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.
Exhaust Catalyst
PCM checks catalyst efficiency by monitoring switching frequency of both front and rear Heated Oxygen Sensors (HO2S). Under normal closed loop fuel conditions, high efficiency catalysts have oxygen storage which makes the switching frequency of the rear HO2S quite slow compared with the frequency of the front HO2S. As catalyst efficiency deteriorates, its ability to store oxygen declines, and the rear HO2S begins to switch more rapidly, approaching the frequency of the front sensor. In general, as catalyst efficiency decreases, the switch ratio increases from a switch ratio of 0 for a low mileage catalyst to a switch ratio of 0.8 or 0.9 for a low-efficiency catalyst. Diagnostic Trouble Code (DTC) P0420 (bank 1) or P0430 (bank 2) will be stored. See appropriate DTC-related testing in appropriate SELF-DIAGNOSTICS article.
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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.