Afternoon all, can someone check my logic, I'm a week or two into this and possibly forgetting things.

55 plate Mk 2 Ford Focus 2lt TDCi with 150k miles on it, as been in the family since almost new
Battery wasn't great but was still ticking along.

Last week the car died when being driven back home, Driver reported engine system error, power steering and possible limp home flashed up then it wouldn't start.

I recovered the driver and their shopping and tried a few things at the road side, mostly a bluetooth ODB2 connector and phone app I had used before with another car. It would not connect.

No crank, fast flashing immobiliser light later showing a 1:6 code and a variety of clear text errors (usually engine system being the main one.)
Tried a number of things at the roadside, purchased a new battery, similar situation, checked fuses, couldn't find any.

Eventually towed the car back home and started to have a proper go at it, put the old battery on charge etc, learning what I could from youtube and looking for wiring diagrams.

Dash error codes (hold reset button as turning key to position 2)
D900 Comms error ECM and Dash
E510 Comms fault Ignition
E200 Invalid data odo
E197 Invalid data vehicle speed
D147 Missing / Invalid Vehicle Security Data
...and ODB2 (BT adapter and phone app) won't connect.
Tried later with Forscan and the same BT adaptor, Forscan can see the BT plug, but won't connect to the car, but show a +12v value in bottom corner.

What I tried so then:
Cleaned corrosion of PCB (GEMs/BCM?) under the fuse box in passenger footwell, cleaned with contact cleaner.
Removed IC and re-soldered joints, I'm no soldering expert and had no flux so don't trust this 100% but pressing the IC face didn't change the problem, and Issue was never intermittent, just stopped working.
Cleaned the more common chassis ground points for PCM, battery etc
Gained access to PCM in wheel well.
(Around this time I found a wiring diagram and @Tdci-Peter posts on here, there was a lot of reading and a little understanding)

Since finding the posts on here, including the wiring diagrams:
Started checking for resistance of Can Bus terminating resistors, think I see 61 Omhs at ODB2 connector, then 122 Ohms when testing:
Fuse box plug C90 pins 1&15, IC and IC plug pins 17&18, PCM C418 plug pins A3&A4
I also see a constant 61 Ohms at ODB2 connector when manipulating the above plugs once all plugged in.
Which I think means my CAN bus continuity is likely to be okay between the IC and the PCM?

Engine bay fuses all seem okay (as in not broken, and can get continuity across the polls with a meter)
Passenger fuse box had a blown fuse #75 10A that I found quiet late, I replaced that and still get the same. (again checked across polls with meter)

Started checking Grounds to PCM
C417 Pin K2, C418 Pins G4 and H4, C419 Pins H4, J4 and K4 all connectivity to to battery neg cable.

Checking PCM power wires (Checking continuity between C90 fuse box plug and PCM plugs)
C90 Pin 13 to C417 pin G4 good
C90 Pin 37 to C418 pin C3 good (since replacing fuse #75)
C90 Pin 41 to C419 I'm confused by, I think there are multiple options from the wiring diagram as to where this pops out (flipping of the pin numbering hasn't helped me TBH), I think I have 3 good connections likely to be K2, K3, and J2 (Still not sure which wiring diagram version I need page 41 for "DW10 DIESEL STG3/4 DPF135PS MMT6" seems to match what I see.)

Replaced R14 with a different relay of the same spec, still the same.

Given my lack of confidence in the correct power pins in plug C419 I'm thoughtful about trying to feed 12v into the back of the PCM by back probing.

Where next?
Apart from re doing the soldering on the IC I'm not sure what to try, I've got access to Plug C112, but can't seem to split it, and so far I've not found plug C113 but continuity seems good through them so wondering what I would be testing by taking them apart.

Anyone any ideas?

Oh, and thank you @Tdci-Peter for possibly getting me this far.

8 hours ago, Cross flow said:

Forscan can see the BT plug, but won't connect to the car,

 

8 hours ago, Cross flow said:

I also see a constant 61 Ohms at ODB2 connector when manipulating the above plugs once all plugged in.

I see those two points above as being key. If the CAN bus is intact from end to end (from the 60 ohm result), and Forscan can not locate the PCM, then it does look like the problem is in the PCM, or the power & earth supplies to it. So it is worth re-checking those results, if you have not done so. Intermittent faults, and false readings due to difficulty in making good contact to the correct places with a multimeter can lead to wrong conclusions. I have been led astray like that more than once!

Some diagnostic systems (I think the Snap-On system is one) can find other modules even if the PCM is broken or disconnected. But Forscan must connect to the PCM first, it seems.

It should not be necessary to feed power into the PCM, back probing can check if that 12v power is present when it should be: that is with Ignition on, and for a short time while R6 remains on after ignition off. The purpose of R6 is to allow the PCM to do its shut-down tests etc. after the ignition has been turned off, then it powers itself down. I hear an audible click as I see all the sensors being powered off, a few seconds after ignition off, and after various other noises during the shutdown.

From what you say, I am not suspecting a problem with the infamous Focus IC. On my car, Forscan still made contact with the PCM when the IC was duff due to the bad soldering on its connector.

As it is a Mk2 as opposed to a 2a, I think the Stage 3/4 schematics on pages 41 & 42 are the correct ones.

C90 (the big connector in the engine bay fusebox) pin out is missing from the schematics, I have found one that seems ok:

The other connector pin-outs are at the end of the schematics.

F75 is called F138 in the schematics, all the Passenger fusebox fuse numbers changed about the time of the 2 to 2A re-design. It simply informs the PCM when the ignition is on. Most of the operating power for the PCM comes via the 3 wires from R6 and C90 pin 41.  That F75 blew is a bit suspicious, it does suggest a wiring fault or pcb fault. Keep checking it.

So you seem to be on the right tracks, but may, unfortunately, be heading for a faulty PCM conclusion. This is rare, so it is worth double & triple checking all alternatives. I am hoping it will be a broken wire or other bad connection, that can be fixed!

 

Cluster solders broken, send cluster off to be re-soldered, if you are getting "Theft Detected" that's the main identifier for this issue.

 

Thanks for the long reply @Tdci-Peter and thank you @DaveT70

Well it turns out you were both right, it did need a better diagnostics set up, a local auto electrician had some spare time and plugged in enough to get/clear the codes I'd been unable to get to.

As well as that he also re-did my re soldering on the IC plug and the large inch square chip also on the PCB, it seems to be working at the moment and I'll go for a test drive later this evening.