[JonGSA]

Does anyone know if the ECU does anything different when the car is in 6th gear (6mt).

Here's my story....

Ok....long time no posting on the subject, but I made a stupid weird discovery the other day. This "could" be considered a cat back issue, but the cat back is fine. No leaks, tight clamps, etc. ...I checked it many a times already.

So I was plodding along the other day, on a stretch of road. Doing 50+ or so. Then up ahead I saw some local LEO's sitting in a parking lot. Having the SW Cat-Back on, in sport mode...and in 4th gear. I decided to lower the RPMs a bit to bring the sound down a bit. So I put it in 5th. Drove by no problem. So a minute later down the road, I thought....well, how quiet does it get in 6th at 55 mph. It didn't lug then engine at all. I got up to 55 before putting it into 6th. I noticed it was very quiet.......and also i noticed a CEL pop up after having it in 6th for about a minute.

So i got to where i was going.....read the code P0420. Cleared it as usual as I get this same code every so often. My latest theory was I only get this code, when im on the highway. But now it seems like when I put it into 6th, the ECU changes something and causes the code, every single time. I tried it a few times already, and it does it every single time.

I can drive the car all day or short trips, easy to hard, fast or slow, highway or traffic. But as long as I just use 1st to 5th, it's fine. Instantly when I use 6th gear.....CEL light. Im...

[Michigan1LTRS]

Take it in. You have bad O2 sensor, cat, and or exhaust leak.

[travislambert]

I had that code briefly after I swapped engines. The exhaust was just a little loose in the joint behind the passenger side converter (right before the second O2 sensor). Given that you have a different exhaust on your car, I'd double and triple check the bolts and gaskets first.

Another possible cause could be a loose or faulty O2 or possibly a loose harness connection.

I'm not aware of anything special for 6th gear other than you are likely hitting different cells in the airflow, timing tables, etc..

This might be a stretch, but did you get the car tuned after the cold air intake install? Cold air intakes often cause the MAF sensor to read incorrectly. The computer will eventually correct below 4000 RPM, but above that it's relying solely on the MAF. I suppose over time this could cause an issue with an O2 sensor.

[JonGSA]

Yeah, I'm close to bringing it in.

Thats what im thinking too, 6th gear changes map tables, rpms, airflow volume, timing...etc.

Ill have to go over the connections on the left side one more time i guess. After this time...it's going in.

I never got the tune, but on the flip side I haven't had the CAI in the car in months. After reading up on all the tests and how its kinda useless without a tune, I took it out and been running the stock intake since october. Actually gonna put up the CAI CAI for sale soon. I'd love to tune it, but the car is a lease...so.... God if i had the balls to mod the car while being a lease and all....it'd have headers and a cam by now.

[travislambert]

Originally Posted by JonGSA

Ill have to go over the connections on the left side one more time i guess. After this time...it's going in.

My problem was right here:

Originally Posted by JonGSA

I never got the tune, but on the flip side I haven't had the CAI in the car in months. After reading up on all the tests and how its kinda useless without a tune, I took it out and been running stock since october. Actually gonna put up the CAI CAI for sale soon.

Good plan. I am not a fan of CAIs unless you are making some serious horsepower AND you have tuned for it. I'm making 610 wheel horsepower (SAE) with the stock intake (at that HP it's within its limits, but just getting close to becoming an airflow restriction).

[Chevrolet Customer Svc]

Hi JonGSA,

We apologize we weren’t able to get to you sooner. Please know that we are only a private message away if you’d like to further discuss your concern. Our team would be more than happy to assist any way that we can.

Bret B.
GM Customer Care