Safety Warning
DIY auto repair can cause serious injury, fire, or vehicle damage. These guides are for informational purposes only. Always follow OEM torque specs, wear PPE, and consult a certified mechanic if you are unsure. You are solely responsible for your safety.
Code P0420, Catalyst Efficiency Low (Bank 1)
"S" & "T" SERIES 4.3L (VIN W)
Check the following:
- Ensure 3-way catalyst is factory original for application. Check converter for dents, severe discoloration, holes, loose internal (rattle) materials, etc.
- Check exhaust for leaks or missing hardware.
- Ensure oxygen sensor is secure and that sensor pigtail is not damaged or touching hot exhaust.
- Check all control module and sensor grounds. Check sensor high and low circuits for intermittent open or short to ground.
If no problems are found, catalytic converter is bad. If converter needs to be replaced, ensure that conditions which may damage new converter do not exist i.e. misfire, high oil or coolant consumption or retarded or weak spark.
NO RELATED
Use the Manual With the Right Hardware
Pair factory procedures with proven DIY tools so the instructions are easier to execute.
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When to See a Mechanic
Stop DIY work and contact a certified mechanic immediately if any of the following apply:
- • You smell fuel, burning insulation, or see smoke.
- • Brakes feel soft, pull hard to one side, or make grinding noises.
- • The engine overheats, stalls repeatedly, or misfires under load.
- • You are missing required tools, torque specs, or safe lifting equipment.
- • You are not confident in the next step or safety outcome.