P0173: Fuel Trim Fault, Bank 2
P0173 means the engine computer has had to push the fuel mixture on bank 2 (one side of a V6 or V8 engine) far from normal to keep it balanced. It is usually not an emergency, but it turns on the check engine light and should be diagnosed soon so it doesn't turn into a bigger problem.
You can drive, but get it checked soon
The data does not list limp mode for this code, and many cars show no symptoms. Short drives are generally fine, but a rough idle or higher fuel use can worsen, and the problem should not be left for long.
What it means
The engine computer constantly adjusts how much fuel is injected, using the oxygen sensor to see if the mixture is right. Short-term fuel trim reacts within fractions of a second. Long-term trim is a learned correction stored for each load and speed range. Positive values mean the computer is adding fuel because the mixture was lean (too much air), and negative values mean it is removing fuel. Around ±10 % is normal. P0173 is the bank 2 counterpart of P0170. It does not say by itself whether the mixture is lean or rich, so a technician has to read the trim values to find out. The code is mainly used by Mercedes-Benz V6 and V8 petrol engines instead of P0174 and P0175. If P0170 is stored together with P0173, a common cause is likely. The code is set after two trips (drive cycles) with the fault present.
Likely causes
- Air flow sensor dirty or faultyVery likely
This sensor measures the air entering the engine, and a wrong reading throws off the fuel calculation; it is named as the most frequent fix for this code on Mercedes-Benz engines.
- Air leak on bank 2 or at the crankcase ventilationPossible
Extra air that was not measured makes the mixture lean on that bank, and the computer adds fuel to compensate.
- Fuel pressure or bank 2 injectorsPossible
Low fuel pressure or a clogged or leaking injector changes how much fuel actually reaches the cylinders on bank 2.
- Exhaust leak ahead of the bank 2 oxygen sensorLess likely
Outside air entering the exhaust before the sensor can make the mixture look leaner than it really is.
What you may notice
- Check engine light on (after two drive cycles)
- Rough idle
- Higher fuel consumption
- Often no symptoms at all
What to check, cheapest first
- 1Read the stored codes and note whether P0170 is also present, since both together point to a shared cause such as the air flow sensor. · you can do it
- 2Look over the intake hoses between the air flow sensor and the engine, and the crankcase ventilation hoses, for cracks, loose clamps or disconnected pieces. · you can do it
- 3Listen for a hissing or whistling at idle and check for exhaust leaks or soot marks near the bank 2 exhaust manifold and oxygen sensor. · you can do it
- 4Inspect the air flow sensor and clean it only with a cleaner made for that purpose, then clear the code and see if it returns. · you can do it
- 5Read the short-term and long-term fuel trim values for bank 2 to see whether the engine is running lean or rich. · mechanic
- 6Test fuel pressure and check the bank 2 injectors, and test the air flow sensor readings if the simpler checks find nothing. · mechanic
Related codes
Frequently asked questions
Can I drive with a P0173 code?+
Short trips are usually possible, especially if the car runs normally. Have it checked soon, because a rough idle and higher fuel use can develop and the cause will not fix itself.
Can a loose gas cap cause P0173?+
The data does not list the gas cap as a cause. P0173 is about the air and fuel mixture on bank 2, so look first at the air flow sensor and air leaks.
Will P0173 pass an emissions test?+
Probably not. The code is emissions-relevant and turns on the check engine light, which normally means a failed test until the fault is repaired and the code stays away.
How hard is it to fix?+
Difficulty is medium and DIY is possible. The estimated time is 0.5 to 3 hours, depending on the cause.
Written with the help of AI from the public OBDex definition (CC0). Each make may use the code differently. Updated: 2026-11-10