Fault Tree: P0420 Catalyst Efficiency

Decision flow

1. Confirm it’s not an active drivability fault
   If you have a current misfire, obvious rich running, coolant loss, or poor performance — fix that first. P0420 can be a downstream symptom.
2. Check for exhaust leaks upstream of the rear O2
   Leaks before/near the front O2 or at the manifold/downpipe can skew readings and trigger P0420.
   If you suspect a leak: cold start listen/feel, look for soot tracks, and smoke test if possible.
3. Look at fuel trims at warm idle and at ~2500 rpm
   Large positive trims (lean) or large negative trims (rich) can overwork the cat and trigger efficiency codes.
   If trims are abnormal, use: P0171/P0172 fuel trim fault tree →
4. Graph O2 sensor behaviour (warm engine, steady cruise)
   • Front O2 (upstream): should switch/trim (narrowband) or show controlled lambda (wideband).
   • Rear O2 (downstream): should be slower/flatter. If it mirrors the front closely, the cat may be weak — or you have another issue.
   A lazy/incorrect rear O2 can mimic a bad catalyst. Confirm sensor function before condemning the cat.
5. Rule out coolant/oil burning
   • Coolant loss, sweet smell, steam after warm-up, oily deposits, or persistent blue smoke can poison the catalyst.
   If there’s any coolant loss, start here: Coolant loss / pressure test flow →
6. Confirm operating conditions
   • Engine must reach proper temperature (stuck-open thermostat can prevent cat light-off).
   • Check for chronic short-trip use; do a proper warm drive before retesting.
7. If all above is OK: suspect catalyst efficiency
   At this point, a weak catalyst becomes likely. Consider an infrared temp check (inlet vs outlet) after a steady drive, and confirm there are no fueling/misfire issues.
   Replacing a cat without fixing the root cause (misfire, rich running, oil/coolant burning) often leads to repeat failures.