Search Auto-Repair-Questions

Tuesday, October 02, 2007

Check Engine Code P0420

Code P0420 : Catalyst System Efficiency Below Threshold This code is set when the o2 sensor behind the catalytic converter sees a lower signal than the set parameters of the OBDII system. English : The Catalytic converter is no longer doing an efficient job and needs to be replaced. Symptoms : Check Engine Light On Drivability : Not affected Causes : 1) Age 2) Running rich caused by failing upstream o2 sensor or failing MAF 3) Fuel Injector Stuck Open Fix : Replace Catalytic Converter and test system


  1. Steven Brown ( 12:46 AM

    It is not smart to replace a converter when the problem can be a sensor, wiring, etc. For example, on my 2001 Toyota Sienna this code can come on with no engine problem, new sensors, and very low emissions by dyno text. Turns out Toyota programmed the original ECM to wrong parameters such that very slight catalyst degradation can cause this code to set even though all relevant parts are within spec. Toyota sent out a 2005 TSB on this, so that people who reported the defect in the right time frame got new ECMs for free. Pretty crooked, since the ECM did not age to become defective, it was installed defective. Why do you advise replacing such an expensive part on such ambiguous evidence?

  2. totally agree, Toyota should extend free ECM replacement offer to customers who are still stuck in the field. Customers can not be held responsible for poor design, especially when the fix is so expensive. My Sienna 2001 has developed this notorious P0420 issue just a tad after 80,000 miles which is a free replacement cut-off line. Very disappointed about Toyota in general..