Catalytic Converter

P0420/P0430 means the downstream oxygen sensor sees the cat not storing oxygen like it used to. Sometimes the cat is genuinely dead — but cats are killed by SOMETHING: misfires, oil, coolant, or rich running. Replace a cat without finding its killer and the new one dies the same death, several hundred dollars later.

What the symptoms mean

The diagnostic order

Work top to bottom — the order is the point. Each step rules out the cheapest, most common causes before anything expensive gets blamed.

1. History first: any misfire codes, oil consumption, or coolant loss? Those are cat-killers and they get fixed FIRST.
2. Watch both O2 sensors live: upstream should switch busily, downstream should be relatively flat. A downstream mirroring upstream = low storage.
3. Rule out exhaust leaks between the cat and sensors — a pinhole fakes cat failure convincingly.
4. Backpressure test if there is power loss: over ~3 psi at 2500 rpm points at restriction.
5. Only after the killers are cleared does the cat itself get condemned — and note the fuel trims on the invoice as your receipts.

Tools that matter here

The classic mistakes

• Replacing the cat while an active misfire is still flashing the light. The new cat is dying in the first mile.
• Skipping the exhaust leak check and eating the comeback.
• Universal-fit cats on California-spec vehicles — know your local rules before quoting.

Catalytic Converter trouble codes (19)