What does code P0172 mean on a Subaru and is it safe to drive?
P0172: System Too Rich (Bank 1). Here's what it means on your Subaru, what usually causes it, what repairs typically cost, and how safe it is to keep driving.
PowertrainMedium urgency
What this code means
Too much fuel relative to air. O2 sensor detects low oxygen in exhaust; PCM is reducing fuel trim.
Common causes for your Subaru
- Faulty MAF or O2 sensor
- Leaking fuel injector(s)
- High fuel pressure
- Faulty purge valve stuck open
Symptoms
- Check Engine Light
- Rich smell, black smoke
- Poor fuel economy
- Rough idle possible
Typical fixes
- Replace MAF or O2 sensor
- Test fuel pressure and injectors
- Replace purge valve if stuck open
Repair cost range
For a Subaru, repairs for P0172 often fall between $100 and $600, depending on the cause and whether you use OEM or aftermarket parts. Labor rates vary by location.
Known issues on Subaru
P0172 on Subaru can result from coolant entering the combustion chamber through a failing head gasket, causing the ECM to see a rich condition from partial misfiring. Always verify head gasket integrity on EJ engines.
Most commonly affected models
Outback 2005–2012Forester 2004–2013Impreza 2004–2014