Powertrain, generic · Fuel and air metering

P0173 Fuel Trim Bank 2

Check engine lightOn
EmissionsRelevant
Drive cycleRequired
Limp modeNo

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.

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

Symptoms

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

Repair estimate

DifficultyMediumDIY possible
Parts and labour€20–600
Time0.5–3 h

Typical range across vehicles; the real cost depends on what the testing finds.

Parts involved

oxygen sensor bank2 sensor1mass air flow sensorfuel systemintake manifold

Related codes

P0170P0174P0175P0101

References: law.cornell.edu, automotivetechinfo.com.