System Too Lean (Bank 2)
In short
P0174 means the engine is running lean on Bank 2 — too much air relative to fuel — and the computer has run out of room adding fuel to compensate. It's the Bank 2 twin of P0171 and only appears on V6/V8 engines. When P0171 and P0174 appear together, the cause is almost always something that affects both banks at once: a dirty MAF sensor, low fuel pressure, or a large vacuum/PCV leak. A leak on just one bank trips P0174 alone.
Is it safe to drive with P0174?
Generally safe in the short term — the car runs, though you may feel a rough idle or hesitation. A lean mixture burns hotter and can lead to misfires, detonation, and accelerated wear over time, so diagnose it soon, especially if misfire codes appear alongside it.
Symptoms
- Check engine light on
- Rough or surging idle, sometimes stalling
- Hesitation or stumble on light acceleration
- Slightly reduced power
- Often no obvious symptom besides the light
Common causes (most → least likely)
How to diagnose it (before buying parts)
- 1 Read live fuel-trim data. High positive Short- and Long-Term Fuel Trims on Bank 2 (e.g. +15% to +25%) confirm the lean condition. Compare to Bank 1: if both are lean, suspect a shared cause (MAF, fuel pressure); if only Bank 2 is lean, hunt a leak on that bank.
- 2 Note when the lean trim is worst — at idle points to a vacuum leak; at higher RPM/load points to the MAF or fuel delivery.
- 3 Clean the MAF sensor with MAF-specific cleaner and recheck; a contaminated MAF is the classic cause of dual-bank lean codes.
- 4 Smoke-test or spray carb cleaner around the intake, PCV, and Bank 2 runners to find unmetered air.
- 5 If idle vacuum and MAF check out, test fuel pressure under load to rule out weak fuel delivery.
Repair options & cost
By manufacturer
P0171 + P0174 together is extremely common on 4.6/5.4 V8s and EcoBoost engines, usually a dirty MAF or a torn PCV/intake hose. Check the rubber intake elbow and PCV first.
Cracked intake manifold gaskets and PCV faults are common dual-bank lean sources on older V6/V8s.
Dirty MAF or brittle vacuum lines on V6s; clean the MAF and inspect the PCV hose before replacing parts.
VQ V6 intake gaskets and MAF sensors are known culprits; verify with fuel trims and check both banks.
Frequently asked questions
What's the difference between P0171 and P0174?
Same lean condition on different cylinder banks — P0171 is Bank 1, P0174 is Bank 2. You only see P0174 on V-configuration engines. When both codes are present, look for a shared cause like a dirty MAF or low fuel pressure rather than two separate leaks.
Can I drive with P0174?
Short-term, usually yes, but a lean mixture runs hot and can cause misfires and long-term wear. Diagnose it soon — sooner if you also have misfire codes.
Will cleaning the MAF fix P0174?
Often, when both banks read lean. A MAF coated in dirt or oil reads low and leans out the whole engine. Clean it with MAF-specific spray and recheck fuel trims; if Bank 2 is still lean, look for a vacuum leak on that side.
Why only Bank 2 and not Bank 1?
A lean code on just one bank usually means unmetered air is entering that bank specifically — a cracked vacuum line, intake gasket, or runner leak on the Bank 2 side — rather than a shared problem like the MAF or fuel pump.