Applies to 4 cyl OBDII Probes, Contours, Mazda 626’s and a few others. 

   As the resident wrench on the LBWCC campus, I get regular requests for assistance with things like flat tires, keys locked inside the car, oil changes, illuminated warning lights, etc. 

A 2001 Mazda 626 I checked a couple of weeks back yielded a P0421 catalyst efficiency code, and a quick trip into the datastream cinched the diagnosis; the aft-cat O2 sensor was almost as lively as the front one, and that can only mean one thing; the catalyst had become incapable of storing oxygen and had outlived its usefulness.  The pinpoint test routine does call for checking fuel pressure and exhaust system integrity, but those tests usually don’t reveal anything to cause both sensors to switch together like these were.  If the catalyst failed at either very low or very high mileage, it’s wise to find out if there were combustion- related anomalies (including oil consumption) that could have toasted it.

But in this case, as far as I was concerned, it was one of those open-and-shut cases.  Some diagnostic trouble codes are like that.  A P0455 usually means the gas cap is loose.  A P0125 means the thermostat needs replacing (provided the coolant isn’t low).  Lean and rich codes aren’t as ‘pat’ and can have indirect causes.  But in this case, the Mazda would need a catalyst.  It was as simple as that.  The good news was that she only had 77k on the odometer, and the catalyst is covered for 8/80, per federal standards.  I called the Mazda dealer and gave the service department a heads-up that she would be coming in for service.  They checked the code when she got there, then ordered and replaced the catalyst.  Melissa (the owner) was tickled pink.

A few days later she was back at my door with the MIL illuminated again.  The Mazda PCM spit out the same code, and I instantly knew what had happened.

When I was working at the Ford dealer, I checked out a Ford Probe with the same setup.  Having seen fore and aft sensor switching together, I had the parts department order a catalyst.  The line techs on the other side of the shop put it on, and the next time I saw the car was about three days later when it came in with the same code.  As I investigated what happened, I realized that the rear catalyst (replaced by the line tech in good faith) isn’t monitored by the PCM.  The cat that triggered the light was the small coffee can-sized converter right in front of the engine.  The shop manual refers this one to as the Warm-Up Three Way Catalytic Converter. There's another catalyst under the car, but that one has no O2 sensors monitoring it.

To his credit, the line tech at the Ford dealer had no idea why I had ordered the catalyst, so he had no reason to do anything beyond a nuts-and-bolts replacement.  Had we been working together when he got the car on the lift, I would have noticed that there were no O2 sensors anywhere near the catalyst he was replacing.  I should have been more careful when I ordered the converter, but the parts guy didn’t ask me which converter I wanted, so I assumed, and you and I both know what that makes the ‘u’ and ‘me’ in a situation like that.

The Mazda was a repeat of this scenario, with one exception.  The guy who checked the car out was the same guy who ordered and replaced the cat.  Why he failed to notice the absence of O2 sensors at the rear catalyst remains a mystery.  Upstream O2    (top photo)                                   Downstream O2 (bottom photo - too active)

After finding the P0421 code in the Mazda, I looked under the hood at the catalyst between the two O2 sensors that should have been replaced but hadn’t been.  Then I ran the car up on the lift for an inspection of the exhaust and found a freshly replaced catalyst that hadn’t needed replacing. I called the Mazda service department the second time and explained the problem and how I had fallen in the same trap a few years ago. The mechanic initially made the huffy statement that there was only one catalyst on the car.  I hung up and called back to talk to the parts guy, and he verified that there were two catalysts, one front, one rear.

It’s easy for any of us to get caught up in the bolt-twisting phase of a job sometimes and forget to reason things out.          

 The right catalyst has an Oxygen sensor before and after it in the exhaust stream - most cars don't have an unmonitored catalyst the way this Mazda package does.

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                                             R.W.M.