996.1 C4, Cabriolet, Tiptronic - P0740 Code

[Andrew Pudimott]

Hi,

 

New to the forum, but not new to classic cars, DIY mechanic projects, etc.  I’ll give a brief history of the car I bought which is throwing a P0740 code.  I have read all of the previous discussions on this topic both here and on Rennlist, but I still don’t see a good diagnostic path or solution.

 

Below occurred prior to my ownership and all work done by a Porsche specialist with 30+ years experience:

 

1. Oct 2021 - 76,386 miles - new long block installed with L&N IMS bearing.  All useable parts from old motor were re-used on new motor

2. Sep 2022 - 79,197 miles - used DME installed

3. Aug 2024 - 81,649 miles - used transmission installed.  Used trans was pulled from another car with 52k miles.  9 liters of Pentosin ATF, new gasket/filter/seal/bolt installed

 

I purchased car in Jan 2026 with 83,013 miles and drove it home 400 miles.  Car now has 83,8xx miles.  

 

Feb 2026 - drove to DMV and while at DMV, had to shut off car so employee could connect scanner.  The car would not turn back on and it was pushed out of the bay.  I had car towed home, and after reading multiple posts, I diagnosed the issue as a faulty starter relay which was replaced.

 

I then hooked my cheap OBD II scanner up to the car to check for any other faults and noticed a P0740 code.  I reset the code, drove to DMV, passed inspection and drove home.  The code returned.  

 

I took the car to a local Porsche specialist who topped off the trans with a “small amount” of fluid.  This was done for free because they owed me a favor.  

 

Now the car is home, and I would like to solve the P0740 issue.  I have read all of the tech posts and noticed a few paths including low/dirty trans fluid, Pressure Regulator # 4 and/or Torque Convertor.

 

Also noted, no CEL, a hard up-shift at times between 3rd-4th when trans is cold and local Porsche specialist “won’t touch Tiptronics.”  So I’m on my own for now, and I live in Delaware.  

 

If you have hung in this long, I appreciate you.  Seeing as though the trans fluid/filter was replaced 2,500 miles ago, and just topped off within the past few days, I ruled out low/dirty trans fluid.  Remember, the used trans installed in this vehicle currently has 55k miles on in.

 

My question is, how do I diagnose if the issue is with the torque converter (which was not replaced when the new motor was dropped in) or, if the issue is Pressure Regulator #4?

 

Please only post constructive comments.  Yes, I can drive a manual trans, but bought an auto trans so the spouse could drive the car as well.  Long story behind that lol.  Thanks so much for your time and comments!

Edited Friday at 02:30 PM by Andrew Pudimott

[JFP in PA]

Welcome to RennTech 

 

Tiptronic problems are never fun, and often complex and difficult to diagnose and repair, which is why many shops shy away from working on Tips.  Potential issues causing P0740:

 

-ATF supply (as you have fresh fluid and assuming it has been topped up correctly, this would only apply if the filter were the problem and not moving sufficient fluid)

-The torque converter itself is bad.  To properly diagnose this, you would need some very specialized and very expensive tooling.

-Pressure regulator 4 is problematic

-The electrical triggering connection to pressure regulator 4 is bad (shorted, disconnected)

-Hydraulic control unit (read valve body has sticking valves)

[Andrew Pudimott]

Hi,

 

Thanks for the input.  Looks like you may be from PA, do you have any transmission shops you would recommend?  The Regulator looks easy enough to swap out, but my guess is a run of the mill shop would just recommend swapping out the whole valve body.  I want someone whom I can have an honest conversation with.  

 

If they’re going to be in the transmission, they might as well swap out the #4 Pressure regulator and the torque converter plus check all wiring connections in the valve body.  The kit on FCP Euro includes all of the solenoids/regulators so I would probably just have them swap everything out (or do it myself).  My guess is a transmission shop would be able to diagnose all of the problems you mentioned and discern if it’s the regulator or the torque converter.  That said, there are no drivability issues right now, and the car is not a trailer queen so I’d be inclined to drive a reasonable distance to a specialty transmission shop.  Any suggestions?

Edited Friday at 05:58 PM by Andrew Pudimott

[JFP in PA]

 I do not have a useful recommendation but would suggest contacting your local chapter of the Porsche Club of America as they typically have background on known Tiptronic repair facilities in your geographic location.

[Andrew Pudimott]

I appreciate it