EPC light, Limp mode, and a bunch of codes turned on overnight and car wont drive.

[Willi_must]

I've got a 2008 S5 and on Saturday 1/25/25 I drove the car to and from work making sure it fully warmed up both times before it was parked. The next day when I went to drive the car, the EPC light was on along with limp mode and there were a bunch of new codes. The new codes were P2101 (Throttle actuator control (TAC) motor - range/performance problem), P2016 (Throttle actuator control (TAC) system - Forced limited power mode), P0174 (System too lean, bank 2), and P0171 (System too lean, Bank 1). There are also a few other codes for misfires but those have been there for while. In case the misfires could have cause limp mode here are the misfire codes. P0300 (Random/multiple cylinder(s) - misfire detected), P0301 (Cylinder 1 - misfire detected), P0305 (Cylinder 5 - misfire detected). We believe that the misfires are caused by bad injectors but are not certain. Like I said though, the misfires have been there for several months. This problem has happened before about 2 weeks ago and we don't know how we fixed it. All we did previously was charged the battery, and replace the main power relay. We did not find any damage to the main power relay before we replaced it but we did it because it was cheap and easy and meant that we know one less thing is broken in the car. I don't think the new power relay was what fixed it.

Like I said, these issues cropped up overnight and I have no idea why. Nothing changed overnight, the car was just sitting there doing nothing, and broke. Gotta love Audi.

As of now I'm pretty sure that the issue is electrical because I doubt some part would just break overnight but I'm open to all suggestions. If you have any theories, Things to check, questions, know anything about common ground points I should check for the sensors throwing the codes, or have had a similar experience, let me know. All help in any form is much appreciated. Thanks!

[Brunzy]

i would start with basics, ive been battling a lot since i got my 4.2 fsi and have learned so much. im new to the forum.

currently im down to a radio dtc and two heater circuits on o2.

check for leakdown on the hpfp rail. it should stay constant and if it is leaking you can literally watch the pressure drop live with vcds. if you dont have vcds its well worth the investment.

also check fuse 5 under the ecu wire bundle. you may have to remove it (the ecu) attached still to access and test the row of fuses. but that #5 is a power supply for a lot of emissions related crap.

check your connections for sensors like the engine mounts. make sure nothings pinched inbetween frame etc. fault to ground situations.

you can access the throttle position calibration and test modes for all kinds of engine components. i would definately look at whats going on with throttle control. (assuming you have or get vcds)