Parts
$30–400
P0172 — System Too Rich (Bank 1)
CAUTIONIs it safe to drive? Running rich — too much fuel. It'll wash oil off the cylinders, foul plugs, and overload the cat. Get it sorted before it does damage.
What P0172 means
The computer is pulling fuel out and the mixture is still rich. Long-term fuel trim sits negative.
Most likely causes (in order)
- Leaking / dripping fuel injector
- High fuel pressure (bad regulator)
- MAF over-reporting airflow
- Clogged air filter
- Faulty coolant temp sensor reading cold
Symptoms you might notice
- Black smoke / fuel smell
- Fouled plugs
- Poor economy, rough idle
What to check first
Check fuel pressure and the coolant-temp sensor reading. Look at fuel trims and the upstream O2; a stuck-rich injector often shows on a balance test.
Repair cost & difficulty
Labor
0.5–3 hr
Difficulty
DIY-friendly