P0175
- Powertrain
- Generic (SAE)
- Emissions
Diagnose soon
Not an immediate hazard, but a sustained rich mixture washes oil off cylinder walls, dilutes engine oil with fuel, fouls plugs and pushes unburned fuel into the catalytic converter, which can overheat and fail. The damage accumulates over weeks, so this is urgent in days rather than minutes.
- Safe to drive
- Short-term driving is usually fine. Fix it within days, and stop if you smell fuel, see black smoke or feel misfires.
System Too Rich (Bank 2)
The computer had to keep subtracting fuel on bank 2 to hold the mixture correct, and it ran out of room to subtract. Something is putting more fuel into that bank than the computer asked for, or is misreporting conditions so that it commands too much.
The upstream oxygen sensor on bank 2 reported a persistently rich exhaust, so the ECM drove short and long term fuel trims negative to compensate. When total correction stays beyond its calibrated plausibility limit — commonly in the region of minus 20 to 25 percent, though the exact threshold is vehicle-specific — P0175 sets. Bank 2 is the bank that does not contain cylinder 1, so on a V, flat or twin-bank engine this code narrows the fault to one side unless P0172 is stored with it, in which case the cause is shared.
How the car detected itClosed-loop fuelling corrects toward stoichiometric using the upstream sensor, and long term fuel trim stores the running average of that correction. The module sets the code when trim stays beyond its limit for a sustained period at given load and RPM cells. It measures the correction, not the cause — which is why a genuine fuel excess and an oxygen sensor that only reports one look identical from inside the monitor.
A trouble code records what a control module measured. It does not identify which part failed. Test before replacing anything.
- Safe to drive
- Short-term driving is usually fine. Fix it within days, and stop if you smell fuel, see black smoke or feel misfires.
- Urgency
- Diagnose soon
- Will it pass emissions
- No
- If ignored
- Fuel economy falls and spark plugs foul, which brings on misfires. Raw fuel reaching the catalytic converter can overheat and destroy it, turning a moderate repair into a much larger one. Fuel washing past the rings dilutes engine oil and accelerates bearing and cylinder wear. The vehicle will also fail an emissions test while the light is on.
- Check engine light
- Solid
- Clearing the light
- A completed drive cycle is normally required before the monitor re-runs and the light can clear.
- Increased fuel consumption
- Black smoke from exhaust on acceleration
- Smell of unburned fuel, fouled spark plugs
Many vehicles show no symptom at all beyond the warning light. The absence of a symptom does not mean the fault is not real.
These are the causes this code can have. For the order to work through them in, see what to check first.
| # | Cause | How often | Component |
|---|---|---|---|
| 1 | Fuel pressure regulator stuck closed | Very common | — |
| 2 | Leaking fuel injector on bank 2 | Common | — |
Ordering reflects how often each cause is responsible in general, not a probability for your vehicle. Confirm by testing.
In order. Each of these is cheaper or faster than what follows it, and each one can make the rest unnecessary.
Whether P0172 is stored alongside it
Both banks rich points at a shared cause — fuel pressure, purge, coolant temperature, airflow measurement. Bank 2 alone points at that bank's injectors or its upstream sensor.
Live short and long term fuel trims at idle and at 2500 rpm, on both banks
Free, immediate, and it shows whether the richness is present at idle, at load, or across the board — which narrows the cause more than any part swap.
Freeze frame conditions
Tells you the load, RPM and coolant temperature where the fault matured, so you can reproduce it deliberately instead of hoping it reappears.
Engine oil level and whether it smells of fuel
An oil level that has risen above full, or oil smelling of gasoline, confirms fuel is reaching the crankcase and raises the urgency considerably.
Whether it started right after a fill-up, especially one topped off past the click
A charcoal canister saturated with liquid fuel purges heavily into the intake and produces exactly this code. It often clears over a few tanks.
Read freeze frame and both banks' fuel trims across the load range
Tools Scan tool with live data
Record short and long term trim for bank 1 and bank 2 at idle, at a steady 2500 rpm, and under road load. Whether bank 1 is normal or equally negative is the branch point for everything that follows.
ExpectedTrims within roughly plus or minus 10 percent. Note whether only bank 2 is negative (bank-specific fault) or both banks are (shared cause).
Check fuel pressure with a mechanical gauge
Tools Fuel pressure gauge, service specification
Measure running pressure, pressure under snap throttle, and whether it holds after shutdown. On a returnless system, confirm the pressure the module is commanding matches what the gauge shows. The specification is vehicle-specific — do not guess it.
ExpectedPressure at spec and stable. Pressure above spec, or pressure that will not fall when the return path is opened, implicates the regulator or a blocked return.
Test the EVAP purge valve for leaking through when closed
Tools Hand vacuum pump, scan tool
Apply vacuum to the closed valve and see whether it holds. Then disconnect it entirely and watch whether the trims move back toward zero within a minute or two of running.
ExpectedThe closed valve holds vacuum, and trims do not improve when it is disconnected. If they do improve, the purge system is adding fuel vapour.
Prove the bank 2 upstream oxygen sensor is telling the truth
Tools Scan tool, propane enrichment tool, hand vacuum pump
Force a known lean condition and a known rich condition — a controlled small vacuum leak one way, a brief propane enrichment the other — and confirm the sensor swings fully in both directions and returns. A sensor that will not report lean is biased rich and is causing the trim rather than reporting it.
ExpectedFull, prompt swing across the sensor's range in both directions, matching the behaviour of the bank 1 sensor.
Compare coolant and intake air temperature readings after an overnight soak
Tools Scan tool, thermometer
With the engine fully cold, both sensors should agree with each other and with ambient. A coolant sensor reading falsely cold makes the module enrich deliberately, and it will do so on both banks.
ExpectedBoth readings within a few degrees of each other and of actual ambient temperature.
Test bank 2 injectors for leakage and delivery balance
Tools Low-current probe and scope, injector flow tester
Pressurise the rail without commanding the injectors and look for dripping. Then current-ramp each injector, or run a flow balance test, comparing bank 2 against bank 1. A leaking injector adds fuel continuously and is the classic bank-specific cause.
ExpectedNo dripping when pressurised but not commanded, and delivery balanced across all cylinders.
Check for fuel dilution in the oil and inspect the crankcase ventilation path
Tools Basic hand tools
Pull the dipstick and check level and smell. Then confirm the PCV path is correctly routed and sealed — raw fuel vapour drawn from the crankcase into the intake adds fuel the module never asked for.
ExpectedOil at the correct level with no fuel smell, and a crankcase ventilation system that is intact and correctly routed.
Read the spark plugs on bank 2 against bank 1
Tools Spark plug socket, torque wrench
Pull one or two plugs from each bank and compare deposits directly. This is the cheapest confirmation available that the richness is real combustion-side rather than a sensor reporting error.
ExpectedBank 2 plugs sooty and black against bank 1's tan colour confirms real richness on that bank. Matching plugs on both banks suggests the sensor is lying.
| Repair | Only after you confirm | Typical cost | Who |
|---|---|---|---|
| Replace an EVAP purge valve that leaks through when closed | Valve passes vacuum when commanded closed, or fuel trims move back toward normal with it disconnected. | $100–$350 | DIY |
| Replace an engine coolant temperature sensor reading falsely cold | Sensor disagrees with actual coolant temperature or with intake air temperature after an overnight soak. | $90–$300 | DIY |
| Replace a leaking or stuck-open injector on bank 2 | Leak-down or flow testing shows that injector dripping or over-delivering compared with the others. | $180–$700 | DIY |
| Replace a fuel pressure regulator holding pressure above specification | Gauge reads above the vehicle's specified fuel pressure, and pressure returns to spec with the new regulator. | $120–$500 | DIY |
| Replace the bank 2 upstream oxygen sensor | Sensor will not swing lean under a forced lean condition, or holds a high voltage while the mixture is confirmed correct by plug reading and fuel pressure. | $160–$480 | DIY |
| Clear a restricted fuel return line or blocked return path | Pressure above spec that does not fall when the return line is opened at the rail. | $120–$500 | Hard |
| Replace a charcoal canister saturated with liquid fuel | Liquid fuel found in the canister or purge line, typically after repeated overfilling of the tank. | $250–$800 | DIY |
| Change fuel-diluted engine oil after fixing the source | Oil level risen above full and smelling of fuel; required after any sustained rich condition is corrected. | $60–$180 | DIY |
US independent-shop ranges, parts and labour, excluding diagnostic time (commonly $90-250 here, since this code needs live testing rather than a code read). Direct-injection injectors and rear-bank access on transverse V engines both push the top of the range considerably higher.
- Replacing the oxygen sensor because the code says 'rich'. The sensor is usually reporting the problem — but confirm it can still swing lean before you trust it.
- Assuming bank 2 is the passenger side. Bank numbering follows cylinder 1, and that varies by manufacturer.
- Cleaning the MAF sensor and clearing the code. A contaminated hot-wire MAF usually under-reports airflow, which causes lean codes, not rich ones.
- Blaming a dirty air filter on a MAF-equipped engine. The MAF measures air after the filter, so a restriction there rarely produces a rich code on its own.
- Overlooking the EVAP purge valve. One that leaks through adds fuel vapour continuously and is cheap and quick to test.
- Diagnosing without a fuel pressure gauge, which is the single most decisive measurement when both banks are rich.
- Replacing a full set of injectors when a flow or leak-down test would have named one.
- Fuel Injector
- Fuel Pressure Regulator
- Oxygen Sensor Upstream
- Definition
- Generic code set (SAE J2012 / ISO 15031-6) via the OBDexopen database, dedicated to the public domain under CC0-1.0.
- Diagnostic guidance
- Written and reviewed in-house. See our editorial policy.
- Review status
- Reviewed
Found something wrong? Tell us — we publish corrections.