PRNDS lit up red.

Tiptronic not working.

Transmission stuck in 4th gear limp home mode.

No crank with key.

Battery tested and good.
Relays tested and good.
Fuses tested and good.
Starter/solenoid tested, momentary push button used to start car (battery>push button>solenoid)

Car will start but then go into limp mode when selecting reverse, complete with thump as it does so.

Fault codes:

17090/P0706/001798 - Transmission Range Sensor (F125): Implausible Signal - 003 - mechanical failure

18161/P1753 - Tiptronic Switch (F189): Implausible Signal

 

Replaced F189 (circuit board under gear selector bevel) and sliding gaitor complete with magnets.

Replaced F125 Transmission range sensor with new one.


Car now works as it should.

 

Before you take your car to a garage, use VCDS (or OBDeleven) to pull codes from transmission ECU (TCM).

If you get either of these codes, try replacing these parts before being told you need a new transmission by a rip off merchant (mechanic)

Read so many threads with these exact symptoms and people saying they shelled out thousands of dollars or pounds for a new transmission.

Spend £100 first, replace the entire top end of the shifter unit (circuit board, plastic housing/bezel, track with magnets) from a scrap car/ online breakers.

Buy a new Transmission range sensor, follow online guides to fit it if you are good at DIY car maintenance.

Save yourself a fortune and have it back working as it should.

If the fault code for F125 range sensor states "mechanical failure" this is referring to the transmission range sensor (AKA neutral safety switch) mechanically failing - NOT THE ACTUAL TRANSMISSION ITSELF.

Water and dirt tend to be the culprits for F125

Bad circuit board or missing magnets are the culprits for F189

HTH.

Get 'er done yourself, don't get ripped off.

Replacing the F125 will take a couple of hours if you spend the time gathering appropriate tools and watch/read some guides beforehand.

 

Just for example:

2004 A4 3.0 V6 Quattro (ASN)

Note that the one youtube video I found for this job is a Jetta, the only difference I found is that the exhaust header for the V6 gets in the way where it doesn't in that video, thanks to that youtuber though, made light work of a potential PITA job.

Tools required:

3/8 ratchet, 2 long extensions, 1 short extension, 1 universal joint.

May also be useful to have 1/2" drive ratchet and sockets to hand for lower bolts and nut as they will be rusted.

13mm socket (2 bottom trans mount bolts)

16mm spanner (single trans mount through bolt)

15mm deep socket (nut bottom end of trans through bolt)

6mm hex bit (inner CV joint guard/shield - 3 bolts)

8mm hex bit (transmission mount bracket 3 bolts, long bolt on lower left corner)

10mm ratchet spanner (trans mount bracket heat shield upper bolt)

10mm socket (trans mount bracket lower bolts)

T27 torx bit (F125 range sensor 2 bolts)

Standard scissor jack or trolley jack for raising car, support on axle stand(s)

trolley jack or bottle jack with block of wood to support transmission while mount and bracket are removed.

2-3 hours and a thin arm, a little patience.

 

Post mortem examination on transmission range sensor p0706 F125

Here's what you'll find when the fault code states "mechanical failure".

In the following photos you can see that two of the contacts have broken off, anyone who has owned a slot car racing set will recognise them, same thing as the cars use to get power from the track.

There is also noticeable charring of the terminals on the wiring, probably as a result of the contacts shorting but also possibly due to ingress of moisture, which could be the cause of the whole deterioration of the sensor unit.

Note that the two contacts which have broken are the two that sit lowest (nearest the road) and therefore would be submerged if water built up inside the unit.