2003 Mazda Protege • 160,000 miles


Question re: Mazda Protege 5, 5 speed manual non turbo. The issue - car is stalling/chugging when idle-ing at stop signs/red lights/stop go traffic. I have replaced the EGR valve, spark plugs, spark plug wires, ignition coils, air filter and the O2 sensors were changed about a year ago. The last fix I tried was the spark plugs and iginition coils - car ran great for a month after the change (check engine light was still on, code p0421). It just started acting up again, chugging but not to the point it dies.

Any idea what the problem could be now?

Thanks for any help!
November 28, 2012.

P0421 - Catalyst system, efficiency below threshhold.

Possible Causes

l Warm-up Three Way Catalytic (WU-TWC) converter deterioration or malfunction.
L Exhaust system leak.
L Loose rear HO2S.
L Loose front HO2S.
L Front HO2S malfunction.

The idling speed is too low or you have a misfiring causing the engine to idle erratically or stall.

Try cleaning the throttle body and IAC and retest.

Nov 28, 2012.