P0172 means there is too much fuel in the mixture on bank 1. The engine computer has been taking fuel away to compensate and has run out of room to do it. You may notice a smell of petrol, a rough idle, sooty exhaust tips and a fuel bill that has climbed. Diagnose soon and drive gently. Indicative UK repair cost is £40 to £500.
A rich mixture is less comfortable to live with than a lean one and it does more damage, which is worth knowing because the two codes are often discussed as though they were equal and opposite. Unburnt petrol does not stay in the exhaust. Some of it washes down the cylinder walls and ends up in the sump, thinning the oil that is supposed to be protecting the engine, and that effect is more pronounced on direct injection engines. The rest goes into the catalytic converter and cooks it. Both of those are quietly expensive while the car still feels broadly fine.
Can I drive with P0172?
Diagnose soon, drive gently. The car will keep going and will probably keep going for months, which is the trap. Two things are happening while it does: the converter is being overheated by fuel it was never meant to deal with, and the engine oil is being diluted. If you can smell fuel strongly, or the oil on the dipstick smells of petrol and is noticeably thin, stop making long journeys and get it looked at properly. Short of that, use it lightly and sort it out sooner rather than later.
What P0172 means for your MOT
Of every code in this library, this is the one most likely to fail the measured test rather than just the light. Three separate major defects in the DVSA MOT inspection manual are in play.
- Carbon monoxide and hydrocarbons. Both are measured at the tailpipe and both rise with a rich mixture. The manual fails a car for emissions levels exceeding either the manufacturer's specified limits or the default limits.
- Lambda. Rich means a lambda coefficient below one, and a coefficient outside the limits is its own defect. This is the figure that moves first.
- Visible smoke. The manual fails a vehicle whose exhaust emits clearly visible black smoke for a continuous period of five seconds at idle. A badly rich engine can do exactly that, and it is judged by eye with no instrument involved.
Plus the lamp itself, under section 8.2.1.2 for petrol cars first used on or after 1 July 2003. So unlike most codes here, booking the MOT and hoping is not a strategy that works with P0172.
What usually causes it
- A leaking injector. The commonest mechanical cause. An injector that drips rather than seals adds fuel nobody asked for, and it will usually leave one spark plug looking sootier and smelling stronger than the other three.
- A faulty airflow or manifold pressure sensor. A sensor over-reporting how much air is arriving makes the computer add fuel to match air that is not there. Same part as several other codes, opposite error.
- High fuel pressure. A failed pressure regulator, or a blocked return line, raises rail pressure so every injector delivers more than its commanded amount. This version affects all cylinders evenly, which is a useful distinction from a single leaking injector.
- An oxygen sensor fault. A front sensor stuck reporting lean will have the computer adding fuel to correct a problem that does not exist, until the correction itself becomes the fault.
What it costs to fix in the UK
£40 to £500. A pressure regulator or a single injector seal is the bottom. An oxygen sensor or an airflow sensor is the middle. A set of injectors, or injector work on a direct injection engine where they are harder to reach, is the top.
Add an oil and filter change to whatever the repair turns out to be if the oil has been diluted, because leaving contaminated oil in after fixing the cause wastes the repair. For the investigation itself, what a car diagnostic costs in the UK gives you the going rate.
These are indicative UK ranges for guidance only. Actual causes and prices vary by vehicle, and a stored code points to a system, not always the exact part.
What to check first
- Pull the plugs and compare them against each other. This is the cheapest way to separate a single leaking injector from an engine-wide fuelling problem. One plug blacker and wetter than the rest names the cylinder. All four equally sooty points at pressure, at a sensor, or at the oxygen sensor instead.
- Smell and look at the engine oil. Oil that smells of petrol, reads above the full mark without anyone topping it up, or feels thin between your fingers has fuel in it. That changes the urgency and adds a service to the job.
- Check fuel pressure against the specification. Static and running. A rail sitting above spec explains everything at once and costs nothing but the gauge.
- Watch the front oxygen sensor on live data. A sensor that is reporting lean while the engine is visibly running rich has identified itself, and that is the cause people most often replace last.
Related codes
- P0171, system too lean (Bank 1). The opposite condition on the same bank, with a different diagnostic route.
- P0420, catalyst efficiency below threshold. The code that frequently arrives a few months after this one, and the reason not to leave a rich engine running.
- P0102, MAF sensor circuit low input. The same airflow sensor seen from the other direction, worth comparing if both codes are stored.
Get it narrowed down before you spend
A stored code tells you which system is unhappy, not which part has failed. If you want it narrowed down before you spend anything, send the code and your symptoms and a mechanic will read them personally. Quick Diagnosis is £20, and if we can't help, you don't pay.
Written by the First Diagnostic mechanic. Over a decade in the trade: a VW-only workshop running diagnostics and electrical repair, then BMW-only through to full engine rebuilds. Now an independent all-makes workshop, plus building and wiring drift cars for competition, including trackside work as head mechanic. More about who reads your codes.