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After putting my 2005 TD6 auto through a carwash, I experienced a loss of drive a mile down the road. The sensation was similar to a slipping clutch, where the engine rpm had no impact.
I attempted to operate the vehicle in both Drive (D) and Reverse (R) modes, as well as in Manual mode and Low range.
They all achieved the same outcome.

After a duration of thirty minutes, I made another attempt and the vehicle proceeded for about one mile before experiencing the same issue once again.

After being discharged, I returned home and used the Gap IID to scan for codes. However, no unexpected results were found.

Based on the observation that it moves intermittently, I have inferred that the problem does not lie in the prop shafts or drive shaft.

There is an absence of oil in the coolwant water.

What would be your approach to diagnosing this?

My current plan is to inspect the electrical connections to the gearbox and transfer box by going under the vehicle. I will disconnect, clean, and then reattach them.
If the previous suggestion is not helpful, I believe I should proceed with the task of inspecting and maybe changing the gearbox oil.

All advice is appreciated.

It seems to be a fortuitous occurrence. Could you provide a brief overview of the history of gearboxes and their maintenance requirements, specifically regarding oil changes? What is the mileage?

Inspecting the level and condition of the gearbox oil should be the first step to take into consideration.
Would a car wash potentially damage any underbody electrical connections? Furthermore, if there was an electrical issue, GAP would have undoubtedly indicated it.

Perhaps, while the vehicle is in motion, connect the GAP device and capture real-time data.

Do you have any specific ideas on which real-time data to monitor?

The history of the gearbox is uncertain.

As @ Sarah_Jane1980 inquires, what is the historical background of the gearbox? Typically, a rebuild is necessary after the vehicle reaches around 100,000 kilometres.

Regrettably, I am unaware of its historical background. However, the vehicle's mileage is recorded as 202,728.

Ensure that there is an adequate amount of oil in the gearbox.

Without inspecting the gearbox menu, I am unable to ascertain its contents and hence cannot provide a definitive answer.

Examine the oil first.

If there are no fault codes present, it is likely that the problem is in the torque converter. Particularly when there is no motivation or incentive.

Perhaps there is insufficient fluid reaching the TC? As the temperature increases, the viscosity of the substance will decrease, which might potentially restrict the ability to operate a vehicle with a traditional combustion engine.

The link provided is: https://santiemidwest.com/blog/hydrauli ... explained/