With closed-loop control the engine module corrects the injected fuel using the oxygen sensor: short-term fuel trim reacts within fractions of a second, long-term trim stores a learned base correction per load and speed range. Positive values mean fuel is being added because the mixture was lean, negative values mean fuel is being removed; around ±10 % is normal. P0173 is the bank 2 counterpart of P0170: a direction-neutral trim fault, so whether bank 2 runs lean or rich has to be read from the trim values. It is mainly used by Mercedes-Benz V6 and V8 petrol engines instead of P0174/P0175; their workshop magazine names the air flow sensor as the most frequent fix. P0170 and P0173 together point to a common cause. No manufacturer threshold was found; the description rests on CARB's fuel system monitoring requirement and that publication. Two trips.
Medium severity. Can I keep driving?You can drive in moderation for a short time. A very lean or very rich mixture can damage the engine or the catalytic converter.
Bank 2 is the side without cylinder 1, so it only exists on V or boxer engines.
Symptoms
- Check engine light on (after two drive cycles)
- Rough idle
- Higher consumption
- Often no symptoms
Most likely causes
- Air flow sensor dirty or faulty
- Air leak on bank 2 or at the crankcase ventilation
- Fuel pressure or bank 2 injectors
- Exhaust leak ahead of the bank 2 oxygen sensor
What to check, in order
Start with the simplest. You can tick off each step.
Good to know
- Repair difficulty: medium
- It affects emissions.
What each character of P0173 means
- P: Powertrain: engine and transmission
- 0: Generic code: same on every make
- 1: Air and fuel
- 73: Specific fault in that system
Frequently asked questions
Can I keep driving with code P0173?
You can drive in moderation for a short time. A very lean or very rich mixture can damage the engine or the catalytic converter.
What causes code P0173?
The most common causes are: air flow sensor dirty or faulty; air leak on bank 2 or at the crankcase ventilation; fuel pressure or bank 2 injectors.
Related codes
- P0170Fuel Trim Bank 1
- P0174System Too Lean Bank 2
- P0175System Too Rich Bank 2
- P0101Mass or Volume Air Flow Sensor A Circuit Range/Performance
Sources
- law.cornell.edu: 13 CCR 1968.2
- automotivetechinfo.com: Causes of DTCs P0170 and P0173 in Mercedes Benz Ve
Values and procedures vary by manufacturer. If it affects brakes, steering, fuel or high voltage, see a professional. Updated: 2026-11-04. How we build these pages.