Generic Automotive Fault Code
Free · no signupP0420
Check Engine Light Only / No Performance Loss
System
Emissions
Symptom
Check Engine Light Only / No Performance Loss
Diagnosis & fix steps
LOGIC: Catalyst System Efficiency Below Threshold (Bank 1).
ACTION: Check for upstream exhaust leaks, engine misfires (P0300), or oil burning before replacing the converter. Monitor downstream O2 sensor switching frequency.
Red Seal field insights
Diagnostic Logic: The PCM compares upstream and downstream O2 sensors. If the downstream sensor mimics the upstream sensor's switching, the catalyst has lost its oxygen storage capacity.
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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