EPC/engine light

[maars]

Went to start the RS4 and the EPC/engine light came on. The car normally fires first time, but now takes 5-10s before it fires. It then appears to run fine. I've not tried driving it in case I get stuck somewhere and it won't start again.

It has decat downpipes and I've run O2 extender blocks in it for a few years without any error messages or issues. Could it be the O2 sensor perhaps, or something else?

[oxblood8]

Let me start by saying I'm absolutely no expert, there are far more learned folk on here

However, when you say 5-10 seconds before it fires, do you mean you have to hold down the start button and wait?

If so, you might need a new lower clutch switch (blue one). I am currently going through this at the moment. Am replacing both upper and lower switches. My cruise control stopped working which I believe is the upper switch. Also, I've had the issue now of having to hold down the start button for a while and just found out that the lower clutch switch could be the cause. From the thread link I posted below, the EPC light will come on if that is the issue (mine isn't on for some reason).

Good thing is they are very cheap (I am in Australia, cost me $45 AU each, I believe they are about 20ish pounds in the UK). It can be a DIY job but PLEASE PLEASE PLEASE read this thread first.

viewtopic.php?f=35&t=114660

Lots of great information on there including the ELSA diagrams (first page, about 4th post) for how to change the upper and lower switches. By the sounds of the thread they are a once only thing, so if you do it wrong, you'll have to buy a new one. There are also a couple of YouTube vids out there you can reference:

https://www.youtube.com/watch?v=BBhvPShYRYo
https://www.youtube.com/watch?v=fpmVbHvDfko&t=186s

The part number for the lower switch is: 7H0 927 189

[Dinosroume]

Went to start the RS4 and the EPC/engine light came on. The car normally fires first time, but now takes 5-10s before it fires. It then appears to run fine. I've not tried driving it in case I get stuck somewhere and it won't start again.

It has decat downpipes and I've run O2 extender blocks in it for a few years without any error messages or issues. Could it be the O2 sensor perhaps, or something else?

Scan the car with a diagnostic tool.
A few months ago my EPC light came on and the result was a brake switch that the diagnostic tool found.
It was very cheap around 20 euros and easy to replace.

EPC light can come on for a lot of issues so scan the car and see what fault code has been stored so you can be sure what to replace and don't waste money.