The DME compared upstream and downstream O2 signals on bank 1 and calculated that the catalytic converter is no longer converting efficiently. On the 987 this can be a genuinely worn catalyst (age/mileage or a prior misfire/oil-fouling event) or a lazy downstream O2 sensor. Rule out upstream faults (misfire, lean codes) that damage cats first.
SYMPTOMS
- Check-engine light
- Usually no drivability change
- Possible faint sulphur smell
- Emissions failure
LIKELY CAUSES
- Aged/degraded catalytic converter (bank 1)
- Lazy downstream O2 sensor
- Prior misfire or oil consumption contaminating the cat
- Exhaust leak skewing sensor readings
DIAGNOSIS STEPS
- Compare upstream vs downstream O2 switching (downstream should be steady)
- Confirm no active misfire or lean/rich codes first
- Check for exhaust leaks
- Test/replace the downstream O2 sensor before condemning the cat
RELATED PARTS
- Catalytic converter (Bank 1)
- Downstream O2 sensor