OBD computer

First post, please be kind. :p
I'm rolling a 99 Century with 120K miles. I moved to GA where they require emissions tests. I'm having a problem passing it. About 5 years ago (I'll give everyone a minute to do the math), the SES light came on and has been on since. I took it in about 6-7 times :banghead: and the dealership told me different things each time, but the most common story was that I was using cheap gas and the injectors are dirty. So, after the warranty was up, I took it to the "family mechanic" who really seems honest. He replaced a couple of O2 sensors over and over again. Eventually, he sort of reset everything and manually fired the heating element in the sensor that the computer said wasn't working. If this doesn' make much sense, I apologize. I'm trying to describe what he told me ... and keep in mind that this short story took about 9 months to develop. Anyway, in MI, we aren't required to pass an emissions test, so the mechanic and I agreed not to worry about anything. He said the computer isn't working properly and that it would cost $3,000 to replace it.

Now that I'm here, I have learned that OBD computer comes from the manufacturer with an 8-year warranty. Has anyone had anything like happen to them? Any suggestions as to what to do next? Any experience replacing the OBD computer under warranty?

First post, please be kind. :p
I'm rolling a 99 Century with 120K miles. I moved to GA where they require emissions tests. I'm having a problem passing it. About 5 years ago (I'll give everyone a minute to do the math), the SES light came on and has been on since. I took it in about 6-7 times :banghead: and the dealership told me different things each time, but the most common story was that I was using cheap gas and the injectors are dirty. So, after the warranty was up, I took it to the "family mechanic" who really seems honest. He replaced a couple of O2 sensors over and over again. Eventually, he sort of reset everything and manually fired the heating element in the sensor that the computer said wasn't working. If this doesn' make much sense, I apologize. I'm trying to describe what he told me ... and keep in mind that this short story took about 9 months to develop. Anyway, in MI, we aren't required to pass an emissions test, so the mechanic and I agreed not to worry about anything. He said the computer isn't working properly and that it would cost $3,000 to replace it.

Now that I'm here, I have learned that OBD computer comes from the manufacturer with an 8-year warranty. Has anyone had anything like happen to them? Any suggestions as to what to do next? Any experience replacing the OBD computer under warranty?


Do you have an extended warranty? GM's basic emissions warranty on the CAT convertor and powertrain control module is 8 years or 80,000 miles, which ever comes first. Therefore with 120K miles it is out of warranty.

You might want to take it to Autozone, etc. and have it scanned for free or invest in your own ODB-II scanner. Be sure to right down the specific generic and manufacturer specific codes and go from there.

BTW - $3,000 for an ECU/ECM/PCM computer to include labor is outrageous.

With 120K miles and failing emissions I'd seriously look at the CAT convertor if it has never been replaced before. Good luck!

:iagree: Autozone will scan the computer for free if the CEL is on. It sounds to me like you need a cat replacement along with a couple of new O2 sensors. If the car is still running and performing well the computer is probably OK but under no circumstances should it cost $3000 to replace one even if it is totally destroyed. You can always purchase a used one from a wrecking yard for less than $200.

If your O2 sensors were not working properly you probably destroyed the cat by putting too much gasoline into the convertor or it is just worn out. Cost to replace varies but you should be able to replace it for $400 including labor. Once that is done and the computer is reset the car should pass emissions with no problems.

:iagree: Autozone will scan the computer for free if the CEL is on. It sounds to me like you need a cat replacement along with a couple of new O2 sensors. If the car is still running and performing well the computer is probably OK but under no circumstances should it cost $3000 to replace one even if it is totally destroyed. You can always purchase a used one from a wrecking yard for less than $200.

If your O2 sensors were not working properly you probably destroyed the cat by putting too much gasoline into the convertor or it is just worn out. Cost to replace varies but you should be able to replace it for $400 including labor. Once that is done and the computer is reset the car should pass emissions with no problems.


You're absolutely correct GTP Dad. Our local Autozone will only scan it if there is a MIL indicator on (SES or CEL). Thats why I broke down and invested in my own scanner. My Actron has paid for itself many times over. Experienced firsthand the old naggy P0420 DTC, went through both 02 sensors and eventually replaced the CAT convertor which cured the error code.

...Experienced firsthand the old naggy P0420 DTC, went through both 02 sensors and eventually replaced the CAT convertor which cured the error code.

Thank you all for great information. I think I'll hang onto the car for another few months while I (we :biggrin: ) figure this out.
I'm all about AutoZone. They have pulled the codes for me before. I guess I've been hanging on the advice of my In-Laws' mechanic too long. So, I'll start over with the information I've gotten here ... good!
Now, the converter can't be too bad, I'm thinking. If it were, wouldn't the car be running poorly? I have some ... some automotive experience (during college) with a national chain muffler shop.

So, please tell me more about the P0420 DTC.

The "other" guy said he was getting an error on one O2 sensor, but he could override it when he was "in the computer." He concluded that there wasn't actually anything wrong with the sensors, so it must be the computer. I am not real sure how/what he meant by "in the computer." Is it possible to sort of "hack" the OBD computer?

OK, it's too late to still be at work. I'll tell my wife that I need the garage this weekend and to leave me alone. Think it'll work ... I mean has that worked with anyone else's wife?

Thanks again!

Thank you all for great information. I think I'll hang onto the car for another few months while I (we :biggrin: ) figure this out.
I'm all about AutoZone. They have pulled the codes for me before. I guess I've been hanging on the advice of my In-Laws' mechanic too long. So, I'll start over with the information I've gotten here ... good!
Now, the converter can't be too bad, I'm thinking. If it were, wouldn't the car be running poorly? I have some ... some automotive experience (during college) with a national chain muffler shop.

So, please tell me more about the P0420 DTC.

The "other" guy said he was getting an error on one O2 sensor, but he could override it when he was "in the computer." He concluded that there wasn't actually anything wrong with the sensors, so it must be the computer. I am not real sure how/what he meant by "in the computer." Is it possible to sort of "hack" the OBD computer?

OK, it's too late to still be at work. I'll tell my wife that I need the garage this weekend and to leave me alone. Think it'll work ... I mean has that worked with anyone else's wife?

Thanks again!

With 120K miles, the CAT convertor would be at the top of my list especially with the P0420 DTC before suspecting the ECU/PCM. It is probably partially clogged or damaged and operating below efficiency. The rear 02 sensor after the CAT would generate that specific error.

Have you had an emmissions test done?

With 120K miles, the CAT convertor would be at the top of my list especially with the P0420 DTC before suspecting the ECU/PCM. It is probably partially clogged or damaged and operating below efficiency. The rear 02 sensor after the CAT would generate that specific error.

Have you had an emmissions test done?

Not really. I need to pass the test in order to register the car here. The test that I failed to pass involved simply plugging in the computer and it automatically fails because the SES light is on. So, no, the car hasn't been scoped ... and yet I have technically failed an emmissions test.

Not really. I need to pass the test in order to register the car here. The test that I failed to pass involved simply plugging in the computer and it automatically fails because the SES light is on. So, no, the car hasn't been scoped ... and yet I have technically failed an emmissions test.

Yeah...same thing in Texas. SES light on is an automatic failure. The P0420 code problem needs to be fixed because not only is it triggering the MIL (SES) but inhibiting the ECU/PCM from completing its emissions testing cycle which it does automatically during the drive cycles.

I have a handheld ODB-II scanner and its show the emissions testing cycle. I run this test myself first before taking the car in for safety/emissions when it is due annually. That way I have a high confidence level of passing.

Yeah...same thing in Texas. SES light on is an automatic failure. The P0420 code problem needs to be fixed because not only is it triggering the MIL (SES) but inhibiting the ECU/PCM from completing its emissions testing cycle which it does automatically during the drive cycles.

I have a handheld ODB-II scanner and its show the emissions testing cycle. I run this test myself first before taking the car in for safety/emissions when it is due annually. That way I have a high confidence level of passing.
I'm thinking about getting a scanner, too. I have a Dell handheld Pocket PC thing. I've been reading about ways to hook a laptop up to the OBD computer with the right cable and software. They say that I can do the same with the handheld (PDA thing). Besides learning if your car is running properly, what else do you get from having your own scanner?

I'm thinking about getting a scanner, too. I have a Dell handheld Pocket PC thing. I've been reading about ways to hook a laptop up to the OBD computer with the right cable and software. They say that I can do the same with the handheld (PDA thing). Besides learning if your car is running properly, what else do you get from having your own scanner?

Piece of mind..lol. Seriously, convenience and the ability to check your car before emissions testing and the ability to clear the DTC codes and get rid of (reset) the SES light. My Actron CP 9145 is a scanner not just a code reader and will readout manufacturer P1XXX codes in addition to generic P0XXX. In my case it really comes in handy because I have 3 GM "W" bodies....two have over 90K miles. It will pay for itself in the long run.

Checkout www.actron.com

Piece of mind..lol.

I know you're right. I think I'll get into something like that. I can get the software for free (or next to nothing) and the cable should be about $20. So, I think that's the way I'll go.

Meanwhile, do I change the O2 sensors and converter? Should I get the scanner first and see what exactly my car is telling me? I'm not sure I want to spend the money for a new converter at this point. I'm also having some engine "concerns" while this debate continues. I think it's a problem with the oil level. The dealership says I have a gasket that is failing. I'm not sure what to do, but I'll start a new thread for that one ... when I'm ready to learn that my car dead. So, you see, I have a couple of issues that I need to address. Any one on its own wouldn't be a show-stopper. But in total, they might be too much for this car. I just don't know. :confused: