Hi guys, I’m looking for a little advice here. I created this thread two years ago when my car didn’t pass smog based on high HC content.

http://forums.corvetteforum.com/c5-t...smog-test.html

I just smogged my car again earlier this month and here are the results from the past several years of smog tests.



So, of course I was happy that I passed, but there is still a high level of HC and now high NO content. So, next plan was to scan the car to check the fuel trims and O2 sensor operation. Below is a screenshot of my O2 sensor voltage at idle. I obtained this by utilizing the “Engine Link” iPhone App, using an ELM 327 wifi OBDII connector….kind of a cool product (however the graphing leaves a little to be desired since they don’t even have a scale on the graph). I have since ordered an Actron CP9185 scanner but it won’t arrive until next week.



From the data it appears that my primary O2 sensors are operating properly and cycling from about 0.2 to 0.9 V at idle. It appears that my Left downstream O2 sensor is cycling (from 0.45 to ~0.85) indicating the left catalytic converter may not be functioning properly. My right downstream O2 sensor is reading very constant at 0.45 V, indicating my right cat is operating properly.

Also, my Short Term Fuel Trims are very close to 0, and the Long Term Fuel Trims are on the positive side with a maximum of about 7-8%. From my research this doesn’t appear to be too out of whack, but it would explain why my car may be running a bit rich (hence the high HC content)?

I replaced the cats two years ago (4,000 miles). I have heard that these high flow cats don’t last very long, but 4000 miles? That doesn’t seem right.

So, is my logic correct here? I wanted to bounce this off of some of the experts here. I’m new at diving into these sorts of tuning issues, but I’m looking forward to learning more and its fun.

Thanks for any input.

 

Looks like the Bank 2, Sensor 2 O2 sensor is T/U. A steady 0.43V does not seem right...

 

Thanks for your reply jcgunn. For the downstream O2 sensors, shouldn't the voltage be fairly constant provided the cat is doing what it's supposed to do? That is, if the cat is not properly filtering the exhaust, the downstream sensor readings would match closer to the upstream readings....cycling rapidly. If the cat is working properly, the voltage readings from the downstream O2 sensors should be fairly constant. That's the way I understand it?

 

That steady O2 reading does not look right. But, the rear O2 sensors are for monitoring so the rear O2 not working doesn't change the emissions.

 

Another thing I noticed last night...I ran a scan from cold start-up until the engine was warm, and during open loop the Bank1 Sensor2 was oscillating from 0.45 to 0.85 as shown above. The other sensors were steady (as they should be during open loop). Then when it went to closed loop, the upstream sensors began oscillating. I also noticed that the Bank 2 Sensor2 was now stuck at 0.05 V. So, it appears that both downstream sensors are not behaving properly (but for different reasons). I know this won't affect my emissions but I would think the computer would set a code for either or both of these scenarios.

Is it possible that when my car was dyno tuned that the tuner turned off the rear O2 sensor input to the PCM?

 

There really is no fixed way the O2 sensors should work in open loop. Only when they are cold, then they will show around 0.5V.

Not the inputs, but the fault codes, the CEL and the SES warnings.