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Discussion Starter · #1 ·
I've had an intermittent check engine light and finally got the code:
P0420 - "Catalyst system efficiency below threshold (rear cylinder head)"

Can anyone translate that into plain(er) english?


Premium Member
2104 Honda Ridgeline SE
1,811 Posts
Try this Joe...

P0420: Rear Bank Catalyst System Efficiency Below Threshold(Bank 1)
There is a test proceedure to see if the problem is intermittent or steady. If steady, the rear WU-TWC needs to be replaced (catalytic converter).
After replacement, the re-learn proceedure should be followed to reset the ECM.
Make sure all lectrical accessories are off (AC, lights radio, etc)
Reset the PCM with the HDS (if you don't access to the HDS, there is a manual method that requires disconnecting the battery)

Turn iginition to on for 2 seconds (dont crank the motor)
Start the motor and run at 3000 rpm until the radiator fan comes on.

Idle the motor for 5 minutes ( don't touch the gas pedal)

If the fan comes on again, dont count the time it is running as part of the 5 minutes ( so if the fan comes on for 30 seconds every 2 minutes, you will run for about 6 1/2 minutes total)
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