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
2004 Acura 3.5RLSECTION DTC P0420: Catalyst System Efficiency Below Threshold
WARNING: This page is about a different car, the 2003 Acura RSX. However, it is still accessible from the selected car via links, so may be relevant.
NOTE:
If DTC P0137, P0138 and/or P0141 is set, troubleshoot those DTCs before proceeding.
- Perform ECM/PCM reset procedure. See ECM/PCM RESET PROCEDURE/CLEARING DIAGNOSTIC TROUBLE CODES under SELF-DIAGNOSTIC SYSTEM.
- Start engine and run at 3000 RPM, with no load (in park or neutral), until radiator fan comes on. Allow engine to idle.
- Test drive vehicle on highway for about 10 minutes without stopping. Speed may vary.
- With A/T in "D" position or M/T in 4th gear, drive vehicle at steady speed between 50 and 62 MPH for 30 seconds.
- Repeat step 4 three times. Between each repetition, close throttle completely for 1-2 seconds. If engine is stopped during this procedure, go to step 3 and do procedure again.
- Using scan tool, check for temporary DTC. If DTC P0420 resets, go to next step. If DTC P0420 does not reset, check for readiness code completion. If readiness is complete, problem is intermittent. System is functioning properly at this time. If readiness is incomplete, repeat steps 2 - 5 .
- Inspect Three-Way Catalytic Converter (TWC) for plugging, melting, cracking, exhaust system leaks or TWC deterioration. Repair or replace as necessary.
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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.