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.
Circuit Description
This test monitors catalytic converter efficiency. This runs only once per ignition cycle and only after the converter has warmed up to operating temperature. Converter efficiency is calculated using data collected from pre- and post-converter oxygen sensors during steady driving conditions. The pre-converter oxygen sensor signal should fluctuate like those of the front and rear oxygen sensor. The post-converter oxygen sensor signal should fluctuate less than the pre-converter oxygen sensor.
The catalytic converter efficiency test is performed in 2 stages. The first stage calculates post-converter oxygen sensor deviations (difference from the average value), compares these deviations to the maximum allowable deviation (based on air flow), and records a failure if those deviations are excessive. The second stage is only run if the first stage fails or DTC P0420 is currently set. The second stage averages the difference between the deviations calculated in the first stage and the maximum allowable deviations. If the second stage average is too high, DTC will set.
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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.