Flunked California Smog

[m610]

Hi. New here. Lots of questions since I've never before owned a remotely modern GM product until now and this one has a few issues.

Mine is a 1996 Tahoe, 4 door/4WD, which I purchased from a nice little old lady that just used it to go to the store to get food for the stray cats she fed (True). Very nice woman, actually, and smart, but way too much vehicle for her, so now I have it and she has a Honda station wagon.

I've cleaned this thing up and have recently started in on the hardware and of course, getting it registered and smogged. Today it flunked smog. CO was too high, NOx was marginal but passed. HC was OK.

I have an AutoTap scanner and took some readings. Everything looked OK except one of the secondary O2 sensors is reading low, 30-50 mV, when the manual says it should be reading 130 to 870 mV. My understanding is that a low voltage output means lean conditions, which should be what you want in the secondary sensor if the CAT is working. The first sensor cycles between around 125 and 850 mV, as expected. On the other bank both sensors cycle between around 125 and 850 mV. To me that says the CAT on that side is not working. Am I interpreting this correctly?

Other items checked:

• MAF: 0.87 lb/min/ (6.5 g/s), 5-7 g/s is normal
• MAP: 9.2 in Hg (31.1 kPa), 38 kPa is normal
• ECT: 194°F, 200°F is normal
• IAT: 91°F, but I had the hood up. It started to climb when I closed it. Normal is 107°F

I didn't spend the big bucks to get the Autotap add-on that gave me EGR values.

So am I interpreting this right? The bank with the low-reading O2 sensor is working, while the bank where the upstream and downstream O2 sensors follow each other.

Either way, it sounds like I need a new CAT.

Thanks,
Mike

 

[lesterl]

Sounds like one of your cats is bad or punched out..... Man dont California suck.... guess there are a few other states that smog check also..... I picked up a couple Walker universals off amazon.com for $50 each and I am back up and running (we dont smog, but they have to be there -punched out or not) and I bought my truck and they had been removed at some point..... Good luck.

 

[m610]

Thanks.

I'm OK with Cali Smog rules in general, but there are things they do that I think are plain rotten. That whole executive order thing (smog approved parts). If the tail pipe is clean, who cares what equipment you run with. Also, gearing and other nonsense.

I'll swap the downstream O2 sensors to see what happens. It the results are the same it is the CAT. If the 40 mV sensor gives the same reading on the other CAT, it's the sensor.

 

[lesterl]

That sounds like a plan, good luck getting the sensors out without tearing them up... might try a dab of antiseize on the threads when reinstalling them and heat the bung up pretty good with a torch before removing them....

 

[m610]

Good point on the difficulty of getting them out. They've been in there a while, plus the repair manual says I'm supposed to use some special kind of goop on the threads when I put them back in.