P0420 on a Honda Accord
P0420 means catalyst System Efficiency Below Threshold (Bank 1). On the Accord, a few causes come up far more than others.
What P0420 usually means on a Accord
On the Accord, P0420 reports that bank 1's catalytic converter is converting exhaust below the ECU's efficiency threshold. High-mileage K24 and J35 cars often reach this honestly as the original cat ages. On V6 cars, remember bank 1 is the firewall-side head. A lazy rear O2 sensor is a common false-alarm cause worth ruling out before spending on a converter.
Most common causes on the Accord
- Aged catalytic converter
Factory cats on 150k+ Accords lose efficiency; the most common root cause on older cars.
- Sluggish downstream O2 sensor
A slow rear sensor imitates a failed cat and is far cheaper to replace.
- Oil consumption from VCM (V6)
J35 VCM engines that burn oil can contaminate and degrade the converter over time.
- Exhaust leak upstream of the rear sensor
A leaking gasket or flex joint skews the readings feeding the monitor.
What P0420 costs to fix on a Accord
On four-cylinder Accords the converter is reachable and moderately priced; on the V6, a bank-specific cat and tighter access raise the bill. Verify the rear O2 sensor with live data first - it is a fraction of the cost. CARB-compliant converters are required in California-emissions states to keep the monitor satisfied.
Year, engine and mileage change the answer. Start a case and Redline weighs these causes against what your car shows, then names the check that would settle it.
Accord owners also ask
Is bank 1 the front or rear cylinder head on my V6 Accord?
Bank 1 on the J35 V6 is the rear head, closest to the firewall - worth knowing before ordering a bank-specific converter or sensor.
Will P0420 hurt my Accord if I keep driving?
It will not damage the engine, but the car will fail emissions and, if the underlying cause is oil consumption or misfire, that root problem can worsen, so it is worth diagnosing.
Other codes on the Honda Accord
Related symptoms
Honda Accord problems & recalls by year
Based on widely reported patterns for this model; individual causes vary by year, engine and history. Generic P0420 behavior is covered on the main P0420 page. Confirm with a scan tool before buying parts.
P0420 on your Accord?
Every problem becomes a Case. Tell Redline what your Accord is doing: it shows what the evidence supports, what’s still missing, and what to check next, before you approve a repair.
“Inner pad ~1 mm. Outer pad 3 mm.”
Next: check the slide pins move freely before buying parts.