GTI 1.8T cat efficiency but only one O2 reporting

[JHZR2]

Hello,

Im trying to help a friend tune up his 2002 GTI 1.8T. It is due a timing belt (never done), which I told him to get done right away. It has also never had coolant or MT fluid, though he religiously changes his engine oil with Mobil 1 0w-40 and the engine is clean inside from what I can see.

Anyway, other issues of maintenance aside, he had a CEL, so I checked it for him. It was actually two, one pertaining to the coolant system, which is a common sensor, and another which was regarding catalyst efficiency.

Thinking that catalyst efficiency metrics are tied to O2 sensor readings, I used the software to poll the O2 sensor voltages. I only got one... the Bank 1, location 2 gave me a constant 0.460V, and didnt fluctuate. The Bank 1, location 1, which is at least where the other O2 sensor is on my saab, didnt report, nor did any other banks/locations that I scanned.

My assumption is that likely (unless he has run rich for so long that he burned up his cat) the cat is OK, its just that one O2 sensor is completely dead, the other does not fluctuate, so the ECU wouldnt even know what the catalyst efficiency is.

The plan is to replace the coolant sensor (a $13 part, easy) and both O2 sensors. Id guess that this will solve the CELs and a new cat wont be necessary. Do you think this is likely?

He has 130k miles on the GTI, it runs good, has decent power, and even with a pcv pipe leak and original spark plugs (just changed them today, other than some rust on the threads, they looked good, granted they are Platinum), he is getting 27 MPG at nearly entirely 75 MPH interstate driving. Not that bad in reality...

Your thoughts???

Thanks!

JMH

 

[brianl703]

It's odd that it's giving a catalyst efficiency code when it knows there is something else wrong with the system. Most will not give you a catalyst efficiency code unless there are no other codes. You might try replacing the coolant sensor and checking the upstream oxygen sensor again.

 

[punisher]

Bank 1, location 2- I would leave that alone for now. No need throwing money at a good sensor. Good rule of thumb for secondary/cat monitors- the sensor is OK unless there is a specific code condemning it. A primary O2 fault can cause a false cat efficiency code, a secondary really can't.

Did you monitor the primary O2 while driving or just at idle? If the sensor is not sending voltage you can be sure the PCM will be launching a few codes for it. If you can't read the sensor, and there are no codes for it, I would suggest there might be a problem with your scantool/software.

Have fun.

 

[JHZR2]

Yes, it seemed odd. In the saab, at idle, I was getting voltage for two, both bank 1, postions 1 and 2.

Then in the GTI, nothing on any bank except for the lazy sensor.

Thanks for the insight!

JMH

 

[punisher]

The second/cat monitor sensor is supposed to be lazy. In fact that is what the PCM is looking for. The problem comes when the second sensor mimics the readings of the primary O2. That is the giveaway that tells the cat is shot.

 

[huffmuds9320]

Yes the new coolant sensor has the green color top, most likely the car has the the black one. Very easy to replace and I would replace that first.

You can use the Vag-Com to check the oxygen sensors. It would give you a value, if the value is not withing the specified range in the service manual then probably needs replacing.

 

[crw]

As others have said, replace the coolant temp sensor first. It's cheap and can be done in minutes. Then I'd reset and take it from there. And don't delay any longer getting that timing belt done!

 

[crw]

Oh by the way, I believe there is a recall on the CTS. You might be able to get it done for free at a dealer.