Has anyone else had these codes? How did you fix them? The cars has 100k so I'm scared to change the trans fluid, it's full but a bit dirty, what do y'all think?

Edit: the p0420 has been on and off for awhile before these new codes showed up. It should be known that the car is "driveable" but feels like it's stuck in third gear, I know it isn't stuck because I can go to second and it will shift very hard but won't accelerate any better.

 

I doubt changing fluid will help here as all 3 codes are electrical. The pcm codes aren't saying the solenoids aren't responding well, its saying it can't see that the solenoids are working at all.

Is the car actually shifting at all? I.e. you come to a stop, does it go into 1st, then up through the gears normally as you drive? Or is it stuck in a single gear when in drive?

You're going to have to drag out the wiring diagram and multimeter and figure out what's not getting the signal it should.

I'd start with the basics - is the transmission plug getting solid ground and ign 12v at the plug?

Then I'd move to the pcm output on that same cable - is the pcm switching the solenoid combination correctly to call for 1st gear when stopped (there are 2 solenoids, each can either be on or off giving 4 combinations total which calls for each of the 4 gears). This will check that the pcm is doing what its supposed to (as pcm failures are not uncommon) and that the external wiring is ok.

If the transmission is powered up correctly and the correct pcm signals are making it to the transmission, then you've narrowed it down to an internal transmission issue. Next step I'd take it ohm'ing out the two shift solenoids, but its pretty unlikely all 3 controls failed at the exact same time - if these all check out ok then the transmission may have mechanical damage.

 

I'm hoping I can start checking things today, but I can't tell if the car is shifting, I can change the selector to a different gear but it'll just clunk loudly and it won't accelerate any differently no matter how hard I'm pressing the gas either.

 

In any gear on the selector, it'll start off in 1st - so its not unusual that they all feel the same. The easy way to tell if its shifting is to go from a dead stop (in D or OD) up to say highway speed. Does the rpm climb, then drop (indicating a gear change) and continue to climb / drop / climb / drop as you accelerate?. Or is it either absurdly sluggish (stuck in a higher gear) or banging off the limiter at a low speed (stuck in a low gear)?

Doesn't really change the diagnostic process, just would be nice to confirm up front that the pcm codes match reality (that the pcm is not getting the response it should- ie a gear change after commanded).

 

Okay, I think I finally have some answers. I remembered that my code reader can tell my what gear the trans is in. In my driveway it said second gear (top right of first picture). In the second picture it shows third gear, I took this one after coasting down the hill back to my driveway. I should note that the car is prone to stalling when coming to a stop and will sometimes stall when coasting through a left hand corner at intersections.

Edit: it will normally show first when at a complete stop regardless of if it's in park, neutral, or drive. I will also note that it seems to accelerate like normal when in reverse.

 

The cat may be clogged. Doesn't explain the transmission codes though.

 

That seems to confirm that the solenoids aren't working properly then since it isn't going down into 1st. Next step is like I'd mentioned above - breaking out the multimeter and wire diagram and isolating the issue (and hoping its not the solenoids themselves or internal transmission damage preventing them from actuating).

 

Awesome, and as I believe I discovered yesterday, the hope is that the solenoids haven't failed because they're attached to the valve body right? It would mean removing the trans to fix the issue correct?

 

They're under the side cover. I know on some GMs people claim to be able to get the side cover off in-car, but Ive got no idea if thats possible with the monte. The bad thing is its not just unbolting the cover and sliding it straight up/ down, its got to be pulled away from the transmission first to clear all of the guts inside, so you need several inches to the side also.

Just seems very unlikely to me that both wouldve physically failed at the exact same time. Thats why I think checking electrical items in common first is worthwhile (power / ground / pcm output).


I will say if the electrical checks out 100% ok, I'd probably give fluid + filter a shot. I know you're hesitant given the mileage, but Id try about anything easy before pulling the whole transmission out (and if you went that far, I'd be very tempted to at least give it a basic refresh- rebuild).

 

This video describes almost exactly the symptoms I'm having (toward the end of the video) but he completely fixed this issue with a junk yard TCM. I'm going to pull the TCM and at least make sure it doesn't sound like a box of Legos, add some connector grease, and see if that changes anything.

EDIT:
I guess Impalas have different computers from the Monte, I'm just going to leave the battery disconnected, unplug/re-plug in the two computers in the airbox and see if anything changes.