Symptom · Petrol engines with pre and post catalyst oxygen sensors
P0420 means the control unit compared the two oxygen sensors and decided the catalyst is not doing enough work. That conclusion depends entirely on both sensors telling the truth and on the exhaust being sealed. Check the evidence before you quote the most expensive part on the car.
Genuinely worn catalyst
Age, mileage or a past misfire has cooked the substrate, so it no longer stores oxygen.
Lazy or contaminated post-cat sensor
A slow downstream sensor mimics the trace of a dead catalyst almost exactly.
Exhaust leak before the rear sensor
Air drawn in through a cracked flexi or a blowing gasket makes the rear sensor read lean.
Rich running from a fuelling fault
Excess fuel overwhelms a healthy catalyst and sets the code as a symptom, not a cause.
Misfire loading the catalyst
Unburnt fuel raises catalyst temperature and destroys it, so the misfire is the real job.
Coolant or oil burnt through the exhaust
A head gasket or heavy oil consumption poisons the substrate over a few thousand miles.
Nothing on that list is the answer yet. A measurement decides which ones survive.
Test 1
Graph both oxygen sensor voltages at a steady 2500 rpm once fully warm.
A rear sensor mirroring the front means poor oxygen storage. A rear sensor that barely moves at all, in either direction, points at the sensor rather than the cat.
Test 2
Check for stored misfire and fuel trim codes, and read long term fuel trim at idle and at cruise.
Any misfire or a large negative trim makes the catalyst a victim. Fix the cause first or the new cat dies too.
Test 3
Smoke test the exhaust from the tailpipe, or listen along the system cold with the engine idling.
A leak upstream of the rear sensor invalidates every reading you have taken so far and is far cheaper to fix.
Test 4
Measure the temperature just before and just after the catalyst at 2000 rpm once hot.
An outlet no warmer than the inlet supports a dead catalyst. An outlet clearly hotter says the cat is working and the code is coming from elsewhere.
Test 5
Swap or substitute a known-good rear oxygen sensor and rerun the readiness monitor on a full drive cycle.
If the code stays away, the sensor was the fault and the customer keeps their catalyst.
Three cheap checks — trims, exhaust leak, temperature across the cat — stand between a lazy sensor and a catalyst quote nobody needed.
Open a Case with the vehicle and the symptom in front of you. It asks the two or three questions that narrow yours, then names one test with the value to expect, and re-ranks the list on what you measure.
Open your first CaseFree and complete. No account, no card. A Bay Pass after that is $12.00 for 90 days.