2003 Buick Century 3.1L V6, 116K miles

I bought the car from second owner (who purchased around 18K miles) for a good price knowing that it had code P0742 - TCC System Stuck On. This is the only DTC showing from the scan. Car was garage kept and is cosmetically very good.

Engine oil and transmission fluid are in pristine condition, but prior owner (an elderly ex-military man) insisted, over the objections of his daughter, on tossing the service records when he decided to sell it. However, she told me that a transmission flush and filter change had been done within the past year and car had not been driven much since then. I suspect it was done to see if it would clear up the DTC P0742.

Car runs and shifts well when cold. After fully warming up, it shifts hard from third to overdrive.

Is there a diagnostic procedure available online for DTC P0742? I found an old thread that referred to one in the third post here: https://www.gmforum.com/performance-...-p0742-295808/
However, the actual diagnostic procedure does not appear in the thread (or I'm just blind).

Some tests that can be performed with a multimeter are presented in this 6-1/2 minute video:
I have also read in some posts that Seafoam Trans Tune sometimes frees stuck solenoids.

I have studied the procedure to remove the side cover to r&r the TCC PWM solenoid, and I have the tools and am prepared to do it. However, before doing so, I wanted to rule out wiring harness issues and faulty PCM. I had also read a thread here in which someone with this code repaired it by replacing a coolant temperature sensor. Not sure how a coolant temp sensor would affect the TCC PWM solenoid. If anyone knows, please enlighten me.

I just want to ensure that I do not leave out any tests that would reveal a failure of anything outside the transmission before I embark of the removal of the side cover.

Moved thread from "GeneralGM Chat" for visibility

Try this:

1. After it warms up, bring the car to a complete stop.
2. DON'T take it out of drive
3. Select 3
4. Accellerate normally up to about 45MPH
5. Keep accelerating normally while selecting Drive

Did it shift hard just then?

This kind of "fix" scares me. Having said that, sometimes it works. I wouldn't try this (yet) for your issue.

Is there a frame rail in the way of removing the side cover? If so, have you prepared for having to remove the transmission from the vehicle to be able to get the side cover off? I just want to catch a surprise for you before it surprises you in the driveway. The valve body in the side pan fills it mostly.

I believe it chooses to NOT use the TCC until the engine coolant has achieved a certain temperature. That said, if your temperature gauge (or better still an OBD2 reader) shows expected temperatures then this is probably not the fix for your car.

Thank you for your help, it is very much appreciated.

The first time I tried your suggested test, I discovered that the coolant level was much too low. So coolant temps were not as expected at first. After adding coolant, bleeding off the air, and checking for leaks, I ran your suggested test. Following are my findings.

1. After warming up, the car shifted with a bump when shifting up in all gears. However, the shifts did not seem as hard as when I test drove the car.

2. When accelerating to 45 mph in 3rd and shifting from 3rd to Overdrive while continuing to accelerate, there was no immediate bump. A few seconds after I made the shift, there would be a slight bump.

Other info:
I did not see a frame rail blocking access to the side cover of the transmission. The 2003 Century seems to be made like the LeSabre. The engine and transmission rest in a subframe that is bolted to the body. It appears that raising the car and lowering the subframe, along with the removal of the other parts on the driver side (shift cable, rack and pinion, drive shaft, tie rod end, strut assembly) will permit access to the side cover. If it will not, I have an engine hoist.

Sorry, I had assumed you had gone through all of the fluids. Whatever else you may not have checked should be checked. Especially the transmission. Check it warmed up and running. after cycling through all transmission range selections for a few seconds each.

This is the hard part of diagnosing across the Internet. By "bump" do you mean normal shifting like pre-torque-management cars did? I'll assume for now that this is the case. If I'm right then it sounds like it is doing as it is supposed to.

Has the transmission ever been serviced? Like the drop pan, change filter, reinstall, refill kind of service? If not, do that first.

Okay, this is good to hear. Another difficult part of diagnosing across the Internet is that it is difficult to know what skill level someone is. It seems you have that solved.

Thank you again for your reply. I sincerely appreciate your help.

I am not sure of what you mean by pre-torque-management, but the shifting does not feel normal to me. I have a 4L80e in my 1994 C2500 pickup and the shifts in my truck are smooth. When this car shifts, it is like going over a small speed bump. You can feel the impact through the steering wheel. It is not a smooth shift but not quite a shudder either.

I believe my skills are up to any necessary repairs. I have done everything that can be done on a modern automobile except rebuild an automatic transmission, and I am not opposed to taking that on.

I will check the fluid levels again when warm as you suggested, then drop the pan and r&r the filter and gasket and refill with fresh ATF. It may be that the flush the prior owner had performed was done without changing the filter.