2002 audi tt just cranks, but doesnt start what can it be?

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!

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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 assume not cranking? Or cranking, but not starting?
No other work done while battery disconnected?

Yes & when the dealer finds the answer they will
share that with you

If a dealer can't repair your car, we certainly can't
even get a start on it without the car

replace the crankshaft position sensor

if vechicle getting spark and fuel.timing could be off due to a worn timing belt.i would do a compression test see what condition the engine is in.

Check for spark-if you have a tachometer, see if when you crank it the tach needle moves-if not, you probably need a crank sensor, which are not too expensive or difficult to replace.

Check coilpack and crank sensor

dealer only ,factory immobiliser is cutting in, try the spare key and if it doesnt start call the breakdown truck to take it to dealer

I would suggest you check oil levels etc. Generally these cars suffer from the air mass meter / air sensor problems aswell as coil problems too. Did you ever get a solution to the problem?