P0170: Fuel Trim Malfunction (Bank 1)
Quick Answer
Safe to drive (short-term)
P0170 means the fuel trim values on bank 1 have exceeded the normal correction range. The ECU can no longer properly compensate for a rich or lean condition by adjusting fuel delivery. This indicates a persistent air-fuel mixture problem that needs investigation.
- Severity
- Moderate · Fuel System
- Estimated repair cost
- From €20 to €500+, varies by vehicle
€20 or €500, depending on your car.
- Clean MAF sensor€20–€55
- Fuel injector cleaning or replacement€90–€500
A scan shows whether P0170 is really stored on your car, which codes sit next to it and how urgent each one is.
A scan needs a Bluetooth OBD2 adapter (Bluetooth LE on iPhone), about €25-60; the built-in demo runs without one. See which adapter to buy
What does P0170 mean?
P0170 is a fuel system-related diagnostic trouble code classified as moderate severity.
What are the symptoms of P0170?
- Check engine light on
- Poor fuel economy
- Rough or unstable idle
- Hesitation or stumbling during acceleration
- Possible misfires under load
- Failed emissions test
What causes P0170?
Vacuum leak (intake manifold gasket, hoses, PCV system)
: high
Dirty or failing mass air flow (MAF) sensor
: high
Faulty oxygen sensor providing incorrect feedback
: medium
Fuel pressure too high or too low
: medium
Leaking or clogged fuel injectors
: medium
Exhaust leak before the O2 sensor
: low
Is it safe to drive with P0170?
Generally yes, for short-term driving. You can drive to a workshop but expect reduced fuel economy and possible rough running. If accompanied by misfires or strong exhaust smell, limit driving.
How do you diagnose P0170?
- Use a scan tool to read short-term and long-term fuel trims for bank 1. Values within about +/- 10% are normal; a long-term trim sustained beyond about +/- 10% warrants investigation, and the code typically sets when the total correction reaches the ECU's limit, often around +/- 25%. Positive trims mean lean, negative trims mean rich
- If fuel trims are positive (lean), perform a smoke test or propane enrichment test to find vacuum leaks. Check intake manifold gaskets, PCV hoses, brake booster line, and all vacuum connections
- Inspect and clean the MAF sensor with dedicated MAF cleaner spray. A contaminated MAF underreports airflow, causing lean fuel trims
- Check fuel pressure with a gauge. Low pressure causes lean trims, high pressure causes rich trims. Compare to manufacturer specifications
- Monitor the upstream O2 sensor data. If it is sluggish or stuck, it may be providing incorrect feedback that forces the fuel trims out of range
- If trims are negative (rich), check for leaking injectors by performing a fuel injector balance test or checking for fuel smell in the oil
How much does P0170 cost to fix?
| Repair | Parts Cost | Labor Cost | Total Estimate | DIY Difficulty |
|---|---|---|---|---|
| Clean MAF sensor | €5–€15 | €15–€40 | €20–€55 | Easy |
| Fix vacuum leak (hose replacement or manifold gasket) | €10–€80 | €40–€200 | €50–€280 | Moderate |
| Replace MAF sensor | €40–€200 | €20–€50 | €60–€250 | Easy |
| Replace O2 sensor (bank 1, sensor 1) | €30–€150 | €40–€120 | €70–€270 | Moderate |
| Fuel injector cleaning or replacement | €30–€300 | €60–€200 | €90–€500 | Professional |
Prices estimated as of October 2026. Costs vary by region, vehicle, and shop.
P0170 on specific vehicles
Mercedes-Benz
Mercedes C-Class
2007-2019
The W204 C-Class four-cylinder petrols were the 1.8 litre M271 (supercharged from 2007, turbo CGI from 2009) and, from 2012, the 1.6 litre M274, which the W205 also uses.
Source: en.wikipedia.org, en.wikipedia.org, en.wikipedia.org
Opel
Opel Astra
2006-2015
The Astra's 1.6 litre Z16XER and A16XER petrol engines are naturally aspirated.
Source: en.wikipedia.org, en.wikipedia.org
P0170 on Ford
Ford Focus
Related diagnostic codes
Common questions about P0170
- What does P0170 mean?
- P0170 means the engine computer has detected that its fuel mixture corrections on bank 1 have gone beyond the normal operating range. The system tries to maintain the ideal 14.7:1 air-fuel ratio by adjusting fuel delivery, but something is preventing it from doing so.
- What is the difference between P0170 and P0171?
- P0170 indicates a general fuel trim malfunction - the trims are out of range in either direction. P0171 specifically means the system is too lean (too much air or too little fuel). P0170 is a broader code that can indicate either lean or rich conditions.
- Can a dirty MAF sensor cause P0170?
- Yes. A contaminated MAF sensor underreports the amount of air entering the engine, causing the ECU to deliver too little fuel. The O2 sensor detects the resulting lean condition and the fuel trims increase to compensate, eventually triggering P0170.
- How do I read fuel trim data?
- Use an OBD2 scan tool to view Short Term Fuel Trim (STFT) and Long Term Fuel Trim (LTFT). Values near 0% are ideal. Positive values mean the ECU is adding fuel (lean correction). Negative values mean it is removing fuel (rich correction). A long-term trim sustained beyond about +/- 10% warrants investigation; the code typically sets when the total correction reaches the ECU's limit, often around +/- 25%.
Read our detailed guides
Sources
- en.wikipedia.org en.wikipedia.org/wiki/Mercedes-Benz_W204
- en.wikipedia.org en.wikipedia.org/wiki/Mercedes-Benz_M271_engine
- en.wikipedia.org en.wikipedia.org/wiki/Mercedes-Benz_M274_engine
- en.wikipedia.org en.wikipedia.org/wiki/GM_Family_1_engine
- en.wikipedia.org en.wikipedia.org/wiki/Opel_Astra
- en.wikipedia.org en.wikipedia.org/wiki/Ford_Focus_(third_generation)
- en.wikipedia.org en.wikipedia.org/wiki/Mass_flow_sensor