ECU code 420
 

Hey guys,

I've been reading up on this stuff a little and can't seem to find the solution to my problem.

I've got a 2000 ES, Installed a header (shorty style) with both the 02 sensor and the EGR all hooked up. It worked fine for about a week then I got the CEL, P420. The manual (at Autozone) said this was "Catalyst System Effeciency Below Threshold (Bank 1)". The guy at Autozone thought this meant it was the first 02 sensor that has gone bad.

SO what do I do now? I saw some things about the SIM resistors, but that sounded like if you were not running catalytic converters- I still have my catalytic converter and precat installed.
Could I just have a bad 02 sensor (I do have over 100K miles)?

I don't know if this applies to the protege, but when I had my 91 CRX, I had to have a "heated 02 sensor" when I had headers on it.

Any help is appreciated

it's because you don't have a cat anymore, both o2 sensors are seeing the same oxy content. put the cat back in and reset cell and it won't come back

has your car ever had misfires which take cats out very fast , also proteges and 626's have had problems with p0420 and p0421 codes probably have a bad converter you should remove precat and you at inlet and see if any damage. will look like it is eaten away in spots. the dealer has a tester can check both o2 sensors while driving car to see if one of the o2 sensors is bad . the rear o2 is for checking cat eff.

The cat is still there-- I never removed it. The header is a "shorty" style that only replaces the manifold.

I don't think the converter is bad- I remember specifically noticing that the "screen" at the top of the cat looked brand new. But thats not saying that it isn't now--- I could check that. Is there a way to test the 02 sensor with a standard voltmeter? The closest dealer to me is an hour away :(

With an OBD2 scan tool you can see what the sensors are doing. Odds are that the upstream and downstream O2 sensors are doing the same thing- that's what triggers the 0420 code. Did you by any chance have problems with the coil pack on the car?
Chris

No trouble with the coil pack. That TSB was not in my VIN range.
I dont' think I've had any misfires either.

I am about due for new spark plugs....Don't know if that would help?

It seemed like the Scan tool that they used at Autozone only tells you the code- it doesn't give specifics like which 02 sensor is bad. Does the dealer use a different tool?

I've never used the Autozone tool. I bought one from here that allows graphing of the O2 sensors and other functions. Yes, the dealer's tool almost certainly would do that sort of thing.
My '99 Miata had the failed coil pack (no TSB on those) and it killed the cat very quickly. Kind of a bummer but covered under the 8 yr/ 80k emissions warranty.
Chris

Chris,

thanks for the link to the OBD tool- that could be a good investment. I was wondering what the "symptoms" of a failed coil pack woud be (besides the CEL)? My car seems to run fine. In my past experience with coils (on my CRX), was that they either work or the car did not run.
Like I said, the car is running OK- I just took 1st place in STS today at an autocross :). Last night, I just changed out the spark plugs and reset the ECU by disconnecting the battery. So far, no light, but I'll be crossing my fingers for a week or so.

Thanks for your help. I'll keep you posted if anything comes up.

I am just having this problem myself and I have had an ignition coil replaced. How is this related to the O2 sensor?
BTW, I also had 2 fuel injectors replaced. My mechanic can't figure out what is wrong...the car runs fine but the CEL keeps coming on with an O2 sensor (bank 2) code.

The coil pack on the Miata and Protege is actually 2 coils. When you get a failure, you'll usually end up with 2 cylinders not firing. It's pretty obvious when that happens, but in case you aren't noticing that the car is barely running, you'll get a nice blinking check engine light. That's usually a P0300 code- random multiple misfire.
It'll be back. There's a certain driving cycle that you need to follow in order to get the car to complete the diagnostics for the cat. If you have a real Mazda service manual for the car, it's explained somewhere in the diagnostic section of the book. I don't have the Miata book here to check where it is in there, though.
Chris

It's only related because the O2 sensor is what the car uses to determine whether the cat is doing anything. The bank 1 sensor is used to determine fuel mixture- it should cycle back and forth from a high to low voltage (the term high is relative- it's only like 0.5v or something) as the computer adjusts the mixture on either side of rich or lean. The bank 2 (the one downstream from the cat) sensor should report a much smaller variation in voltage but there should be some variation- if there is none, or if an abnormal voltage level is reported, the bank 2 sensor isn't working. If the voltage levels are virtually the same as what the bank 1 sensor reports, then the cat isn't working and the car should generate either a P0420 or P0421 code- I think which one depends whether your car has the "California" emissions equipment.
The factory manual has very clear troubleshooting trees for these problems and it's a very useful tool in these cases. What is the exact code you get?
You should also confirm your car isn't still covered by the federally-required emissions warranty. On my '99 Miata, it's like 8 years or 80k miles and it bought me a brand new cat after the rest of the car was out of warranty.

get one of the BEGI 02 signal simulators(think thats what its called) should fix the problem right away...3rd gen pro's with headers always seem to have this problem...even ones with an 02 sensor bung either that or deal with the CEL...your car is running fine

any pics of the shorty header?
Who makes it?

I got the header from Wagner Motorsports (looks like you know them from your signature). Sorry, no pics at this time.