Diagnostics · System playbook

Catalytic Converter

The cat is the victim, not the suspect.

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

P0420/P0430 alone, runs fine
Aging cat OR a lazy downstream sensor — verify before quoting
Rattle from underneath
Broken substrate — this cat is actually dead
Sulfur smell + poor mpg
Running rich and cooking the cat — fix the rich condition first
Power loss up hills, glowing manifold
Melted/clogged cat restricting exhaust — do a backpressure test

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

Scan tool with dual O2 live dataBackpressure gauge (or a vacuum gauge and experience)Infrared thermometer — outlet should run hotter than inlet on a working cat

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.
Pro tip: A cat efficiency code that only sets on the highway with everything else healthy is often a genuinely aged cat. One that sets around town with fat fuel trims is a symptom, not a diagnosis.

Catalytic Converter trouble codes (4)

P0420Catalyst System Efficiency Below T…P0421Warm Up Catalyst Efficiency Below …P0430Catalyst System Efficiency Below T…P0431Warm Up Catalyst Efficiency Below …
Worked the whole playbook and it still isn't right? That's what the Diag Desk is for — a vetted master tech looks at YOUR readings on YOUR vehicle. $25, free if unanswered in 24h.