01 Eclipse GT P0421 - Warm-Up Catalyst Efficiency Below Threshold (Bank 1) Diagnosis and Repair Guidance
I'm experiencing fault code P0421 on my 2001 Eclipse GT, which indicates 'Warm-up Catalyst Efficiency Below Threshold (Bank 1)' as reported by my PocketScan Plus. The code has been present for approximately one week before I cleared it manually. The diagnostic information from the OBD-II system includes: - Misfire - Fuel system monitor - Comprehensive components monitor - Catalyst system monitor - Evaporative system monitor - Oxygen sensor (O2) monitor - Oxygen sensor heater monitor - Exhaust Gas Recirculation (EGR) I understand that my 2001 Eclipse GT is equipped with three catalytic converters, and I'm currently unsure whether the issue lies with the pre-cat under the manifold or one of the oxygen sensors. Before investing in a replacement part—either an O2 sensor or the pre-catalyst—I'd like to gather more expert input on potential root causes. Any insights into how to properly diagnose this P0421 code, especially distinguishing between catalyst and sensor-related failures? I'm looking for reliable, factual advice based on actual vehicle systems.
If the issue were solely with an oxygen sensor, a different fault code (such as P0135 or P0141) would typically be set. A P0421 code specifically points to catalyst efficiency issues during warm-up, suggesting that the problem may lie in the catalytic converter system rather than the O2 sensors. It's advisable to inspect the pre-cat under the manifold and consider testing for exhaust flow or backpressure before replacing components.