Hi guys, I have a strange and annoying issue with my Fiesta 1.4 tdci: the car starts and runs perfectly when cold, but if the engine gets hot and I switch it off and on again, runs for 3 seconds, it stalls and the yellow wheel with an exclamation mark appears.

Here's some history:
Sometimes the car used to go into limp mode and run in 3 cylinders, had to restart it to work fine again, but then the issue got worse and one day the engine just started and stalled right after. This was back in the autumn of 2021 and persisted till February 2022. The error code was P1204, so I've decided to replace injector number 4. After replacing the injector number 4, the engine has no more hiccups and doesn't go into limp mode anymore. The power is just fine, it has no injectors leaks and the car runs smooth.

So the car starts fine when cold and runs smooth, but if I run it for more than 10 minutes and switch it off and then I switch it on again, the glow plugs indicator will flash 3 times and then I get "engine malfunction" error without event trying to start the engine. However, if I start it, it starts and stalls after a few seconds. If I rev it to 3000rpm let's say, it revs, but every 4-5 seconds the revs decrease by ~500rpm and then go back to 3000rpm and so on (I tried this twice and the engine started to sound strange after 3-4 cycles). In order to be able to start it again I have to wait for some hours for the engine to cool down and then it starts and runs fine again. When it starts, I can drive it for 1000km without any issues, but without trying to switch the engine off and on. I've diagnosed it and it shows P0605 code when the fault occurs.

So I took to Ford service, mechanics, electronic engineers, nobody knows where the issue comes from.

I'm thinking that it could have a fault on injector 2 as well (because I know they run in pairs), or some issue with the high pressure fuel pump or one of its regulators, or some strange electronic wiring issue.

Any help / idea would be really helpful! Thanks in advance!

15 hours ago, paul_st said:

I've diagnosed it and it shows P0605 code when the fault occurs

Sadly, that error code does seem, to me, to stack up with the various odd faults.

P0605: Internal Control Module Read Only Memory Error, this seems to be the most common definition, though they do vary. More basically it seems to be pointing to some sort of internal control module error. There are several "control modules" in modern cars, any electronic unit with a processor can be called this. But the ECU (aka PCM or ECM) is the most likely one here. 

It could well happen just with key-on, before cranking, it could do various odd things while running, and it could well be temperature dependant. I think on the Fiesta the ECU is mounted at the top of the engine bay, and when stopped, heat will quickly build up here from a hot engine. While moving, or just with the engine running and perhaps the a/c fan on, there will be air movement under the bonnet, but when stopped that looks to be a rather warm place.

Some diagnostic systems, like Forscan, will tell you which module each error code is found in. If the P0605 was in another module (eg ABS or IC), then that would change things a bit. Also if the code keeps coming back or if it was just an odd once off would make a difference,

A diagnostic check might be to raise the bonnet as soon as the engine is stopped, and even get a fan to direct cool air at the ECU. If that made a big difference, then it would confirm the diagnosis.

I rather hope I am wrong here, but that is the first thing I would try to investigate.

Thanks for you reply Peter.

So the full error shown by Forscan is this one:
Code: PCM P0605:00-28 - Internal Control Module Read Only Memory (ROM) Error, Module: Powertrain Control Module

If I clear it, it will appear again when I start the engine hot. Tried this few times already.

The ECU is placed right near the battery. Not sure if the temperature of the ECU is related, because I've kept the car running on idle for 2 hours with the bonnet closed and it worked perfectly, no error at all. The issue appears only on start, probably the ECU runs some control checks and some calculations go wrong.

2 hours ago, paul_st said:

The issue appears only on start, probably the ECU runs some control checks and some calculations go wrong.

You could well be right there. It is possible that data (eg maps) are loaded from an EEPROM or similar device into the main processor RAM at switch on, and it is this operation that causes the error. Whether it is fixable or not I can't tell. Mending surface mount pcbs where you don't have all the technical info to hand is well nigh impossible. It is hard enough even on stuff I have designed, and (supposedly!) have all the info.

So after hours of investigations on sensors and other stuff, I took it to an ECU specialist, changed the ECU with a second hand one and rewrote the software and works perfectly again.

Seems the ECU software happens to have bugs and somehow has rewritten itself, which led to wrong checksum calculations when the ECU was hot. Strange, I know.

Moral of the story: if the ECU says it has a problem, then it has a problem. So don't ignore it and look into other places.