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.
DTC P0420: Catalyst System Efficiency Below Threshold: Description
Three-Way Catalyst (TWC) efficiency is measured by how well it can store oxygen. PCM monitors converter efficiency by comparing voltage values of Heated Oxygen Sensor 1 (HO2S1) and Heated Oxygen Sensor 2 (HO2S2). Under normal operating conditions, HO2S1 should vary between 10 millivolts and 1065 millivolts and HO2S2 should remain relatively steady 500-800 millivolts. This steady reading of HO2S2 indicates a correctly functioning catalyst. When all parameters have been met, PCM will run a 5 second diagnostic at idle. PCM will command rich and monitor time it take HO2S2 to go rich. It will then command lean and monitor the time it takes HO2S2 to go lean. The longer it takes HO2S2 to change rich/lean means converter is storing oxygen and is functioning properly. If 5 second test fails, PCM may take several tests during several ignition cycles to set DTC. DTC sets when PCM has determined that catalyst is no longer efficient.
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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.