Fuel trims on bank 1 (cylinders 1-3) have gone lean beyond the correction limit. On the 987 flat-six the usual cause is unmetered air - a failed air-oil separator or an intake/vacuum leak - or a dirty MAF. Note: 987.1 M96/M97 cars often log the Porsche codes P1128/P1130 for the same lean condition, whereas 987.2 DFI cars typically set P0171/P0174.
SYMPTOMS
- Check-engine light
- Rough or high idle
- Hesitation / stumble
- Reduced economy
LIKELY CAUSES
- Air-oil separator (AOS) failure
- Intake/vacuum leak (boots, hoses, manifold O-rings)
- Dirty or failing MAF
- Loose oil cap / dipstick-tube leak
- Weak fuel delivery
DIAGNOSIS STEPS
- Read short/long-term fuel trims (high positive LTFT confirms lean)
- Smoke-test the intake and replace the 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