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 DTC P0421. Following conditions must be met during the test:
- Atmospheric temperature is 14°F (-10°C) or more.
- A/C switch is off.
- Selector lever is in "D" range (A/T).
Monitor takes about 20 minutes to complete. To run monitor, use illustration. See Fig 1 . Perform steps as follows:
- Remove all test equipment from vehicle and install all components and connectors that were removed or disconnected during testing.
- Start engine.
- Accelerate vehicle to 45 MPH.
- Travel 300 seconds or more while keeping speed at 45-60 MPH. Use 5th gear on M/T.
- Decelerate to 35-40 MPH.
- While keeping accelerator pedal opening degree constant (use 4th gear on M/T), keep speed at 35-40 MPH for 90 seconds or more.
- Fully close throttle and decelerate for 10 seconds. Then, quickly accelerate until speed is 35-40 MPH. Then repeat steps 5 and 6 five more times.
- Return vehicle to shop. Turn engine off. Recheck for DTC(s). If any DTC(s) is set, diagnose affected DTC(s). See DIAGNOSTIC TROUBLE CODE DEFINITIONS .
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.
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