Common Symptoms
- Check Engine Light illuminated
- Slight decrease in fuel economy
- Rotten egg smell from exhaust
- No noticeable driveability issues in most cases
- Failed emissions test
Most Likely Root Causes
- Catalytic converter internal degradation or contamination
- Engine oil consumption fouling catalyst
- Exhaust leak before or at oxygen sensors
- Faulty downstream oxygen sensor
- Engine running rich or lean causing catalyst overload
Full step-by-step tests, DVOM guidance, and pinout data are available inside the AI Diesel Tech diagnostic session.
AI walks you through every test — one step at a time. Plans start at $12.95/mo.
Component Location
Catalytic converter is in the exhaust system between the exhaust manifold and muffler, with oxygen sensors before and after it.
Manufacturer Notes
Many manufacturers require confirming catalyst efficiency failure over multiple drive cycles before replacement under warranty.
Also diagnosing J1939 SPN/FMI codes?
Heavy-duty trucks often throw both OBD-II and J1939 faults simultaneously. AI Diesel Tech handles both protocols in a single guided session.
AI Diesel Tech — The Technician's Diagnostic Engine
AI Diesel Tech diagnostic logic is built by an ASE Certified Diesel Technician. Professional-grade OBD-II and J1939 fault analysis — trusted by technicians nationwide, used in shops, fleets, and training programs.
Get AI-Guided Diagnosis for P0420
The diagnostic engine walks you through every physical test — step by step, on your phone, at the truck.