Audi A6 2004, 2.0TDi won't start when ignition switched on. Battery is 100%, all lights are on. Recognises when clutch not depressed. Just silence. Problem is intermittent. First time I bumped started it and all was fine for 4 days then next time just waited 10 mins and it started fine. What is the problem? Thanks

hi Roy.
The starter won't crank. If nothing happens when you turn the ignition key to the "Start" position, it means that the starter motor doesn't turn over the engine. Most commonly this could be caused by a dead battery; here is How to check the battery. ... The ignition switch could be bad - it's a common problem.13 Aug 2013

Why a car won't start - troubleshooting tips - Samarins.com

How do you fix a car that won t start?
When your car responds with silence or clicking:

1. Turn on the car lights. ...
2. Tighten and clean the battery connections and try again. ...
3. Jump start the car.
4. If this works, immediately have the battery tested, especially if it's old. ...
5. Things could still be worse - such as a bad starter or alternator.

Car Won't Start? Here's What You Should Do ' GEICO

https://www.geico.com/more/driving/auto/auto-care/what-to-do-if-your-car-wont-start/ Search for: How do you fix a car that won t start?

It sounds like current is feeding into the start wire off the switch. Trace the wire from the clutch switch to the ignition switch connector. Does the wire look good? Examine closely the connector and see if you see a burnt terminal, or some broken plastic in the connector, some way for the start wire to pick up current. If you see nothing wrong, IDK, probably will need a shop to chase the wiring down and find it. I don't know if a bad starter relay (stuck on) could do that, likely not if it only happens when you depress clutch and have clutch switch on.
Good luck.

Sorry bud you need a starter, relay contacts in the starter are very dirty or just plain worn out.

When it will not crank, a mechanic would check for voltage at the starter, the clutch safety switch, and the key switch.

Can you push it off and pop the clutch in 2nd gear to start.

So it's a no crank condition? Do dash lights, gauges, and everything else work when the car won't crank over? If so, buy a cheap $5.00 testlight, and when the car won't crank, pull the small black wire off the starter solenoid. While someone holds the key in start (cranking) position and clutch pedal depressed, see if the black wire has power as it should. (Ground the testlight and just touch the wire's end terminal. If powered, the testlight will light up.) If power there, then the starter solenoid is failing. Toyota starters are very good units, but the solenoid's internal contacts get old and pitted.
If no power at the black wire, then the start circuit from ignition switch to clutch safety switch (on the clutch pedal) to starter relay to starter solenoid will have to be diagnosed. To double-check: take a single battery jumper cable and put it on the positive battery post connection. Put the other end of the jumper cable on the solenoid terminal where black wire was removed. Key can be off. Make sure car is in neutral, brake set. If starter now operates, solenoid is O.K., and it is a problem in the start circuit, -ignition switch, neutral start switch, or in the wiring.
Now find a grounding place for testlight under the dash. When car won't crank, check for power coming out of ignition switch on the black wire with white stripe. This is the start wire for the starter solenoid. Key must be in crank position. Wire shoud be powered-ignition switch is working. If no power, switch is failing. You can do the same for the clutch start switch-it's a simple switch, that when clutch is depressed switch is closed and passes power on from ignition switch to the starter relay (thus allowing power to the starter solenoid). Black wire, red stripe-this is wire from clutch switch to starter relay. When key is in crank, clutch depressed, this wire should be hot.
With me so far? Still haven't found a problem, or any loose connections? Next and last is the starter relay. It is located under dash, close to gas pedal, but under the center console, just forward and left of gear shifter. It can be tested and the wiring diagnosed there, but relays are cheap, so just swap in another and see if that helps.
If you have tried everything, and still no help, you could try a new wire from the relay to the starter solenoid. This is that black wire we started with. Older toyotas sometimes have a problem with degraded wiring not passing enough current to operate the starter

It's called the clutch start switch. Yes, it could be bad or needs adjustment.

Push the clutch in and the switch closes contact, so power flows from the ignition switch start position to the clutch switch and on to the starter motor-well, actually to the starter solenoid on the starter motor. If you have a voltmeter or test light, you can test the clutch switch . A red and white wire to it and a red wire from the switch to the starter solenoid. When this wire gets to the solenoid, it is a black wire with red stripe. It should have voltage when key is held in start position and the clutch pedal is depressed.

Go under the dash and bypass the clutch pedal safety switch and see if it starts.

you may have a bad clutch switch which will only allow the car to start with the clutch pedal depressed.it allows power through the switch only with it depressed to start the car so you won`t start the car with it in gear causing an accident.there will be a switch that the clutch pedal pushes against that allows power to flow through it to start the car.there should be power on one side with the key turned on and power on the other side with the clutch depressed.

Either your clutch safety switch has gone bad, or the starter is defective.
Remove the plug for the safety switch(on the pedal bracket next to the clutch pedal). Jump the wires together, try to start.
If it doesn't start, you will need to check out the starter