Generic Fault Code
Free · no signupP0420
MIL Illuminated / No Performance Loss
Variant 1
System
Emissions
Symptom
MIL Illuminated / 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 is failing to store oxygen.
Variant 2
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, or oil burning. 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 is failing to store oxygen.
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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