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Recurring P0420 Code on 1999 Honda Odyssey - O2 Sensor and Cat Replacement Issues

Model: '99 Odyssey Fault Code: P0420 Posted: 2006-11-25 10:53

New user with difficulty posting in the correct section. Vehicle: 1999 Honda Odyssey LX, 94,000 miles. I recently performed a fuel injector cleaning to resolve a P0401 code. After 1,000 miles, the vehicle triggered a P0420 code. I replaced the catalytic converter and downstream O2 sensor. Approximately 150 miles later, a P0401 reappeared. This time, I replaced the Exhaust Gas Recirculation (ERG) valve and did another fuel injector cleaning. However, just 7 miles later, the P0420 code returned. This vehicle, once reliable, has now become a significant financial burden. Any insights or troubleshooting steps would be greatly appreciated? Key points: P0420, P0401, 1999 Honda Odyssey LX, catalytic converter replacement, O2 sensor issues.

Related fault codes

P0420

Comments (3)

Anonymous 2006-11-25 15:57

The P0420 code is most commonly triggered by a failing upstream oxygen sensor. Over time, these sensors degrade and become sluggish, which can cause the engine control module to detect improper exhaust gas conversion and set the P0420 fault code.

Anonymous 2006-11-25 16:40

That makes sense—thank you for clarifying. I replaced the upstream O2 sensor, and after 20 miles, the check engine light returned. What should I do next? Any other potential causes to consider?

Anonymous 2006-11-25 21:59

Was the catalytic converter installed an aftermarket unit? Aftermarket converters are generally not reliable, especially on Honda vehicles. Honda's engine control system has very tight programming for monitoring catalytic converter efficiency, and aftermarket converters often fail to meet these standards, causing the check engine light to remain on. Mike P.S. Have you checked for exhaust leaks or cracked exhaust manifolds? These can also interfere with oxygen sensor readings and trigger P0420.