P0420 on a 2015 Honda Accord Hybrid
P0420 means the downstream oxygen sensor pattern suggests the bank 1 catalytic converter is not storing oxygen properly.
Not your vehicle? See general P0420 info.
Most common causes on the 2015 Honda Accord Hybrid
Ranked from 609 NHTSA owner complaints filed for this vehicle.
- 1
Prior cylinder misfires overheating and damaging the catalyst substrate
High severityThis batch shows a documented multi-cylinder misfire (P0304) and a case of two dead cylinders reaching the converter as unburned fuel, which overheats and melts the substrate, dropping conversion efficiency.
Reported in 2 NHTSA complaintsParts:catalytic converter - 2
Worn or failing catalytic converter
High severityAn owner had a mechanic directly diagnose a faulty catalytic converter after the check-engine light appeared shortly after purchase — direct confirmation of the exact failure this code monitors.
Reported in 1 NHTSA complaintParts:catalytic converter - 3
Oil contamination of the catalytic converter from oil consumption
Medium severityAn owner reports the engine burning about a quart of oil every 2,500-3,000 miles; oil ash passing through the exhaust coats and poisons the catalyst's active surface.
Reported in 1 NHTSA complaintParts:catalytic converter - 4
Failing downstream (post-catalyst) O2 sensor
Low severityA degraded rear O2 sensor can mimic reduced catalyst efficiency even when the converter itself is still good, and is worth testing/replacing before condemning the converter.
Common causeParts:oxygen sensor - 5
Exhaust leak near the downstream O2 sensor
Medium severityA leak that lets outside air reach the rear sensor skews its reading and can falsely trigger a catalyst-efficiency code.
Common cause - 6
Coolant contamination of the catalytic converter (head gasket leak)
Medium severityCoolant burned through a leaking head gasket coats and disables the catalyst, a less common but well-documented cause of P0420.
Common cause
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These causes are aggregated from NHTSA owner complaints and general repair knowledge — this is not a diagnosis. P0420 on your car still needs shop confirmation before any repair.
Common symptoms
- Check engine light
- Usually no drivability issue
- Failed emissions test
- Sulfur smell in some cases
- Reduced power if converter is clogged
What to check before replacing parts
Check for misfire and fuel trim codes first
Look for: Any active or recent misfire, or lean/rich trim codes, means raw fuel or an off mixture has likely already damaged or is currently overworking the converter — fix those before touching the catalyst.
Inspect for exhaust leaks ahead of the converter
Look for: A leak before the converter (cracked manifold, loose flange, blown gasket) lets in outside air that confuses the downstream O2 sensor into reading as if the catalyst isn't working, even when it is.
Compare upstream and downstream O2 sensor activity
Look for: A healthy converter makes the downstream sensor read flat and steady while the upstream sensor swings actively. If the downstream sensor switches nearly as fast as the upstream one, the converter isn't storing oxygen properly and is likely worn out.
Check converter temperature and performance
Look for: A converter outlet noticeably cooler than expected relative to its inlet under load suggests little catalytic reaction is happening inside — supports a genuinely failed converter rather than a false reading.
Hold off on replacing the converter until root causes are ruled out
Look for: If misfires, exhaust leaks, or fuel trim issues caused this code, a new converter will fail again quickly — confirm those are clean first.
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