P0172 — System Too Rich (Bank 1)

What it means

Long-term fuel trim on bank 1 has hit its lower limit: the control module is cutting injection time because there is too much fuel relative to air. The rich mixture is confirmed by the upstream oxygen sensor holding a high voltage signal. Typical symptoms are black exhaust smoke, a strong fuel smell, rough idle, sooted spark plugs and high consumption.

Likely causes (most common first)

1. Leaking or dripping injectors that do not seal when closed
2. Excessive fuel pressure: faulty pressure regulator or restricted fuel return line
3. Clogged air filter or restricted intake reducing the amount of air
4. Mass air flow sensor overreporting airflow, or a faulty intake air or coolant temperature sensor
5. Faulty upstream oxygen sensor that permanently reports a lean mixture
6. Evaporative purge valve stuck open or a carbon canister saturated with fuel

Can you drive with it?

Driving is possible for a short while, but a rich mixture washes oil off the cylinder walls and destroys the catalytic converter, so repair it without delay.