P0173 code: Fuel Trim Malfunction (Bank 2)
P0173 means fuel Trim Malfunction (Bank 2). The code says where to look, not what failed — the fix depends on which cause your car actually has.
What P0173 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.
P0173 symptoms
- 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 P0173
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 →How to fix P0173
Moderate — doable at home with common tools and patience
- Read the freeze frame to learn the trim direction
P0173 doesn't say lean or rich, so the freeze frame and any companion code have to. Bank 2 long-term trim strongly positive means the computer was adding fuel to fight a lean condition; strongly negative means it was pulling fuel out. Every step after this depends on that answer, so don't skip it.
- Confirm bank 2 and compare it with bank 1
Bank 2 is the side without cylinder 1, so this only applies to V and flat engines. At warm idle and again at a steady light cruise, watch total trim — short plus long — on both banks. Both drifting the same way says a shared cause; bank 2 drifting alone says something local to that side.
- If lean, smoke-test bank 2 and check its exhaust
If bank 2 is adding fuel, smoke the intake and pay close attention to the bank 2 intake gasket edges, runner vacuum ports, and hoses on that side. Also check for an exhaust leak at the bank 2 manifold ahead of its upstream sensor — outside air drawn in there makes the sensor report lean when the mixture isn't.
- If rich, look for an injector letting fuel by
If bank 2 is pulling fuel, read the spark plugs on that bank for a sooty or fuel-wet one, and watch fuel pressure after shutdown for a quick bleed-down. A single leaking injector richens only its own bank. Shared rich causes like a stuck purge valve or high fuel pressure usually drag bank 1 along too.
- Clean the MAF and test fuel pressure for shared causes
When both banks drift together, clean the MAF with dedicated cleaner and verify fuel pressure at idle and under load. Then clear the code and drive several cycles, watching that bank 2 trims settle near zero and stay there across idle, cruise, and acceleration.
DIY or shop? The thinking work on P0173 — reading freeze frame, comparing banks, plug inspection, MAF cleaning — is within reach of a home mechanic with a live-data scanner, and it decides whether you are dealing with a ten-minute hose or a bigger job. The repairs that follow often go to a shop: bank 2 is frequently the harder side to reach, intake gasket replacement means a partial teardown, and injector testing or a fuel pump replacement needs equipment most people don't own.
Tools for the DIY steps: Scan tool with fuel trims for both banks · Smoke machine · Fuel pressure gauge · MAF sensor cleaner · Spark plug socket
Not sure which step applies to your car? Get the likely cause for your exact vehicle before buying anything for P0173.
Start a case with P0173 and your car. Redline weighs these causes against what your car shows, and names the check that would settle it before you buy parts.
Can I drive with P0173?
Redline can’t determine from a symptom or code alone whether your car is safe to drive.
P0173: common questions
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.
Related trouble codes
Don’t have a scanner yet? A ~$25 Bluetooth adapter reads codes like P0173 from your phone. See our guide to the best OBD2 Bluetooth adapters. Run a repair shop? See Redline for shops.
Generic OBD-II reference. Manufacturer-specific behavior varies. Confirm with a scan tool and, for safety-related codes, a professional inspection. Cost ranges are reference ranges, not quotes.
Seeing P0173 on your car?
Every problem becomes a Case. Tell Redline what the car is doing: it shows what the evidence supports, what’s still missing, and what to check next, before you approve a repair.
“Inner pad ~1 mm. Outer pad 3 mm.”
Next: check the slide pins move freely before buying parts.