OBD codes / P0173
P0173
SeriousFuel Trim Malfunction (Bank 2)
Usually drivable short-term, but treat it like its lean and rich siblings: a mixture the computer can no longer fully correct runs the engine outside its design range, which can foul plugs, invite misfires, or overheat the catalytic converter over time. Get a diagnosis before buying parts — the cause could be a five-dollar hose or a fuel pump.
What this code means
Fuel trim is the running correction the engine computer applies to the fuel mixture, and P0173 means that correction has hit its limit on bank 2 — the cylinder bank opposite the one containing cylinder 1, so this code belongs to V6, V8, and flat engines. Unlike P0174 (too lean) or P0175 (too rich), P0173 is the general version: the trim is out of range, and the direction has to be read from freeze-frame data or from whichever companion code came with it. Lean-direction causes include vacuum and intake leaks, a mass-airflow sensor under-reading, and weak fuel delivery; rich-direction causes include leaking injectors, high fuel pressure, and a MAF over-reading. If only bank 2 is affected, look for causes local to that side, like an intake gasket leak or a vacuum hose on that bank; if bank 1 codes appear too, suspect something shared, such as the MAF or fuel supply.
Symptoms you might notice
- Check-engine light, sometimes with no other symptom
- Rough or unstable idle
- Hesitation or reduced power under load
- Worse fuel economy
- Fuel smell from the exhaust if the trim ran rich
What it costs to fix
Varies by vehicle — get it diagnosed.
The repair cost for this code depends too much on your exact vehicle and the underlying cause to quote a meaningful range. Confirm the cause first so you only pay for the fix you actually need.
Diagnose P0173 for my vehicle →Frequently asked
How is P0173 different from P0174 and P0175?
All three say bank 2's fuel trim went out of range. P0174 specifies too lean and P0175 too rich, while P0173 is the generic version some vehicles use without naming a direction. The freeze-frame trim values stored with the code reveal which way it went.
Why is there no cost estimate for P0173?
Because the code does not point at a part — it reports a mixture the computer could not correct. The fix can be as cheap as a cracked vacuum hose or a MAF cleaning, or as substantial as an intake gasket or a fuel pump. A proper diagnosis is what turns this code into a price.
Only bank 2 has a code — does that narrow things down?
Helpfully, yes. A problem shared by both banks, like a dirty MAF or weak fuel pump, usually sets codes on both sides. One-bank trim trouble points toward that bank specifically: an intake gasket section, a vacuum line, injectors, or an exhaust leak near that bank's upstream O2 sensor.
Can a bad O2 sensor cause P0173?
It can — the trims are computed from the upstream O2 sensor's readings, so a skewed sensor on bank 2 can drive the trims to their limit even with a healthy mixture. That is one reason to diagnose with live data rather than replacing parts on a guess.
Seeing P0173 on your car? Get a diagnosis specific to your exact year, make and model.
Diagnose P0173 for my vehicleRelated trouble codes
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Generic OBD-II reference. Manufacturer-specific behavior varies — confirm with a scan tool and, for safety-related codes, a professional inspection.