2008 Pontiac G6 automatic transmission

The car is already at a garage, but the mechanic is apparently fairly booked, and I wonder if anybody wants to guess at how bad this is?

The automatic transmission on my friend’s 2008 Pontiac G6 GT Convertible went out on him at work about a week ago. Transmission fluid is full…actually too full. I managed to drive it to my home using the Sportronic feature (push forward for the next gear, pull back to downshift). I’m his occasional car guy, but know nothing about automatic transmissions. It didn’t drive perfectly. First gear didn’t seem to do anything, and shifting from 1st to 2nd didn’t seem to work reliably. I would be at a stoplight, 1st wouldn’t respond, then I’d shift to 2nd and try again with still no motion, but glance at the LED gear selection on the dash and realize it’s still in 1st. A second try to 2nd got the car moving.

It sits at my house for roughly 36 hours and I want to take it for a second test drive to solidify my thinking. My code reader doesn’t find anything but a P0420, which is unrelated, but a local garage offers to check it himself for free, so that’s one stop. He doesn’t want the actual transmission job. To my surprise the Sportronic works perfectly now, even in 1st. All gears and all shifting work as expected, and I’m enjoying the drive. Automatic still doesn’t work. The garage can’t find any additional codes, so the problem appears to be purely mechanical. On the drive home, after no more than 30 minutes of running time, the transmission goes out completely. Only reverse still works. I use reverse to move it out of the way, but Sportronic is dead. I can shift to 2nd and see this on the dash, but the car won’t move. It downshifts itself back to 1st over time (normal) and I can see this on the dash and hear it, but still no motion. I walk home and we get a tow truck that evening. After sitting for a few hours the transmission is still dead.

My best guess is the transmission shifts normally, but doesn’t actually “catch” to put the car in motion. Not sure about the Sportronic shifting issue on the first drive.

Might anybody want to guess what this is, along with the co$t? Thanks.

The radiator leaked into the transmission cooler and filled the trans with coolant. Rebuilt trans, new radiator, fresh coolant.

Parts… $2200, Labor… maybe another $1500. This is a rough guess. A 16 year old car in ??? condition with ??? miles… might not be worth the repair cost.

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First guess would be the clutches are shot or the converter. At any rate needs to go to a transmission shop. Either way if the converter goes, it can send debris through the transmission so has to come apart anyway. When I had the 30,000 mile fluid change done on my G6, the shop said an overhaul is about $3-4000 but not needed for me. A shop will pull the pan, take a look, check pressures, and there you go. I’ve never been afraid to have them overhauled if you use an ATRA member shop. I’d have to try and count the number I’ve had overhauled over the years. Not the end of the world.

One winter I was parked in the ramp, fine in the morning, but at night had only reverse. Backed it out of the ramp to where it could be towed to the dealer. Can happen fast at 180,000.

With transmission “too full” I’ll go with @Mustangman 's guess. What does the fluid look like? It should be red and largely translucent. If it looks more like strawberry milkshake, then the trans is doomed.


What color is the coolant?

If the tranny cooler leaked, the coolant would be contaminated with transmission fluid.


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My guess, the owner put more trans fluid in hoping that would solve some symptoms. Such a thing might work, for a while anyway. At this point however I’m thinking the trans seals have hardened and leaking (internally) under pressure. Faulty shifting is the result, as high hydraulic pressure is needed for the shifting function. You could try a hail-Mary, replace the trans fluid a couple of times. Filter too, if applicable. It might come back to life. Fresh fluid will have fresh seal-conditioning chemicals. Automatic transmissions have a lifespan, and no matter what you do they will eventually fail. But they can last a very long time if driven gently & serviced frequently. The best way is to replace the trans fluid with fresh stuff, more frequently than the owner’s manual specifies.

Re-read the post but didn’t see anything related to the cooler leaking. Just looked over filled which might be normal since it has to be hot and in park after engaging all the gears. Plus does that have a dio stick in 08? Forward clutches could be shot but still have reverse clutch which might indicate the converter still works. Still don’t overhaul without putting a new converter in.

How did it GET overfilled? No mention. My guess was coolant from radiator. Testers was trans fluid to the coolant… either way bad things are happening.

What engine is in the vehicle, I’m show 2 different 4 speed autos (4T45E & 4T65E) and one 6 speed auto (6T70), they are all fully electronically controlled and should throw a code anytime the it is not working right, may just need a better scanner…

Cooler bursting in the rad on the Cobalt with the 4T45E is the reason why I am dealing with it’s crap now, or part of it anyway…

The 4T45E trans does not have a dip stick, but the other 2 transmissions show to have one, the 4T65E should have a dipstick tube and stick, the 6T70 should have a cap with dipstick attached to the cap and no tube… It would not be hard to add ATF in any of them but the 4T45E is much harder to check (remove plug in case)…

But coolant making it’s way into the trans is probably how it was over filled, if it really was…

It can be saved, but only if it is not driven and the fluid has to be changed multiple times or flushed out one… but driving it around trying to figure it out will kill it… lol

Hey, thanks for all the replies!

Some more details. The automatic transmission fluid seemed okay to me. I wondered if being over-full was a clue, but would have mentioned if it looked off. It’s too late to check the coolant reservoir, as the car is now at the garage about 17 miles away, but that was a good idea which I’ll file away for the future. I did check the fluid when the car was cold, sitting for roughly 24 hours.

The owner didn’t know where the dipstick was even located, so he didn’t add any fluid. He had a bit of work done on it within the last two years (emissions control, suspension, probably other things) and my best guess is that garage topped it off as part of some “full service” job.

Incidentally, the comment about the seals going bad took me back to my first car, a used 1979 Mustang. In the last year before I got rid of it, I was changing the transmission fluid (and filter and gasket) every few months to keep the transmission working normally. Never realized the root cause.

The car has only 93K miles and is in reasonable shape otherwise. Most of the issues are cosmetic now, the worst being that the TPMS/keyless entry system is dead. Will fix that myself if it survives. The engine is a 3.9 liter, which may be the weird variant. I discussed the car with a former mechanic friend a few years ago and at first he insisted that I must be wrong, and then suggested the engine came from Canada.

I don’t know if the garage where it got towed has any professional certifications, but the mechanic who checked for codes said that he wouldn’t trust the repair to anyone local. It’s primarily a transmission shop. I was looking over the mechanic’s shoulder and he did check the transmission module specifically, which mine doesn’t do. His reader was a tablet-style and looked professional enough, although he complained about it being slow. Not an Autel…a 3 or 4-letter acronym beginning with “M”.

Will re-read all the comments again when I have time. I still think it’s weird that the Sportronic went from working acceptably on drive #1 to working 100% at the start of drive #2 to completely dying within an hour. But per a comment, apparently that can happen. :frowning:

Was the engine running?? If not then you did not check the fluid level correctly and it will show over full…

So it should have the 4T65E transmission with an old school dipstick tube and dipstick, very easy to check…

If this shop doesn’t have the correct and updated equipment to even pull the codes on a 2008 transmission, you need to find a much more qualified shop to do the repairs… I just had to buy a jumper harness to do a basic check on a very common and simple 4T45E transmission, the trans is acting all kinds of stupid and it is the TCM not the transmission, but the code is showing it to have a bad internal part… The scanner also has to be able to read live data on the transmission for proper diagnostics…

This is one of those cases where the owner needs to have a real shop look at the vehicle. Todd , it seems you are not able to solve this and the current shop does not do transmissions . All you should do is possibly help the person choose a shop and then back off.

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From what I can tell, the 08 Pontiac G6 had these engine options

  • L4, 2.4L
  • v6, 3.5 L
  • v6, 3.6L
  • v6, 3.9L

So nothing weird about your engine. You just have the souped-up version! Suggest to avoid drag racing though … lol …

Ah, no it wasn’t. So ultimately a simple answer and sorry for the red herring. I drive a stick shift myself and haven’t interacted with automatics in quite a few years.

Yes, that’s it. And now I see it’s a 4-speed. I never got fast enough to try shifting beyond 4th and also didn’t want to stress it.

Maybe a misunderstanding? The garage that checked the codes for me isn’t the garage doing the work. They passed and recommended a transmission shop about 17 miles away. He was trying to be nice and didn’t do more than the basics.

So while there isn’t a consensus, I am glad to hear that it likely isn’t a worst case scenario. And kudos for the comments on what sorts of work to expect. Will tag the closest guess once the garage has a chance to look at it and gives an estimate.

Well I guess that stands to reason. You can’t go further than 4th on a 4 speed. :rofl: :rofl: :rofl:

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this is deleted

and what would this code be?