Asian Fault Code
Free · no signupP0420
No driveability symptoms usually; Check Engine Light active; Failed emissions test.
System
Not specified
Symptom
No driveability symptoms usually; Check Engine Light active; Failed emissions test.
Diagnosis & fix steps
Catalyst System Efficiency Below Threshold (Bank 1).
LOGIC: The downstream Oxygen Sensor is "switching" too fast, mirroring the upstream sensor, meaning the catalyst is no longer storing oxygen.
ACTION:
1. Check for exhaust leaks upstream of the cat.
2. Log O2 sensor data.
3. Check for engine oil burning (blue smoke).
FIX: Repair exhaust leaks; replace Catalytic Converter.
Red Seal field insights
On many Asian vehicles, a faulty A/F (Air/Fuel) sensor can mimic this code. Always check for exhaust leaks at the manifold heat shield before condemning the expensive converter.
Same code keeps coming back?
That's a program problem, not a part problem. Book a 30-minute uptime review with Grant, a Red Seal Heavy Duty Technician.
Read before you touch anything
- This is general reference information, not a repair manual. It is for trained, qualified technicians.
- Fault codes, meanings and procedures vary by model, serial number and software version. Confirm the code and the procedure against the manufacturer's service manual before acting.
- Follow the OEM service manual, torque specs and your site's safety procedures: lockout/tagout, block raised equipment, relieve hydraulic, fuel and air pressure, and let hot systems cool.
- You use this information at your own risk. Adventure Mechanical and AM Fleet Integrity accept no liability for damage, injury, downtime or loss arising from its use.
- Not affiliated with or endorsed by any manufacturer. Brand names are used only to identify the equipment a code applies to.
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