Hi , i am new to the forum and would like to thank the members for letting me join ! ...Hope this is the correct place for this , if not , can the admin/s please put my questions into the correct category ... Firstly i own a late 2007 Audi A4 automatic convertible , i am having crank , no start issues with the car , I am hoping that any Audi auto sparks can give me some advice or constructive criticism on this issue .. the car prior to the non starting ran really well , it never missed a beat tbh, i have owned it for over a year it did lie up for a few months with a flat battery after i purchased it (due to poor weather and the car being a convertible ) altho once a new battery was fitted it started first time everytime and ran well , the vehicle is a late 2007 2.0tdi .. last week i went out to start the car and it cranked and wouldn't start , this is the first time this has happened since i fitted the new battery , i noticed a code on the instrument panel L3 2-3 where the miles should be , i also noticed that i am getting a PRNDS red flashing light on the dash now as well .. I googled the code and i read that it would need the key rematched with some sort of software like ODIS or something .. I got a guy out to link the key to the car , he was able to delete the L3 code and my miles are now appearing back on the dash but the PRNDS is still flashing and the car still wont start ? the auto spark suggested that i source a replacement Ecu and he may be able to take the information ( files ? ) of the old Ecu and put it onto the replacement unit , he is not filling me with confidence tbh and he wasn't terribly helpful or forthcoming about why the car won't start nor did he mention the red flashing PRNDS and if this issue would have anything to do with the non start . AFAIK the red flashing PRNDS has something to do with the TCU transmission brain ? am i correct to assume that altho it is a separate issue it arrived at the same time the L3 code came up so if i get the ECU sorted will this fix everything and the car go back to normal ? .. any advice would be appreciated ..I am concerned that this is going to turn into a money pit , i do like the old car and would like to be able to run it if and when we ever get some kind of dry weather ...

Ha e you had a further diagnostic check to see if any further fault codes flag up after the key recoding?

There could be an issue with the gearbox megatronics unit but I'm not sure if this would stop the car from cranking.

Taking it back to basics, it needs to be established if the car is getting fuel and air. As such, could it be an issue with the high pressure fuel pump?

It could also be an electrical issue relating to a crank sensor, can sensor, injector or immobiliser.

I would advise getting a second opinion from another mechanic before you start throwing money/parts at it randomly in hoping to fix the problem.

When you fitted the new battery did you have it coded to the car? If not, that may be the problem. I suggest you get the battery professionally tested to check that it has not been degraded by poor charging as a first step.

If after proving the battery is well the faults still exist, get the car scanned on a code reader to remove redundant codes and diagnose any new codes.

On 4/3/2026 at 6:08 AM, Steve Q said:

Ha e you had a further diagnostic check to see if any further fault codes flag up after the key recoding?

There could be an issue with the gearbox megatronics unit but I'm not sure if this would stop the car from cranking.

Taking it back to basics, it needs to be established if the car is getting fuel and air. As such, could it be an issue with the high pressure fuel pump?

It could also be an electrical issue relating to a crank sensor, can sensor, injector or immobiliser.

I would advise getting a second opinion from another mechanic before you start throwing money/parts at it randomly in hoping to fix the problem.

Thx for the Advice Steve Q , gonna try my old trusty Topdon OBD2 reader on the car as soon as my original Ecu is returned to me .. the dude who was looking at it said something about the Ebay replacement Ecu was not compatable even tho it is from a 2007 Audi A4 2.0tdi Automatic convertible ? looks like i am gonna have to try and source an ecu kit for the vehicle and remove it from a car that i can hear running ! . no one locally seems to know what they are talking about regarding key reading / linking / Ecu Eeprom reflash etc ... the car ran perfectly fine and autobox was perfect before the dreaded L3 code came up ,this co-incided with the PRNDS flashing on the dash ? i do believe the car is immobilised but not completely certain , the autospark / locksmith mobile mechanic seems stumped , it is completely above my paygrade as a dopey old mechanic .. I don't have access to ODIS software or any advanced way of plugging it in that isn't gonna cost me the price of the car ... will let u know if i have any luck but for now it is not looking Financially feasible to sort it ..

On 4/3/2026 at 2:02 PM, cliffcoggin said:

When you fitted the new battery did you have it coded to the car? If not, that may be the problem. I suggest you get the battery professionally tested to check that it has not been degraded by poor charging as a first step.

If after proving the battery is well the faults still exist, get the car scanned on a code reader to remove redundant codes and diagnose any new codes.

Hi Cliffcoggin , many thx for your input , to be honest i did not match the battery to the car after replacing it a few months ago , car was running and driving perfectly fine after replacing the old battery and Autobox was also 100% , i put quite a few miles on the car since replacing the battery so i doubt it is a battery issue ( the car is spinning over no problem but not still not starting unfortunately ) if i had to say i think it is immobilised or is not recognising it's own key ... an auto spark has the original ecu as well as one i purchased offa ebay , when i get it back i will try my obd2 on the car and see what new faults ( if any , show up ) ..

Please keep.us posted on how you get on. I believe ecu are not plugged play. They are coded to each car as they are part of the immobiliser system.

I empathise as I feel most mechanics don't understand the coding and it's complexities.

2 hours ago, Cyberjack said:

( the car is spinning over no problem but not still not starting unfortunately )

That does not prove the battery is in good condition, nevertheless I'd agree the ECU/immobiliser is the more likely culprit.

Hi,

I have just fixed such an issue due to code P0687 ECM/PCM relay voltage HIGH ..My car ( A4 2006 3.2 Cab ) turned over but no fire. I changed the relay ( nightmare of a job due to HEX screw locations ) and low and behold it fired on 1st crank ..

Happy to talk through this if you wish , or see my post on it that has pics of where the relay is located

Hi Mark , ( BC6 _ cab man ) many thx for your input , I haven’t had the time to plug my reader into the Audi and I still haven’t got my Ecu back from the Auto spark locksmith.. when I do ,I will check for codes , glad the relay fixed your issue , I can see the relay you are referring to as it sits underneath where the Ecu sits , I think is a large white one and has a 3 digit number on it from memory ( 645 or something like that ) they are less than a 10er on Fleabay so I might purchase one for piece of mind , I think it governs the Ecu and the fuel pump and is a 4 leg affair ?

645 was the number on my relay, but I took it to Audi ( costly ) and they said they not made anymore, but they supplied one of the same type. I also put in the number on the relay in Amazon and got one for under 9 Pounds ..

1 hour ago, B6_Cab_Man said:

645 was the number on my relay, but I took it to Audi ( costly ) and they said they not made anymore, but they supplied one of the same type. I also put in the number on the relay in Amazon and got one for under 9 Pounds ..

Thx Mark , will most likely swap the relay out for future piece of mind anyways . 645 relay sounds right , the car is outside my uncles house ( and is causing a tad of mayhem where I have it parked lol ) out of my control unfortunately until I can get her running again .. These Forums are handy and always good when u get someone who actually wants to help , once again many thx .. Will let u know if a relay helps sort the issue , Kindest regards Jack ..