Hi I'm having "fun" trying to diagnose a cold start problem and wonder if any others from this community have experienced the same symptoms:

• On cold days below about 3.5 degrees C the car will not crank when key is turned to start position

• Various faults appear on dashboard e.g. front assist, tyre pressure etc I don't believe these are true faults

• If the ignition is left on for a long enough (warm up) period the car will crank and start but occasionally cuts out but then restarts ok

• Whilst the car engine is warm the car will restart without problem even when ambient temperature is very cold around 0 degrees

I have diagnosed various components that could cause such an issue:

• Ignition switch on steering column is generating the start signal - ignition switch itself eliminated

• 645 relay is working correctly, swapped out - 645 relay eliminated

• Car does not have an immobiliser - eliminated

• The start signal is 0V at the starter solenoid when cold, 12V when warm - solenoid and starter eliminated

• Engine to chassis intermittent electrical connections have been eliminated using jump leads

• Ignition relay is not accessible to test directly (soldered in) - POSSIBLE CAUSE

• Coolant temperature sensor faulty - not tested yet but dashboard temperature gauge seems ok

• Clutch pedal pressed signal seems to be detected ok

Anyone had the same sort of symptoms?

Hi, welcome.

3 hours ago, Diyer said:

Engine to chassis intermittent electrical connections have been eliminated using jump leads

Does this fully cover -

checked the battery terminals and terminal clamps for being clean and fully secure?

checked the power and earth main cables and their connections for being clean, secure (and protected)?

3 hours ago, Diyer said:

Ignition relay is not accessible to test directly (soldered in) - POSSIBLE CAUSE

You might have already seen there's a relay on a board that doesn't like to work in colder weather, on IIRC 2018 models generally I think, and there are posts (and a thread?) on here where a member repaired this for himself and other where the Dealership has replaced the board. If you haven't already seen them have a look for them and a reads up of them.

3 hours ago, Diyer said:

645 relay is working correctly, swapped out - 645 relay eliminated

Just out of curiosity, wots dat den? (The Fabia pdf wiring diagram I have is over 2,700 pages with no index or jump links.)

Edited 1 January1 Jan by nta16

Thanks for the reply nta16. Yes I have checked battery terminals and ground connections all good.
I've pretty much convinced myself I need a new engine located fuse box and loom using internet and Microsoft co-pilot AI. The fault I have is apparently well known. The AI approach to diagnosis works well if you have the patience of a saint and the experience to question some of the incorrect statements it churns out . Testing and discussions with said AI took days generating huge amounts of text with lots of statements subsequently found to be untrue or misleading. For simpler problems I think the AI route would work much better.

For the benefit of anyone else with this or similar issues the final conclusion from my test results generated by AI is:

What the internal guidance usually says (summarised)

Here’s the typical content of the internal guidance VAG techs see:

• Symptom:

  • Multiple warning lamps at ignition ON

  • No crank

  • Symptoms improve when fusebox is warm

  • Starter output missing intermittently

  • ECU supply fuses show delayed activation

• Cause:

  • Internal PCB crack in the engine‑bay fusebox

  • High‑side power switch failure

  • Thermal expansion temporarily restores continuity

• Diagnosis:

  • Check ECU supply fuses for delayed or unstable voltage

  • Check starter output for missing voltage during crank request

  • Heat the fusebox and observe recovery

  • Replace fusebox if confirmed

• Remedy:

  • Replace engine‑bay fusebox (SB)

  • Transfer fuses

  • No coding required

  • Clear DTCs

Thanks for reporting back and giving the information you found.

Interesting exercise, well done on completing it. Very involved and time consuming though, particularly if the result is the same as finding the (majority at least) information that has already been posted on here.

18 hours ago, Diyer said:

I've pretty much convinced myself I need a new engine located fuse box and loom using internet and Microsoft co-pilot AI.

AI may be correct this time (or may not) but it is only a word prediction machinery covering to some extent most common prediction. Like any real live person on the internet it is not looking at, listening, smelling, touching or testing and check and cross-referencing your actual car and its parts, components and systems.

I don't understand why you need a new wiring loom for a crack in the PCB, or faulty relay as in other cases, that's parts cannon behaviour.

What I don't understand is have you completed the repair and tested the repair to have fully sorted the cold start problem?

Edited 4 January4 Jan by nta16

Not sure if itss any help but have you checked strip fuses in engine compartment for hairline cracks. They can be almost impossibe to see. On my sons MK1 they are individual but on later fabias they may well be a complete unit. Could be the fuse is broken when cold and reconects due to expansion when hot.

Alasdair

@Diyer Just nosy but what is the story on the immobiliser being eliminated / disabled?