It would be nice if he'd tell you the actual fault code rather than saying "pressure related code." A low line pressure code (P0868) will NOT put you in limp-in (at least, not on a 2007 truck). For that matter, neither will ANY of the line pressure faults. So something else is going on (again, I'd bet it's internal slippage). Not that low line pressure couldn't be your problem - it just won't (by itself) put you in limp-in. Low line pressure will allow the trans to slip, and THAT will put you in limp-in. And by the way, limp-in is 4th gear (direct gear).
You really need to find out exactly what fault codes are set. It is possible to have a hardware failure in the TCM, but that usually presents itself as a solenoid circuit error, which is strictly an electrical (not a "pressure related") fault. So I don't think you have a TCM problem.
When your tech says "pressure related" does he related to overall line pressure, or related to the pressure in one particular clutch?
I'm gonna assume here that you are getting low line pressure. If your tech thinks that is control-related, there is a simple test: Connect a mechanical pressure gauge to the line pressure sensor port (your dealer has a special tool for doing this). Start the engine and let it idle in Park. Is the pressure LESS THAN the desired line pressure (DLP)? If not, warm it up until it starts going into limp, then check it again at idle. Odds are it will be less than DLP (this is not a problem by itself, since normally as soon as you open the throttle and engine speed comes up, line pressure jumps up to match DLP). Now, with the truck still idling in Park, disconnect the 23-way electrical harness from the transmission (driver's side). This disconnects the line pressure control (TCM) from the trans. Did the line pressure on the gauge jump up to a much higher value? If not, then the TCM and wiring are NOT causing a low-line-pressure problem. The pressure you're seeing is all the pump can make (mechanically). If the pressure DOES jump up, then the TCM was deliberately "turning down" the line pressure.
Note that if this test says the pump is bad, there is still a SLIM possibility that the solenoid (which controls line pressure) is failed mechanically, so it is "turning down" line pressure even when the TCM is not commanding it to do so. But that would be a VERY RARE failure (not likely). And if the solenoid is suspect, then at your mileage you should replace the entire valve body assy anyway (new VB
will include a new solenoid).
Also note that this test must be properly understood. Normal "max pump pressure" in Park will be 160-170 psi. DLP (at idle in Park) may be 60-120 psi. If DLP is 60 psi, the TCM should be "cranking down" the line pressure (from 160 to 60 psi). So if you pull the 23-way and line pressure jumps to 160, that doesn't mean the TCM is bad. Under these conditions, that would be normal. But let's say DLP is 120 and actual line pressure is only 90 psi. The TCM SHOULD be commanding max line pressure (to try to crank it up to 120). So either the pump can't make more than 90, or the TCM isn't commanding the LP like it should. At that point, pulling the 23-way will show whether the pump is the cause (line pressure stays at 90), or the TCM is bad (pressure jumps up to 120 or more). Hope this makes sense.
The dealer can also read the duty cycle of the LP control solenoid (also known as the VFS) using the scan tool. At normal temps, the min duty cycle is about 5.1%. So if he sees a 5% duty cycle, that means the TCM is calling for "max" line pressure. At that point (no matter what DLP is), pulling the 23-way should result in little or no change in actual line pressure. On the other hand, if the duty cycle is, say, 15%, then pulling the 23-way should cause a change in LP (and would not mean the TCM was necessarily bad).
A few more notes: Pulling the 23-way will blow a bunch of trans faults (duh), so you'll need to clear them afterwards. You must have the engine running before you pull the 23-way, since once you disconnect it, you'll have no Neutral safety signal and the engine can't be started. As I said, "low" LP (below DLP) at idle is NOT a problem by itself. The true test is, does LP match DLP whenever the engine speed is around 1200 RPM or higher? If LP matches DLP above 1200 RPM, then your line pressure is OK.
Sorry for the long-winded response..... but then again I imagine you guys probably LIKE a long-winded response like this....