Miata throwing different codes

I am helping a friend with his "new to him" 99 Miata. The day after he bought it the CEL came on and showed code P0401 - insufficient EGR. We did the vacuum line reroute to see if that would help and the next day the CEL came back on. So we cleaned out the EGR passage in the upper intake and cleared the code. When I went to clear the code I realized that the code was now actually P0420 which is something like catalyst emissions below threshold. Cleared that to see if it had been caused by the other issues or perhaps from the throttle body cleaner that we used to clean out the EGR passage. The car ran without throwing a code for a couple of days, and today the CEL came back on, but this time it is P0171 ! Which is a lean condition!

Is there something that ties these 3 codes together? Or is this car just messing with me?

It's possible that the front O2 sensor is involved in both the P0420 ad P0171 codes. Possible. Does your scanner read real-time info? If so, take a peek at the front O2 voltage and see if it's behaving properly.

It could also possibly be a funky ground. I'm not sure if all of those guys share a sensor ground, but it's quite possible. One thing to check there would be for aftermarket accessories that have been tied into the same ground - using a sensor ground to also ground out your fog lights could potentially cause problems.

Or it could just be a car with problems

Thanks, I thought that the O2 sensor could trigger both codes, but I was confused by the P0401(but I haven't seen that one for a while.)

I can scan the O2 sensor real-time so I'll take a look.

P0420 is an ecu tuning issue, p0420 is an insufficient catalyst code. The ecu sees set catalyst parameters and when it is outside of those parameters it throws the code, but those set paramters are too stringent for vehices over 120k and you dont necessarily need a new cat but an ecu reflash to correct those un realistic parameters.

The P0420 code has not recurred, but the P0171 keeps coming back now.

I looked at the O2 sensor with the scanner, and it looked like the values were a little lower than they are on my '99 but I wasn't exactly sure what I should be seeing. I was hoping that the difference would be obvious.

What should I look for when I am watching the o2 values?

Check for vacuum leaks.

Well, we finally got this car to stop throwing codes and it passed e-check!!! This whole process was confounding, and I am still not sure if it had multiple problems or if I kept missing the actual problem.

Here is the chronology of the problems / codes and the solutions. The first code was P0402. I had done the vacuum line reroute on my '99 and failed to remember that my code had been P0401. So I moved the vacuum line from the top of the intake by the TB to the back of the intake. The previous owner had already moved the line from the OEM port to the top. So when it threw a code again, we took off the top half of the intake and cleaned out the EGR passage. Unfortunately we didn't check the code until after we did the cleaning. Turned out that the code was actually P0420 which is Catalyst something. We decided to clear the code and see if it came back, thinking that it might have been caused by the throttle body cleaner that we used to clean the intake.

The CEL came back on and we checked the codes again. This time it was P0171 which is a lean condition. I added P0171 to P0420 and added Keith's advice and figured it had to be the O2 sensor. Replaced the O2 sensor and was SURE that we had fixed the problem! A half hour later the CEL came back on.

Last night we got the car back to my shop and it was throwing P0171 again! Since the O2 is new what is left? When I lifted the hood you could hear a loud hissing! I swear that it never made that sound before!!! The OEM vacuum line port for the EGR was leaking. The PO had capped it of with a cap that had a split in it. New cap, no leak, passed e-check.

I think that the port was so clogged that it didn't leak until after we cleaned out the EGR passage. I really don't think that this leak was the problem all along, but it was the final piece to the puzzle.