Q7 "Limp Mode"

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Hi there.
Just joined the forum as I have just purchased a Q7
Its a 56 plate and has developed a problem within days of purchase.
Stability control (DSP) light comes on and puts car into a 40mph "limp mode" in the morning. It goes away as the car warms up. It does not seem to be related to rain as it has happened in dry conditions. It may be a simple earth fault.

I have contacted the garage and they have said that the limp mode issue might be related to the EGR valve in the engine.
It is going to an Audi dealer tomorrow so hopefully it will get sorted.

Anyone else had this type of problem?
If I turn off the DSP then put it back on, the limp mode clears.

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The car was taken to Audi in Edinburgh who re-set the system. Problem back within 2 days. This time its worse and lasts for much longer. Its a safety nightmare trying to merge with traffic and the car limits itself to 38mph. If you can drive below 40mph for the first 10 miles then the problem goeas away. If you have to get into fast moving traffic forget it. So its now with an Audi dealer in Glasgow. Lets see what happens this week.

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Well I discovered that the garage had fitted 275 x20x40 tyres on the front. The back had 275x20x45. Audi believe this could be tricking the system as the front wheels and back wheels are different size hence switching on the limp mode. I hope that no long term damage has been done!! They did however update all the software and the garage fitted new 45 size tyres so I will get it back tomorrow.
Anyone else had a tyre miss-match cause problems? What happens when a spare is fitted?