P0174: System Too Lean (Bank 2)
P0174 means System Too Lean (Bank 2). The engine computer constantly adjusts fuel delivery (fuel trim). This code means bank 2 (the side opposite cylinder 1) is running lean (too much air or too little fuel) and the computer has reached the limit of how much fuel it can add to correct it.
When P0174 appears together with P0171, the cause is likely shared by both banks: MAF sensor, fuel pressure or a large central vacuum leak.
Can I keep driving? Drivable with care. Diagnose soon: performance, economy or other components can be affected.
Common symptoms
- Check engine light on
- Hesitation or stumbling on acceleration
- Rough idle
- Possible misfires
- Reduced power
Possible causes
- Vacuum leak: cracked hose, intake gasket, PCV hose or brake booster hose
- Dirty or faulty mass air flow (MAF) sensor under-reporting air
- Low fuel pressure: weak pump, clogged filter or faulty regulator
- Clogged or restricted fuel injectors
- Exhaust leak ahead of the upstream oxygen sensor
How to diagnose P0174
- Read short- and long-term fuel trims at idle and at 2,500 RPM. A lean condition that improves at higher RPM points to a vacuum leak; one that gets worse under load points to fuel delivery or the MAF.
- Smoke-test the intake for vacuum leaks.
- Inspect and clean the MAF sensor with a dedicated MAF cleaner.
- Test fuel pressure and volume against specification.
- If both banks are lean (P0171 and P0174), look for a shared cause such as MAF, fuel pressure or a central vacuum leak.
What the code means, digit by digit
Generic codes have the same definition on every OBD-II vehicle (petrol cars from model year 1996 in the US, 2001 in the EU; diesels later). Manufacturers can add their own enhanced codes; those are not listed here.
Related codes
General guidance only. Fault codes describe what the engine computer saw, not which part failed: confirm with testing before replacing parts, and follow the vehicle's service information.