P0173: Fuel Trim Bank 2

What does P0173 mean?

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.

The code tells you which monitor failed, not which part is broken. Test before replacing parts.

Symptoms

• Check engine light on (after two drive cycles)
• Rough idle
• Higher consumption
• Often no symptoms

Common causes

• High Air flow sensor dirty or faulty
• Medium Air leak on bank 2 or at the crankcase ventilation
• Medium Fuel pressure or bank 2 injectors
• Low Exhaust leak ahead of the bank 2 oxygen sensor

Likelihood is a general estimate across vehicles; your make and model may differ.