Number P0173: 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. Positive values mean fuel is being added because the mixture was lean, ne
Can you drive with P0173?
Severity: Plan the repair. Yes — this is an emissions-system fault. The car will generally drive normally, but it will fail an emissions inspection with the light on.
If you ignore it: Fuel economy and emissions creep up, and some faults in this family (rich/lean running) stress the catalyst over time.
This is the #72 most common code in our top-150 library, in the fuel trim & air metering family (Powertrain system). Severity is Number Nest's editorial triage — if the light is flashing or the car feels unsafe, stop driving it whatever any page says.
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 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.
Systems involved: oxygen sensor bank2 sensor1, mass air flow sensor, fuel system, intake manifold.
Causes, ranked by likelihood
| # | Cause | Likelihood |
|---|---|---|
| 1 | Air flow sensor dirty or faulty | most likely |
| 2 | Air leak on bank 2 or at the crankcase ventilation | possible |
| 3 | Fuel pressure or bank 2 injectors | possible |
| 4 | Exhaust leak ahead of the bank 2 oxygen sensor | less common |
Check them in order — the list is ranked so the likely, cheaper causes come before the expensive part.
Symptoms you may notice
- Check engine light on (after two drive cycles)
- Rough idle
- Higher consumption
- Often no symptoms
Misdiagnosis traps
- Replacing the part named by the code title without testing. The code reports a failed self-test; the cause list above is ranked so the likely, cheap causes are checked before the expensive part is condemned.
- Clearing the code and declaring victory when the light stays off for a day — the monitor that sets this code runs only under specific driving conditions.
(General diagnostic guidance for this code family — no code-specific trap is published for this code.)
How a good shop confirms it
- Confirm the code and record any companion codes — companions narrow the system fast.
- Inspect the affected components and their wiring/connectors for damage, corrosion and looseness.
- Test the top-ranked cause with live data or a multimeter before replacing anything.
(Standard diagnostic path for this family, labeled as general guidance.)
Repair options & costs
| Repair | Shop total (parts + labor) | DIY (parts only) | Difficulty |
|---|---|---|---|
| Mass air flow (MAF) sensor | $158 – $420 | $120 – $320 (parts only) | 1/5 |
| Intake / vacuum leak repair (gaskets, hoses) | $338 – $838 | $150 – $400 (parts only) | 3/5 |
National ranges from Number Nest's cost model (midsize car, independent shop, US-average labor). Published cross-check — Mass air flow (MAF) sensor: CarMD 2026 national average $346.
OBDex repair data for this code rates the job difficulty medium with a typical labor time in the 0.5–3 hour range and DIY possible: yes.
Price this repair in your state
Estimated total: — (midsize car, independent shop)
Frequently asked questions
How much does it cost to fix P0173?
The fix most associated with this code — mass air flow (maf) sensor — runs $158 – $420 at an independent shop for a midsize car in our cost model (parts + labor, US average). The code names a system, not a part, so confirm the cause before spending: the ranked causes above are ordered by likelihood.
Can I drive with P0173?
See the can-you-drive verdict above — it is based on the system involved and the cascading-damage risk, and it is our editorial triage, not a manufacturer instruction. If the light is flashing or the car feels unsafe, stop driving it regardless of any code.
Will P0173 clear itself?
If the fault was a one-off (a loose cap, a cold-start glitch), the light can go out after several clean trips and the code ages into history. A fault that is still present re-sets the code as soon as the car runs that self-test again — monitors run under specific conditions, which is why "it hasn't come back in two days" is not proof of a fix.
Will my car fail inspection with P0173?
In states with OBD-based emissions inspection, yes — an illuminated check-engine light or an incomplete readiness monitor fails the test regardless of what the tailpipe would show. C-codes (ABS) fall under safety inspection instead in states that have one.
What should I check first for P0173?
The first item on the ranked-causes list above — it is ranked likelihood-first for exactly this reason — plus the free checks: recent work in the area (connectors left loose are a classic), fluid levels, and the battery and grounds on electrical codes.
Where this data comes from
Code facts, causes and symptoms come from OBDex — an open dataset of independently authored descriptions of the standardized generic OBD-II codes (CC0 1.0, public domain; github.com/foerbsnavi/OBDex). The SAE J2012 document itself is copyrighted and is not reproduced here. Cost ranges are Number Nest's own US planning model: national parts and labor ranges cross-checked against the CarMD Vehicle Health Index 2026 published averages, with labor adjustable by the BLS OEWS May 2025 state wage index for auto technicians (occupation 49-3023) — a wage index, not a surveyed shop rate. Severity and can-you-drive notes are our editorial triage, not a manufacturer instruction.