New to the forum and I am appreciative for everyone’* help.

In Feb 2022 I bought a 2006 Lacrosse CX, V6 3.8L, 4T65E tranny, w/61,000 miles. A diag performed before purchase showed a “#1 Solenoid” code, but I continued to drive. Shortly thereafter after driving for ~30 minutes, I experienced progressively longer delays downshifting from 2nd to 1st until the car only started from a stop in 2nd. Had a similar issue on highways, stuck in 3rd and not advancing to D (overdrive).

Took to a local reputable transmission repair shop and they replaced the solenoids, shift sensors, replaced certain OEM tranny parts with upgraded parts, installed a TransGo shift kit and replaced with Dexron VI. I allowed for a reasonable relearning period after the repair but the problem continued.

I took back and they replaced the solenoids and sensors, replaced a transmission manifold sensor. The problem continued.

The next time I took back they deduced the problem to be electrical instead of mechanical and we replaced the PCM (programmed by dealer). Once installed, the scanner began throwing codes pointing to the throttle body, never saw these before. My 3.8 requires an electronic fuel injected throttle body with built-in TPS. Hard to find but located one in China. Once installed for the next 500 miles, short and long trips, no issues at all and the car drove beautifully. Then, the problem returned.

Small town where people know each other, I had the transmission guy compare notes with a trusted mechanic (in a language they understand). The mechanic threw out the possibility of a bad torque converter but the transmission guy dismissed. He gave an explanation why but I don’t recall it. Stumped and never having seen this before, the next step was to replace the valve body with a Sonnax which includes solenoids in the kit. He and I test drove yesterday after the installation and the problem is still there. Upwards of almost $3,000 to this point, suffice to say – I’m frustrated.

Doing searches on this website related to 4T65E, I saw these things:

• Torque converter (what the mechanic above mentioned)
• Torque converter clutch solenoid
• Pressure control solenoid
• Blocked tranny fluid lines (pretty sure we these were replaced or checked initially)
• Dexron 3 vs. Dexron 6

Any thoughts for help on next steps would be truly appreciated and thank you in advance for any and all replies.
Matt

Do the 'Trusted' mechanics have a good scan tool. Need to see what it'* doing with the solenoids.

We can't teach you the language by way of forum posts. If the subject matter were that simple, there wouldn't be transmission specialists.

I agree it'* likely not the torque converter. This issue appears to be a control issue, so solenoids and valve body.

Transmission guy should be able to test line pressures to figure out what is incorrect while the issue is occurring, then troubleshoot from there.

Also, the PCM should have codes stored for this severity of issue. What fault codes are stored in the PCM?

Thank you @carfixer007 and @CathedralCub for your responses.

I will report back on any codes from the PCM and line pressures. To the line pressures, researching from this site over the weekend, I found this thread from June 2016 4T65E help please. A Buick Rendezvous, earlier model year, but same transmission and the same problem dropping from 2nd to 1st when warm. A long, detailed thread from a gentleman who performed his own repairs, his final post indicates he performed an air test which "failed miserably", ended up locating a used tranny at a junk yard which he pulled and installed himself.

I'm still baffled as to why the car performed great for ~500 miles and especially on the long drives with the car warm, especially if it is a control issue and before we took measures to replace the valve body.

Anyway, thank you for the responses and I'll report back when I know more.

They should have performed air checks when they worked on it just to make sure all the piston seals and plumbing are good.
Your intermittence could be electrical which is good and bad due to a bad connection.

I suspect that the transmission started to fail, and then someone poured in a couple of cans of [name your favorite transmission-fix in a can]. This stuff got it through being sold and then a little, and now the original problems pop back up, along with complications added by having a bunch of over-expanded seals everywhere that transmission fluid touches. If this is actually what happened, a full overhaul is the only way to overcome the effects of this stuff.

So, after talking with the transmission people, a few things. First, no solenoid codes and they indicated they cleared the PCM code pointing to the throttle body - hence, I don't have a code number associated with that. The Sonnex as I understand came with all solenoids, including an EPC. So, if you count the two replacements and the Sonnax solenoids, three sets of solenoids.

Also, on the electrical, I asked if they checked the integrity of the wiring harnesses which they assured me they did.

As we talked, the one sensor they did not replace was a mass airflow sensor. We will take care of that tomorrow then test drive with the scanner attached.
One other item discussed but they felt was rare was the transmission housing opposite the valve body and possible cracks running in the crevasses in the housing. I shudder to think about that possibility.

I don't know what you do with this but sensors concern me. The mechanic I mentioned above said it is not uncommon for them to replace a new sensor with another new sensor given failure rates.

Anyway, again, thank you for all the feedback and suggestions thus far. I'll report back when I know more tomorrow.

Thanks CC. At this point, I'm feel I'm pretty close to a full overhaul. On the initial repair, they dropped the transmission, disassembled everything then built back from the ground up. I may be wrong (or hopeful) but I would like to believe that any damage that could have been done within the transmission by a "liquid fix" would have been identified and corrected in the reassembly.

As for the history, I bought the car from a neighbor of mine in her mid-70s who sadly lost her husband and she didn't need a 2nd car. They obtained the car a couple of years earlier from her sister-in-law, who as I understand purchased it used from a large local used dealer. But the car only had 61k miles in a 16-year period, leaving me to wonder if what I'm experiencing is a result of the car sitting for extended periods.

I've never heard of this, even with the supercharged Regal folks that put 400+ horsepower through them in drag racing . . . or the Lacrosse Supers that had 5.3 V-8'* . . . or all of those Impala police cars that had this transmission. I wouldn't worry about this unless you know of some severe and extended abuse that it has seen.

I'd wonder what manufacturer they are talking about here. Seems odds to me that this would be the case more than once in a very long while.

Probably not, unless it sat in a flood for extended periods or something. There is the remote chance that a defect found its way to the surface, but 2006 was pretty late in production for that version of 4T6x and they generally had the big bugs worked out.