P0172 and P0175: rich mixture — why it smells of fuel and where the fuel goes

P0172 “System Too Rich” is the mirror of the lean code. The module sees excess fuel and keeps removing it. When long-term trim bottoms out around −25 %, the code appears.

The symptoms are recognisable without a scanner: a fuel smell from the exhaust, dark smoke under acceleration, consumption up 15–30 %, black deposits on the plugs.

Causes by probability

1. A leaking injector

A worn nozzle or a sticking seat. Usually one cylinder suffers, and P0172 arrives with misfires in that cylinder — the fuel does not burn.

Signature: a strong fuel smell and consumption that jumped rather than crept up.

2. An airflow meter over-reporting

The sensor over-states flow and the module fuels for air that is not there. Rarer than contamination, but it happens after a poor aftermarket replacement.

3. Excessive fuel pressure

A stuck regulator or a pinched return line. The injector opens for the calculated duration but delivers more.

Signature: trim evenly negative across every operating state.

4. A lying coolant temperature sensor

The most treacherous cause and the cheapest to fix. The sensor reports 14 °F instead of 195 °F, the module believes the engine is cold, and enriches for warm-up — permanently.

Signature: hard hot starting, very high consumption, odd cooling fan behaviour. Temperature sensors are covered in their own article.

5. A clogged air filter

Obvious, but it works: less air for the same fuel delivery.

6. Crankcase ventilation faults

Excess oil vapour reaching the intake.

7. An oxygen sensor under-reporting

Last again. An aged sensor can permanently report excess fuel.

Narrowing it down

Picture	Most likely
Trim equally negative everywhere	Fuel pressure or temperature sensor
Worse at idle	Leaking injector
Worse under load	Airflow meter or filter
One bank of two	Local: that bank’s injector or sensor
Alongside misfires in one cylinder	That cylinder’s injector

Always check coolant temperature in live data on a warm engine. If it does not read 185–205 °F, stop looking anywhere else.

What it risks

A rich mixture threatens your wallet more than a lean one threatens the engine:

• The catalyst. Unburnt fuel finishes burning inside it and overheats the ceramic. A direct route to P0420.
• Spark plugs. Fouled with deposits, producing misfires — P0300.
• Engine oil. Fuel washes the film off the cylinder walls and drains into the sump. The oil thins, loses its properties, and within a couple of thousand miles that is a bearing life question.
• Oxygen sensors. Working permanently in a rich environment, they age faster.

That oil point is the most underestimated. If the oil level has risen and it smells of fuel, do not drive at all.

What to do, in order

1. Check coolant temperature in live data — the cheapest and most common cause.
2. Inspect the air filter.
3. Read trims across three operating states.
4. Look at misfire counters — they identify the injector.
5. Check fuel pressure.
6. Inspect crankcase ventilation.
7. Smell the dipstick: fuel means stop now.

In short

• The code means excess fuel, not a failed oxygen sensor.
• The cheapest and very common cause is a lying temperature sensor.
• Evenly negative everywhere means pressure or temperature; worse at idle means an injector.
• The main risk is not the engine but the catalyst and diluted oil.
• Fuel in the oil is a reason to stop, not to drive home.

Download the app — fuel trims, temperature, and misfire counters are read with an ELM327 adapter over Bluetooth SPP on Android.