Audi TT Mk1 will not start.

There could be several reasons why this is happening, including a faulty spark plug, clogged fuel injectors, or a faulty ignition coil. This will likely require a diagnosis by a professional mechanic. The mechanic can run a test for such. Is Your Engine Misfiring? Here are 6 Possible Causes ' RepairSmith Blog

A bent cam shaft will do that.

Mechanic needs to go back to basics. Engine on the TT isn't easy to get around so checking is no small thing.

Basics, in case your mechanic has forgotten are - timing, compression, the right amount of fuel in the combustion chamber at the right time and a good spark also at the right time.

If you have all those the engine will start and run, the engine doesn't so at least one of those essentials is absent, discovering which one(s) is what checking is all about.

I suggest the immobiliser is checked first. If the mechanic has some decent test gear he should be able to see some live data, oscilloscope patterns, etc., even at cranking speed, which would take some of the backache out of the job.
If there is no live data, etc. or what you are getting doesn't make sense maybe the management ECU died or there is a wiring or supply fault. An ECU is often weakened by welding, jump starting, boost charging and other stuff done in the name of car stuff - such weaknesses rarely show up straight away but cause trouble later.

It is also worth keeping in mind it isn't unknown for a catalytic converter to become blocked.
Good luck!

hi starter relay of Audi TT 2002 its located under the carpet where the passenger beside driver all the relay is there

Naaah...

Pretty sure you are looking under the wrong rock...

A BATTERY CHANGE should not cause a FOB to fail ..

I'd suggest you take it to the dealer to get it read
for what-ever-else is going on
(wrong CAR perhaps?)

Carnac the Magnificent

audi tt sensors - forum about car parts and car advices

abmforum.com/audi-tt-sensors.html
audi tt sensors 2004 audi tt coupe windows roll down hi all only new to forum 2001 ...2002 audi tt purchase advice newbie looking to get a tt. ... Used on cold starts towarm engine and cat faster. .... replaced and sending my car into limp mode. my ac just shut off one day and will not turn back on. the esp in the car is stuck off.

I would try a new relay anyway sometimes they will route the circuit through different points of the relay. I also would check the up/down switch itself to make sure it is working on both sides I had the same problem on a window switch for a car I was working on. Just because one side of the switch works doesn't mean the other side does. If the relay and switch are both good then I would switch the wires on the up/down switch to see if it functions then you are basically checking the continuity of the switch again but this time you will be able to tell if the switch is able to handle the amperage also required to operate the motor. You can read 12 volts

See picture below:

could still be the clutch make sure it releases good or is not just done afdter testing new engine good luck