GPHusker

I bought a 2000 Trans Am a few weeks ago and it had been worked on by a pretty shoddy mechanic. I have been going through fixing the messed up stuff that was done to it. The previous owner had Texas Speed shorty headers installed with a Corsa catback. It previously had cats and both sets of O2 sensors installed from what I can gather. The car is running in open loop only and is running pretty rich. I hooked up HP Tuners to it just to look at some things and it runs better when you manually command closed loop. So I started doing some digging around and it brought me to the O2 sensors. I am getting fluctuating readings from both sets of O2 sensors even though it looks like only one pair is installed. I'm confused on how that can be. Before I get crazy tearing things apart I was hoping for some advice on what to do next from people with way more experience than me on this.

GPHusker

So upon further inspection there is a rectangular box that is about a half inch wide by 2 1/2 inches long that has a pigtail that looks to be plugged into the secondary O2 connector. There is one on each one of them.

Edit: It looks like those are O2 sensor simulators of some sort. I unplugged both of them and the voltage goes to .445 on each one indicating it is unplugged.

SouthernRex

I'm no expert, but I know that the rear O2's don't have any actual effect on engine performance, they just exist to throw a light if there's a problem with the cat. Given the shoddy mechanical work you've seen so far, I should mention that the exhaust shop that installed my long tube headers put the rear O2's in place of the front O2's, and I couldn't figure out why my car was running like garbage and stuck in open loop. When I picked up my car they said they "didn't need" the extensions I provided, and I was like "Huh. Okay." Well once I figured out why, I made them fix it for free and use the extensions I had originally bought on the ACTUAL front O2's. (It was either that or I had to get the Corvette O2s to reach.)

The simulators are for passing inspection right? But I wonder if they mixed up the 4 sensors like my shop did the first time.

By the way, it's also possible your front O2's are just .. bad. But I don't want to suggest just throwing money at it, someone else with more experience and knowledge might be able to help better. You're not getting any O2 low voltage codes from the front ones (Bank 1 x Sensor 1, and Bank 2 x Sensor1 I believe) are you? My long tube headers destroyed my factory front O2 sensors pretty fast, and I had to replace them.

GPHusker

Thanks for the response! I did check what you were talking about with swapping the O2 wiring. It is definitely hooked up correctly. The O2's were supposedly new. So this brought me to last night. I was able to hook up HPTuners to it and take a look at the tune. It looks like all the fuel trims are disabled. Closed loop is set to be activated at 285 degrees. I manually turned on closed loop and the O2's seemed to be working correctly. The secondary O2's warnings were all turned off already. From comparing the tune on the car to another stock tune I found on HP tuners everything looks similar except some extra timing and fuel and fuel trims disabled. I think I may bite the bullet and buy some credits and enable the long term trims just to be on the safe side.

RevGTO

iTrader: (12)

You can run full-time open loop if the car is properly tuned for it; I've done it for years at a time. You don't need O2's at all in that case. MAF, VE, and AFR tables have all to be in the ball park. If so, you shouldn't see a decrease in fuel economy. Since there is no feedback, fuel trimming is turned off as you found.