)). The second case is the useful one to understand: the code means the computer ran out of correction, which is why the underlying fault is often something it was compensating for silently for a long time. This is the threshold the monitor is designed around, not a description of your specific fault — the regulation says what the system must detect, not how to find it on your vehicle.
Mechanics Guide Step-by-step diagnosis
Step-by-Step Diagnostic Procedure
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Check short-term and long-term fuel trim values (STFT and LTFT) on Bank 1. Values beyond plus or minus 25% trigger this code.
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If LTFT is very positive (+25%), the engine is running lean: check for vacuum leaks and MAF sensor issues.
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If LTFT is very negative (-25%), the engine is running rich: check for leaking injectors or high fuel pressure.
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Perform a smoke test for vacuum leaks.
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Test MAF sensor, fuel pressure, and injector pulse width.
Sources 3
This page is built from documented references. Verify against your own service info before repair work.
- California Code of Regulations, Title 13 section 1968.2 (OBD II)malfunction criteria, section (e)
- SAE J2012Diagnostic Trouble Code Definitions (defines the generic OBD-II code ranges)
- Cross-checked against the SAE J2012:2002 DTC table (2026-08-27)archived scan of the SAE standard — readable copy, not SAE’s current edition
Commonly Affected Vehicles
- BMW 3 Series and 5 Series
- Volkswagen Jetta and Passat
- Audi A4 and A6
- Mercedes-Benz C-Class
- Volvo S60
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