Bank 2 (cylinders 4-6) counterpart to P0171. Fuel trims are lean beyond the correction limit. Because both banks share the intake, P0174 usually appears with P0171 and points to a common unmetered-air source (AOS, vacuum leak, MAF) rather than a single-bank fault. Most characteristic of 987.2 DFI cars; 987.1 tends to log P1128/P1130.
SYMPTOMS
- Check-engine light
- Rough or high idle
- Hesitation
- Reduced economy
LIKELY CAUSES
- Air-oil separator (AOS) failure
- Intake/vacuum leak
- Dirty or failing MAF
- Loose oil cap / dipstick-tube leak
- Weak fuel delivery
DIAGNOSIS STEPS
- Read fuel trims on both banks
- Smoke-test the intake; replace AOS if torn
- Clean/test the MAF
- Check oil-cap and tube seals
- Verify fuel pressure
RELATED PARTS
- Air-oil separator (AOS)
- Intake boots / vacuum lines
- MAF sensor