P0170
PowertrainFuel Trim Malfunction (Bank 1)
Bank 1 fuel trim malfunction: the ECU's fuel trim values for bank 1 are outside the acceptable correction range, but the system hasn't yet decided whether the engine is running too lean or too rich. P0170 is the broader 'something's wrong with mixture' code that often precedes a more specific P0171 or P0172.
ⓘ Information only. This page provides general educational information about fault code P0170. We do not assess the urgency or safety implications of any specific fault. That requires in-person diagnosis by a qualified mechanic. Full terms.
Recommended next steps
Whether a fault is urgent, drivable, or routine depends entirely on the cause on a specific vehicle, and that can only be determined by a qualified mechanic with diagnostic equipment. If a warning light is illuminated, the most reliable next step is professional diagnosis.
What does P0170 mean?
P0170 is a Powertrain (engine, transmission, fuel system) fault code. It indicates: Fuel Trim Malfunction (Bank 1).
This is a standardised OBD-II code. The technical definition is the same regardless of the make or model of vehicle, although specific causes and symptoms can vary between vehicles.
Symptoms commonly associated with this code
Symptoms that drivers often report alongside this code. Not all may apply to every case:
- • Engine warning light
- • Rough idle, hesitation, or stalling
- • Worse fuel economy
- • Possibly black smoke (rich) or rough running with hissing (lean)
- • Sometimes intermittent symptoms that come and go with engine load
Possible causes
Causes commonly associated with P0170, listed in approximate order of typical investigation. The actual cause on a specific vehicle can only be confirmed by professional diagnosis.
- 1. Vacuum leak (lean condition pulling trim positive)
- 2. Failed MAF sensor reading high or low (mismatched air calculation)
- 3. Leaking injector (rich condition pulling trim negative)
- 4. Faulty fuel pressure (too high → rich, too low → lean)
- 5. Exhaust leak ahead of the front oxygen sensor faking a lean reading
- 6. Failed coolant temperature sensor reading too cold (ECU runs rich)
- 7. Failed front oxygen sensor reading inaccurately
How mechanics typically diagnose
A typical diagnostic sequence used by mechanics, provided here for educational reference only. Diagnostic work should be performed by a qualified mechanic with the appropriate tools and training.
- 1. Read live fuel trim data, the direction (positive or negative) tells you whether the engine is lean or rich
- 2. Investigate accordingly: lean (trim positive) → vacuum leaks, MAF, fuel pressure low; rich (trim negative) → injectors, fuel pressure high, coolant temp
- 3. Use carb cleaner around suspect vacuum leak points, idle change confirms a leak
- 4. Check MAF sensor for contamination
- 5. On older cars: check fuel filter (rarely the cause on modern cars but a £15 fix if overdue)
Common questions about P0170
P0170 versus P0171 versus P0172? +
P0170 is the broader 'fuel trim malfunction' code, the trim has gone outside spec but doesn't specify direction. P0171 is specifically lean (trim positive). P0172 is specifically rich (trim negative). P0170 can appear before a more specific code as the fault develops, or alongside one. Treat them all the same way: read live trim data to determine direction, then investigate accordingly.
Could a recent service have caused this? +
Sometimes. After spark plug or air filter replacement, mistakes can introduce vacuum leaks (loose vacuum line knocked off during work), incorrectly fitted air filter creating an air leak, or wrong-spec parts creating fuelling issues. If P0170 appeared shortly after a service, check the work area first.
Will my MOT pass? +
The code itself doesn't directly fail MOT, but if the engine warning light is on at MOT time, that's an automatic fail post-2018. Also, a rich or lean condition can affect emissions readings during the test. Fix and clear before booking.
Information only, not professional advice
The information on this page is provided for general guidance and educational purposes only. It is not a substitute for diagnosis or repair advice from a qualified mechanic. Always verify any fault before paying for repairs. carfaultcodes.co.uk accepts no liability for decisions made based on this information. Full terms →