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
The DTC P0420 Catalyst System Low Efficiency diagnostic monitors the operation of the exhaust system catalytic converter. To control exhaust emission of Hydrocarbons (HC), Carbon Monoxide (CO) and Oxides of Nitrogen (NOx), a three-way catalyst converter is used. The catalyst within the converter promotes a chemical reaction which oxidizes the HC and CO in exhaust gas, converting HC and CO into harmless water vapor and carbon dioxide. The catalyst also reduces NOx by converting NOx into nitrogen. Powertrain Control Module (PCM) has the capability to monitor this process using a Heated Oxygen Sensor (HO2S) located in the exhaust stream past the three-way catalytic converter. PCM compares the signal from Heated Oxygen Sensor 1 (HO2S 1), located before the catalyst with sensor 2 (HO2S 2) located after the catalyst to determine if the catalyst has deteriorated. If catalyst is functioning correctly, HO2S 2 signal will be far less active than HO2S 1. Both HO2S signals changing at similar rates indicates the catalyst is deteriorating.
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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.