could be oxygen sensor. there actually are two on the car. could be alot of different things. vacuum leak possibly, fuel pump.
if it has spark plugs check the distributor cap and/or rotor button for corrosion
Based on the information, I assume it is the fuel filter you changed, that you have good spark and have done a fuel pressure check. I doubt that ALL the injectors would be clogged. I would think the fuel pump is faulty.
my 92 camaro is fitted with the anti theft device i changed the ignition switch and the car will not start need codes what are they??
Most likely the fuel filter is plugged up from gas tank sediment. Change the filter.
Do you have fuel pressure ? It must have 9 to 13 LBS. No higher or lower are you will have starting and running problems.
is the fuel filter istalled correctly in from fuel line out to engine. PS won't be the first person to install it backwards
is your a4 diesel ?
it may be hard to start, stall, or stop running altogether.
Check the ignition coils and the crankshaft position sensor. One of these could be failing as they heat up.
Check your Air Filter connection. Many times if you go to the shop to get your oil changed they will also check and see if you may be in need of a air filter. Sometimes they forget to tighten the hose connected to the Air Filter, which could cause the car not to run. It might start, but will not stay running!
Replace the fuel filter.
check the fuel regulator. check the fuel regulator. check the fuel regulator.