Been researching this error code for my 2001 626 ES. Is this a definite bad catalytic converter? Could this be an O2 sensor? If so, which one. There are two. One in front
of converter and behind. Can someone help?

 

Since you have the ES model am assuming that the engine is the 2.5l V6?? Assuming also if is a V6 then must be a california spec emission as the Federal only has one catalytic converter and would have set a P0420. The P0421 indicates the right bank catalyst efficiency is below limit. A faulty oxygen sensor is very, very rarely responsible for efficiency codes with Mazda. They all have codes themselves in case a fault is detected. The only Mazda I am aware of that has this problem is the 1997-2000 Millenia S which Mazda claims that the rear O2 sensors could be overly sensitive to set this code.
The front or upstream O2 is used for rich/lean detection and rear is only used as a catalyst monitor. You are aware that the cat converters are covered under emissions warranty for 8 years or 80,000 miles? Hopefully you are under those limits.

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sorry for the long delay. The mazda has a 4cyl 2.0 engine in it. Unfortunately the mileage has just passed 80 K miles. One muffler shop told me the pre -cat is what is bad. He quoted me $838 dollars just for the pre-cat. Is this guy trying to rip me off?

 

You shouldn't have to replace that. if the code says warm up Catalyst below threshold bank 1 try replacing the 02 sensor after the Catalyst. I had the same issue with my 2000 LX and now my 01 LX 2.0liter.. both manuals. When I finally found one similar to mine and bought it.. I asked my kids dad to replace them with the ones in my 2000 cause they were newer than the ones that were in my 01. Anyways, he said he had to connect the wires together on bank 1 for whatever reason.. im pretty sure that's where the issue is.. Anyways my point is in my first sentence. Lol good luck.

 

GRAMMA... you do see that the OP last response was Old Feb 27, 2006 | 03:23 PM
You are a bit late for a response?
Ok so here is the thing. First they are HO2 sensors. Its actually important to get that correct. O2 do not have any heating device so their reporting g to the ECU is different than with a O2 sensor.

You should NEVER randomly replace any part when diagnosis a OBDII ECU that generated a DTC without first testing the part to see if has failed. Doing so can and often will create added problem to the unresolved one.

The steps to diagnosis are to first go into the ECU and see all the area's and then determine a diagnostic tree of part and trouble solving.

The other thing ius stick with advise the is directly related ONBLY to your platform and year! Comparing other planforms is a BAD idea!