The catalyst monitor checks how well your catalytic converter is cleaning exhaust. It needs steady highway-speed cruise to run, and any code related to the catalyst will prevent it from setting.
If the catalyst monitor sets but immediately throws P0420, the catalytic converter has actually failed. A new cat is $400-$2,000 installed.
15-20 minutes at 55-65 mph in top gear with minimal throttle changes is the standard catalyst monitor condition. Use cruise control on a flat highway for best results.
Coolant temperature must be at operating range (~190F) for the entire monitor period. Short trips that never fully warm up will not work.
The catalyst monitor compares upstream and downstream O2 readings, so the O2 monitors must be ready before it can run. Drive cycle order: O2 first, then catalyst.
A pending code blocks the monitor from completing. See P0420. Address the cause - worn cat, bad O2 sensor, or exhaust leak - before retesting.
If the cat is worn or contaminated, the monitor may run but fail. Replacement is $400-$2,000 depending on the car. Hybrids and luxury models cost more.
| What You Notice | Likely Diagnostic Step |
|---|---|
| Only drive in town | Take a 30 min highway drive |
| Cat monitor sets, then P0420 | Cat is bad - see P0420 |
| Stop-and-go traffic only | Find a flat highway with cruise on |
| O2 monitor not ready | Reset O2 first, then catalyst follows |
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Drive a complete drive cycle with 15-20 minutes of steady highway cruise (55-65 mph in top gear, cruise control if possible). Many cars need this twice over two days.
Most often you are not driving in the conditions it needs - steady highway cruise. Alternative causes: a pending P0420 code blocks completion, or O2 monitors have not set yet.
About 30-45 minutes per cycle, with 15-20 minutes of that being steady highway. The cycle may need to repeat over 2-3 days for the monitor to confirm.
If the O2 sensors are sluggish, yes - both the O2 monitor and the catalyst monitor depend on healthy O2 sensor response. Replace upstream O2 sensors over 100,000 miles old.
The monitor passing means the test ran. If it then throws P0420 or P0430, the catalytic converter has actually failed efficiency. Replacement is the only fix - see our P0420 page.
No, tampering with OBD-II is a federal violation, and "cat simulators" or downstream O2 spacers do not actually fool modern systems. Fix the cat or fail.