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Code P0421 after tuneup and fuel service

A month ago I took my 1998 Mazda Protege to a local mechanic/tire store for a tune up, oil change, and added a fuel service that they recommended. The car was running sort rough and occasionally stalling at idle, but otherwise things seemed OK. I was getting gas mileage in the mid 30’s as I always have with this car.(even at 145,000 miles) When I picked up the car, there was a bottle of clear fuel additive in the passenger seat with no instructions, so I went back in and asked about it. The guy at the desk told me that they just forgot to put it in, and came out and dumped it in the gas tank. I drove for about 200 highway miles and the check engine light came on. I stopped at another outlet of the same shop on my way home, and when we opened the hood found no oil cap and oil all over the place. He replaced the cap, and sent me on my way with instructions to get my home shop to clean up the engine compartment and make sure everything was ok, which they did the next day. In the meantime, I noticed that my gas mileage was below 30 for two tankfuls (27.6 , and 29.1 to be exact) for the first time since owning this car. One month later, the check engine light came back on and this time they told me I needed to replace the catalytic converter and the downstream O2 sensor to the tune of $900.00!

1 - could driving 75 mph for 200 miles with no oil cap caused the P0421?

2 - could the fuel additive that was supposed to be added with a full tank, but was added with less than 1/4 caused the problem?

3 - do I have any recourse??

So, they changed the oil (maybe) but never put the fill cap back on the engine, and they forgot to add the fuel system additive in the tank. Great shop you’ve got there. Wonder what they did for the tune-up, or was the fuel system additive it?

How much oil was left in the engine when the tire store guy finally opened the hood and discovered the cap was missing? You didn’t say they added any oil to replace what was all over the engine, just that he replaced the oil cap and sent you on your way.

Amazing! Have you been dealing with this place for a long time, or was this your first visit?

Has anyone checked the oil level in this engine?

Q1: 200 miles of highway driving with less and less oil in the engine as the miles accumulate has the potential to cause LOTS of engine damage. Without knowing how much oil remained in the engine it’s really hard to say what might have happened.

Did any other warning lights come or, or was it just the Check Engine Light? How long did you drive after the light came on?

Q2: The fuel additive is unlikely to have done anything, positive or negative, to the car.

Q3: Find a mechanic (tire chain stores don’t count) and get another opinion about the catalytic converter and O2 sensor. My guess is the O2 sensor will fix the problem and you don’t need a cat. A faulty O2 sensor can also result in reduced fuel mileage.

They checked the oil and it was still at the full line. He said that a little bit of oil really looks like a lot when it’s splattered all over the engine. I wondered if it was initially overfilled. There were no other warning lights, and I called the tire place at home from the car as soon as the check engine light came on. My desk guy said that because it seemed to be running well, to just drive it on home (75 more miles) and they would check to see if it was a “ghost code” or something.

I agree with McP…this shop should be avoided. I probably would not even go back for an engine cleaning. There are places that will safely do an engine steam cleaning and that might be a better option.

I also agree with McP that the additive is unlikely to have made one iota of difference in anything.

It’s great to hear that the oil level was still good and there were no other CEL lights. That’s a real good sign. And yes, a small amount of oil blown everywhere can look like a lot more than it actually is.

The code is for low catalyst efficiency. A shop (NOT the one you went to first) can actually look at the upstream and downstream oxygen sensor readingas and make a more definitive diagnosis.

However, I noticed that you had the tuneup done because the engine was running sort of rough and occasionally stalling. You need to also tell the new shop about this. There’s a very good possibility that this erratic operation and the CEL light are related. Rather than tell a shopp to do a tuneup, you should tell them the symptoms and let them diagnose the cause(s). That’ll save you money and aggrivation in the long run.

Post back with the results. We care.

I took it to another mechanic today and he checked the computer code (no charge) and recommended changing the O2 sensor. (the tire store charged $65 to read the codes) Using an aftermarket part it would run $150.00 parts and labor. Because the tire store wanted to replace both the sensor and the Catalytic converter, I figured I would try the O2 sensor and then if necessary replace the catalytic converter. Either way, I am done with the tire store. What will happen if I don’t replace the Catalytic converter? We don’t have emissions testing in NC.

What will happen if you don’t replace the catalytic converter will only be known after you replace the oxygen sensor(s). You’re asking a question we can’t answer.

There are two O2 sensors; one upstream of the catalytic converter, and one downstream. Don’t assume the cat is bad unless you’ve replaced both sensors and the CEL remains on after miles of driving.

“They” checked the oil, and, of course, they said it was full. No surprise there.

“You” never checked the oil, did you?

Be honest.

“Ghost Code?” What the hell is a “Ghost Code?”

OK - first of all, yes I did watch them check the oil, and did it again later at home. And while I do have the ability to check my own oil, I have no idea what he was talking about regarding computer codes. I may be ignorant, but I’m not stupid!

The catalytic converter is affected by what happens in the engine before it. Too much fuel, or burned oil, or burned antifreeze can poison a catalytic converter and oxygen sensor.
Clean up the exhaust gases before they get to the pre-catalyst and catalytic converter. Replace the spark plugs (and wires) with NGK plugs. Have the cooling system pressure checked for internal (in the engine) antifreeze leak. Several sensors can (negatively) affect how too rich an engine runs: MAF (Mass Air Flow) sensor, coolant temperature sensor, and the O2 sensor. These can be checked for proper voltage and ohm values, by the proper mechanic with a digital multimeter.
After the above repairs, and checks, erase the DTC and see if the check engine light returns.
Here is a good response, to another driver with this code: