Rastoff

I did a search and it didn't yield any help for me.

I have a 1996 Firebird Formula LT1. The previous owner installed headers and a Borla cat back exhaust. He also disconnected the Post Converter(s) O2 sensor and installed a Sim(s).

Here's my issue: The SES light came on and the code was P0138 O2 Sensor High Voltage Bank 1 Sensor 2. If I clear the code and reset the SES, I can drive for about 35-60 miles before it comes back on. It doesn't seem to be dependent on RPM, acceleration or speed. Every time it's come on, I was traveling at a constant speed usually around 60 or 70 MPH in 6th gear.

Here's the questions:
1. If the Post Converter O2 sensor isn't functioning, can I damage my engine?
2. Is it better to run with an O2 sensor instead of the Sim?
3. What does the Sim do?
4. How many O2 sensors are there? 3 or 4? (I only see three.)

Any help y'all can provide would be great.

fidel trujillo

Well are the cats still there? No you will not harm your engine but you will get bad gas mileage becuase your car will not go into closed Loop meaning the car will not run it's "optimum". The reason the SES light comes on when you drive at steady speed is becuase the computer (PCM) runs the monitors and checks every system that has to do with emissions and driveablity and since O2 sensors have to do with air fuel ratio your car is running in limp mode. Did it always have the SES light when you got it? What the SIM is suppose to do is trick the computer it suppose to show the computer what in wants to see, a perfectly good working CAT. The O2 sensor before the cat indicates air fuel ratio and the sensor after the cat indicates CAT effeciency meaning, its checking to see if the CAT is burning the unburned fuel. I think there is suppose to be four one in front and behind the CATS. I hope I was some help to you. Sorry if my explanation was too long

Rastoff

Thanks for the help. It was quite useful and what I thought was happening. Yes, my CATs are still there. They have to be to pass emissions.

I will search for the other SIM tonight and try to reconnect the O2 sensor to see if it fixes it. I'm just glad I'm not screwing up my engine.

I have one more question and I hope I can express it clearly:
Once the SES light comes on, does that mean it will never go into "Closed Loop" mode? If I clear the code does that allow the engine to run properly until the SES light comes on again? Right now I'm getting about 18MPG and I don't think that's too bad.

I honestly believe this is an intermittent problem and isn't an issue except every now and then. Still, it's annoying.

fidel trujillo

Well your engine will run properly and once the engine warms up I think it will go into closed loop ( not too sure) I dont think you will have any driveability problems though. I dont understand if your cats are still there and work why not put normal O2 sensors in it? If you clear the code it will reset the monitors.Eventually the computer will run the monitors and check all the systems again and see that fualt in the O2 sensors. It will do that continously in every "Trip" or Drive Cycle" ( sorry smog tech language) Eventually it will wait for the car to warm up and wait for you to be at a crusing speed to run all those monitors and then the SES light will pop up again. I guess 18 MPG is not bad
at all. I had a friend in school and he has o2 simulators on his cammed Z-28 as well once his SES light came on and it turned out to be a loose o2 simulator or something like that. I think they have batteries them too. Any way I hope I was some help to you. Good luck.

Rastoff

You've been a great help.

Wednesday I jacked up the car and spent some time under there searching for the O2 sensors. I finally found the one I was missing. For the record this car is not easy to work on. Anyway, I disconnected the second sim and reconnected the O2 sensor.

I've been driving it for the last two days without the SES coming on. Maybe that was the fix. Only time will tell.

Again, thanks for your help. I now understand the system a lot better.