P0175
PowertrainSystem Too Rich (Bank 2)
Most of the time this comes down to a tired or contaminated air flow sensor feeding the ECM duff numbers, so it dumps in more fuel than the engine actually needs on bank 2. Technically the code sets when the long-term fuel trims on that bank swing hard negative, meaning the computer has been pulling fuel back as far as it can to compensate for a rich mixture and has run out of room. Bank 2 is the side of the engine that doesn't have cylinder number one, so this only ever shows up on V6, V8 or flat engines. On four-cylinder cars you won't see it.
ⓘ Information only. This page provides general educational information about fault code P0175. We do not assess the urgency or safety implications of any specific fault. That requires in-person diagnosis by a qualified mechanic. Full terms.
Recommended next steps
Whether a fault is urgent, drivable, or routine depends entirely on the cause on a specific vehicle, and that can only be determined by a qualified mechanic with diagnostic equipment. If a warning light is illuminated, the most reliable next step is professional diagnosis.
What does P0175 mean?
P0175 is a Powertrain (engine, transmission, fuel system) fault code. It indicates: System Too Rich (Bank 2).
This is a standardised OBD-II code. The technical definition is the same regardless of the make or model of vehicle, although specific causes and symptoms can vary between vehicles.
Symptoms commonly associated with this code
Symptoms that drivers often report alongside this code. Not all may apply to every case:
- • Engine warning light on the dash, sometimes that's all you'll notice at first
- • Fuel gauge dropping quicker than usual, which most people clock at the pump
- • Lumpy idle, worse from cold and often settling once it's warmed through
- • Strong petrol smell or black sooty smoke from the tailpipe
- • Flat spot or hesitation when you put your foot down
- • Sluggish throttle response when you're just feathering it around town
Possible causes
Causes commonly associated with P0175, listed in approximate order of typical investigation. The actual cause on a specific vehicle can only be confirmed by professional diagnosis.
- 1. MAF sensor gone dirty or lazy and over-reading airflow, so the ECM matches it with too much fuel. The single most common cause and the first thing to rule out
- 2. A leaking or stuck injector on bank 2 letting raw fuel into the cylinders
- 3. Fuel pressure too high from a duff pressure regulator or a blocked return line
- 4. Air filter so clogged it's choking the airflow and skewing the mixture rich
- 5. Upstream oxygen sensor on bank 2 reporting nonsense, so the ECM trims fuel the wrong way
- 6. A vacuum leak after the MAF letting in unmetered air and throwing the readings off
- 7. ECM calibration or firmware fault, rare but it does happen on a handful of cars
How mechanics typically diagnose
A typical diagnostic sequence used by mechanics, provided here for educational reference only. Diagnostic work should be performed by a qualified mechanic with the appropriate tools and training.
- 1. Read the live short and long-term fuel trims for bank 2. If they're sitting well negative, say -15% or worse, the ECM is fighting a genuine rich condition and you've confirmed the code is real
- 2. Pull the MAF sensor and look at the hot wire. Oil mist or a grey film means it's reading high. Clean it with proper MAF cleaner only, never brake cleaner, and recheck the live airflow grams per second against spec
- 3. Check the air filter while you're in there. A filter caked solid will choke airflow enough to push the mixture rich, and it's a five-minute swap
- 4. Put a gauge on the fuel rail and compare against the factory figure. Pressure above spec points at the regulator or a restricted return
- 5. Check for vacuum leaks after the MAF and inspect the bank 2 injectors for weeping or a stuck-open condition
- 6. If trims, MAF, pressure and injectors all check out, scope the bank 2 upstream oxygen sensor and check its wiring before you condemn anything expensive
Common questions about P0175
If I clear the code, will it just come back? +
If you only clear it and change nothing, yes, it'll be back within a few drive cycles once the ECM relearns the fuel trims and sees the same rich bank 2 again. Clearing the code is fine as a way to confirm a repair worked, but on its own it does nothing. The exception is if the cause was a one-off, like a single tank of contaminated fuel, in which case it might not return. Most P0175s come back because the MAF or injector that caused them is still fitted.
What am I risking if I just leave it? +
A sustained rich mixture washes the oil film off the cylinder bores and dilutes the engine oil with fuel, which is bad news over a few thousand miles. The bigger and quicker worry is the catalytic converter. All that unburnt fuel burning off in the cat cooks the substrate and can melt it, and a cat is far dearer to replace than the sensor that caused the problem. You're also chucking money away at the pump the whole time.
How quickly do I need to sort this? +
It's not a roadside breakdown, the car will usually still drive. But I wouldn't sit on it for months. Aim to diagnose it within a week or two, sooner if you can smell fuel or see black smoke, because that's the cat taking a beating. If it drops into limp mode or the idle gets so rough it's stalling, deal with it straight away.
Is it the sensor itself or could it be the wiring? +
Both are worth checking before you spend money. A MAF or oxygen sensor can read wrong simply because of corroded pins or a chafed wire in the loom, especially on older cars where the connectors have been baking under the bonnet for years. Wiggle-test the connectors with the engine running and watch the live data for the readings jumping about. Plenty of so-called sensor faults turn out to be a poor earth or a green, crusty plug that a clean and a dab of dielectric grease sorts out.
Information only, not professional advice
The information on this page is provided for general guidance and educational purposes only. It is not a substitute for diagnosis or repair advice from a qualified mechanic. Always verify any fault before paying for repairs. carfaultcodes.co.uk accepts no liability for decisions made based on this information. Full terms →