P0170: Fuel Trim Malfunction, Bank 1
P0170 means the engine computer on bank 1 (the side of the engine with cylinder 1) has had to add or remove more fuel than normal to keep the air-fuel mixture correct. It is usually not an emergency, but it turns on the check engine light and can make you fail an emissions test.
You can drive, but get it checked soon
The car usually keeps running normally and limp mode is not expected with this code, so short trips are generally fine. Because the mixture is off, fuel economy and emissions can suffer, and a rough or hesitating engine should be checked soon rather than ignored.
What it means
The engine computer constantly adjusts how much fuel is injected, using the oxygen sensor in the exhaust to check the result. Short-term fuel trim reacts within fractions of a second. Long-term fuel trim is a learned correction that is stored for different loads and engine speeds. A positive trim value means fuel is being added because the mixture was too lean (too much air or too little fuel). A negative value means fuel is being removed. Around plus or minus 10 percent is normal. P0170 is an older, direction-neutral code: it says the correction on bank 1 is out of range, but not whether the mixture was lean or rich. That has to be read from the trim values with a scan tool. The code is mainly seen on Mercedes-Benz gasoline engines, which often set P0170 and P0173 instead of P0171 through P0175, and on the Ford 6.7 liter diesel, where it flags a fuel quantity error above 15 percent in either direction. GM and Ford gasoline engines do not use it. The light comes on after two trips (drive cycles) with the fault present.
Likely causes
- Air flow sensor dirty or driftingVery likely
The air flow sensor measures the air entering the engine, and if it reads wrong the computer calculates the wrong amount of fuel; cleaning or replacing it is reported to cure this code most often on Mercedes-Benz engines.
- Air leak or leaking crankcase ventilationPossible
Extra air that enters after the sensor, or through the crankcase ventilation system, leans out the mixture and forces the computer to add fuel.
- Injection quantity errorPossible
On gasoline engines worn injectors or wrong fuel pressure can deliver the wrong amount of fuel, and on diesels injector drift or a faulty rail pressure sensor can do the same.
- Oxygen or NOx sensor offsetLess likely
A sensor that reads slightly off makes the computer correct for a problem that is not there.
What you may notice
- Check engine light on (after two drive cycles)
- Rough idle or hesitation
- Higher fuel consumption
- Often no symptoms at all
What to check, cheapest first
- 1Read the code and the freeze-frame data with a scan tool, then note whether the fuel trim values are positive (lean) or negative (rich). · you can do it
- 2Check for other stored codes such as P0173, P0171, P0172, P0101 or P0263, since they point toward the cause. · you can do it
- 3Look over the intake hoses, air filter housing and crankcase ventilation hoses for cracks, loose clamps or disconnected parts. · you can do it
- 4Clean the air flow sensor with a cleaner made for that purpose, or replace it if cleaning does not help; compare its live reading with expected values. · you can do it
- 5Test for vacuum or intake leaks, for example with a smoke test. · mechanic
- 6Check fuel pressure, injector quantity (or rail pressure sensor on a diesel) and the oxygen or NOx sensor readings. · mechanic
Related codes
Frequently asked questions
Can I drive with a P0170 code?+
Usually yes for a short time, since limp mode is not expected. Have it checked soon, especially if the engine runs rough or uses more fuel.
Will P0170 pass an emissions test?+
Probably not. The code is emissions relevant and turns on the check engine light, so it should be fixed and the code cleared before testing.
Can P0170 be caused by a dirty air flow sensor?+
Yes. It is the most common cause in the reference data, and cleaning or replacing the sensor is the first thing to try.
Can I fix it myself?+
Often, yes. Repair difficulty is medium and takes roughly half an hour to three hours, depending on whether the cause is a sensor, a leak or a fuel system fault.
Written with the help of AI from the public OBDex definition (CC0). Each make may use the code differently. Updated: 2026-11-10