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From a user on another forum, good information in here that I have validated as accurate. Sharing for the greater good...
Our gearbox, a ZF type 5HP19FL, is nothing special; it's in oodles of Audis, BMWs and VWs through the same years as the 986 Boxster.
Submerged in the fluid inside is a wiring harness that connects to various bits.
There is a 2-wire thermistor that pokes its head out of this wiring harness; this is the ATF temperature sensor. It would appear that its plastic shrouding breaks down with heat and age, causing the device to malfunction and transmit spurious and/or no signals to the transmission control module.
In my case the removed harness also displayed evidence of similar brittleness with the plastic electrical connectors that connect to the various solenoid valves.
To sum up, if you have
the D-4 flashing thingo happening on your dash AND
the transmission does not go into 'limp' mode during those occurrences then the ATF temperature sensor in your Tiptronic is playing up.
Replace the gearbox internal wiring harness and your problem will be solved. (You have to buy the whole thing as the thermistor is intergral and cannot be purchased separately)
Don't take notice of comments about the transmission fluid level or purity, it's a furphy - in any case you'll have to drop the bottom to access the harness, so you will need to replenish with new fluid.
Happy motoring!
Wiring Harness Part Nos
Porsche 986 325 611 00
ZF 1056 227 046 01
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