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.
Catalytic Converter Monitor
This monitor applies to DTCs P0421 and P0431. Following conditions must be met during test:
- Ambient temperature is 14°F (-10°C) or more.
- A/C switch is in OFF position.
- Selector lever is in "D" range.
This one trip monitor, takes about 20 minutes to complete. Remove all test equipment (except scan tool) from vehicle and install all components and connectors that were removed or disconnected during testing. Select throttle opening angle data. Find a flat surface (no hills) to run monitor. To run monitor, use illustration and perform following steps. See Fig 1. Step numbers in procedure will correspond to step numbers in illustration.
- Start engine and quickly accelerate to 45 MPH or more.
- Drive 5 minutes or more while keeping speed at 45-60 MPH. You are allowed to brake for a few second during this period.
- Decelerate to 35-40 MPH.
- While keeping accelerator pedal opening degree constant, keep speed at 35-40 MPH for 90 seconds or more.
- Fully close throttle and decelerate for 10 seconds. You are allowed to brake during this period. Then, quickly accelerate until speed is 35-40 MPH. Then repeat steps 3 and 4 fully 5 more times.
- Stop vehicle and turn ignition off.
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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.