P0173 code meaning
Fuel Trim Bank 2
Powertrain system Generic code P01xx codes Check engine light
What P0173 means
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 gasoline 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.
P0173 at a glance
- Check engine lightYes, usually on
- Emissions testCan fail
- Limp modeNot expected
- Repair difficultyMedium, DIY possible
- Typical labor0.5 to 3 hours
- Clears after repairAfter a drive cycle
Symptoms of P0173
- Check engine light on (after two drive cycles)
- Rough idle
- Higher consumption
- Often no symptoms
Common causes of P0173
- Most likelyAir flow sensor dirty or faulty
- PossibleAir leak on bank 2 or at the crankcase ventilation
- PossibleFuel pressure or bank 2 injectors
- Less likelyExhaust leak ahead of the bank 2 oxygen sensor
Check the most likely causes first. Replacing parts by guesswork is the most expensive way to clear a code.
Parts involved
Oxygen sensor bank2 sensor1Mass air flow sensorFuel systemIntake manifold
What each part of P0173 means
| P | Powertrain system: engine, fuel system, ignition, emissions and transmission. |
| 0 | Generic code: it means the same on every make (SAE standard). |
| 1 | Area: fuel and air metering. |
| 73 | The exact fault in that area. |
What to do next
- Note P0173 and any other codes stored with it: several codes together often point to one cause.
- Check for loose connectors and damaged wiring around the parts the code names.
- Check open recalls for your car: some faults are fixed for free by the dealer. Enter your VIN.
- Clear the code only after the repair. If it comes back, the fault is still there.
Buying a used car? Scan it for stored codes before you pay. A recently cleared memory can hide a problem.
Questions about P0173
What does P0173 mean?
P0173 means "Fuel Trim Bank 2". 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. It is a generic code, so it means the same on every make.
Is it safe to drive with P0173?
Usually yes, for a short time, if the check engine light is steady and the car drives normally. If the light flashes or the car runs badly, stop and have it checked.
Will P0173 fail an emissions test?
Yes, it can. P0173 is emissions-related, so most inspection programs fail a car while it is stored or while the check engine light is on.
How hard is P0173 to fix?
Difficulty: medium. A typical repair takes 0.5 to 3 hours of work. A careful DIY repair is possible with basic tools and a scanner.
Will P0173 clear itself after the repair?
After the repair the car has to complete a drive cycle (a mix of city and highway driving) before the test runs again. Until then the readiness monitor shows "not ready", which can fail an inspection.