cliffcoggin

It's disturbing that a professional could not discover such a simple fault for so long, but having happened once it is possible it has happened again. I suggest you take that cable off and clean the terminals with sandpaper or a wire brush. Ensure the screws or clamps are tight when you refit it.

Maria. It may be a defective instrument panel. See this topic for more information: https://www.audiownersclub.com/forums/topic/21817-electrical-fault/

Glad to hear the end is in sight.

Well that confirms there is a real problem. Do let us know the outcome.

I had forgotten about the filter which may be blocked. It is normally changed on a regular basis, perhaps every two years if I recall correctly.

I believe you are right about the flap being stuck, but I know of no way to test it without dismantling. On second thoughts, how about wafting a little perfume by the external air intake. If you then can not smell it inside the flap must be closed.

Have you tried the tip immediately above your post? Have you even read it?

I am puzzled how the cooling system can have been flushed as you claimed earlier without opening it. Be that as it may, your other information confirms that the cooling system, including the thermostat and pump, is functioning as it should. Your heater problem then can only be either a physical blockage such as a kinked hose, a closed valve, a piece of debris, a clogged matrix; or an airlock.

Stephen, you have not answered Gareth's question about how soon the engine gets to normal temperature, to which I will add two of my own. Does the engine overheat, and has the cooling system been opened or drained?

In addition to Stevey's comments, check the fuse. As far as I recall there are separate fuses for left and right headlights.

If the cooling system has been opened or drained there is a good chance you have an air lock preventing flow through the heater matrix. An internet search will show a few methods to shift the airlock, but I have not faced the problem myself so I can not say which ones work.

In summary: cold starting is fine, hot starting is erratic, and the engine runs well once started. If that is correct I suggest there is an electrical or mechanical problem, though that doesn't narrow the field down much. ECM, mass flow sensor, crankshaft sensor, exhaust sensor, fuel injector, and many more could be at fault. I doubt we shall arrive at a firm diagnosis from discussion on this forum, and guessing at answers will be expensive for you, so I suggest you get it tested at an Audi dealer or independant Audi specialist. Do not use on an OBD reader from Ebay or the like, they are unreliable.

Petrol or diesel?

A simple voltage test will not always reveal internal battery faults, nor will such faults necessarily interfere when used in another vehicle. If the problem occurs again I recommend you fit a new battery before spending a lot of time and money on sensors.

Was that a new battery you fitted or a secondhand battery? Dodgy batteries are known to disturb the cars electronics leading to apparently unrelated warnings, false error messages, and symptoms such as limp mode.

Good. How much did it cost to have repaired if I may ask?

Tested by whom, and in what way? That the fuses are OK is irrelevent; it is the circuit board or its components that get damaged as I understand the matter, though I must add that is the full extent of my understanding.

It is not an unusual fault on A3s. They can be repaired by specialist companies for about £200, much cheaper than a new one from Audi. Search on line for such companies.

No it's not normal for a bearing to fail after 18000 miles, which is why I think the noise is caused by something else. A worn bearing would in any case make a very different noise. You can check the bearing by jacking the wheel off the ground, then rotating it. A low pitched rumble would indicate wear; a scraping or rubbing sound is something else. Grab the wheel top and bottom and try to rock it. There should be no movement if the bearing is sound. (Doing the same thing holding the wheel at left and right can be deceptive because of movement in the steering joints.) While the wheel is in the air look for damaged tyre tread, rubbing brake pad, a warped disc. While at 40 mph is there any vibration at the steering wheel? Does the car steer true or does it try to wander at that speed? Let us know the outcome of these tests and we can advise further.

Phew. It's like pulling teeth today.

That's a good result Gareth. I see the replacements are less than £16 on Ebay so quite a saving on Audi.

You mean the car has been modified? Please give some more details about the "hot fix".

Gareth. A Google search came up with https://www.a1-diagnostics.com/audi-specialist-cardiff/ You can bet they are not cheap, but a phone call might get you a diagnostic cost even if you don't get them to do the repair.

That sounds like a poor ground (earth) at the aerial.

Ah, that could be a problem. I no longer have my A3 and can't recall whether it had a key lock. Sorry, I am stumped.