Breezy_Pete

That's it. Make sure the drive includes conditions where the bad behaviour was happening. Be careful not to break the connector latch when you disconnect the EGR. Push the connector firmly onto the valve as you release the latch (you may hear it click open), then pull the connector off/away.

I can only suggest replacing the cable from alternator to battery then. May be a part number printed on it.

The cable arrangement looks a little strange to me. A 16mm-sq cable from battery positive to starter motor, then another, same size, going to alternator from there. Try splitting your measurement into, batt + to starter, and starter to alternator B+ terminal, see which half is worse. Cleaning the connections at the starter junction might be the key.

I've edited your first photo a little to point a green arrow at the EGR valve, and put a red box around a port on the PCV valve that you could try experimenting with. With engine running try putting a finger over the small hole that I've put a red square around next to where PCV is written. See if anything changes about the way the engine is idling. The other thing you could try, since the fault code points to an EGR fault, is to unplug the electrical connector from the EGR valve and see how the engine behaviour changes when in the conditions where it was previously having trouble.

@J.R. I suspect the lengthy dwell time at vacuum is to give the dessicant in the drier time and opportunity to give up any/all contained water. That's my best guess anyway.

Glad you found a fix, good initiative. I'd be wary of having both dipped- beam lights on one relay, as a single point failure leaves you in a bit of bother. No worries about same situation with full beam though. For bulb recommendations you'll need to define your criterion/criteria for 'best'. There's a big design tradeoff between brightness and life expectancy, so work out which of those matters more to you. Some people even get excited about them looking particular colours, so mention your level of 'GAF' on that aspect too. ☺️

I think big spark gaps cause the coilpacks to fail. I wouldn't change a working one, just carry a spare in the car and a means of reading fault codes (handheld scantool). My 2003 Polo has all four factory units still, I believe.

Is your cooling system performing OK? Just thinking that anything above normal 90°C operation isn't going to help things. Thermostat ever been changed? Rad fan(s) fully working? I had to change the smaller fan on our Fabia a few weeks back. Once I'd performed an autopsy on the old one, it was clear that only one out of the four brushes wasn't stuck, immobile in the holder. It would start up sometimes, but often not, when the other fan did.

When fuel quality is OK, I can see that working, but when poor?

OK, but if you had less pointy central electrodes, the gap wouldn't open up as rapidly as the material would wear across a bigger cross-sectional area? And the tip wouldn't be as fragile? Maybe?

I guess you mean pinking has caused the electrode erosion, rather than the other way round?

Those gaps look huge; have you measured to see what they currently are? Don't think I'll be rushing out to buy any plugs like those. What's the part number? BKR6EIX? Great to hear you've finally got it running better.

Arguably it's a symptom rather than the cause, but yeah, you'll have to change it anyway, even if you don't want to delve deeper and try to stop it happening again. Our Fabia still has its factory original resistor pack, 15 years on, so they can last well when everything else is behaving itself. Or perhaps they just made them better way back then.

His table above shows 4 bar G65 pressure with engine not running, that does indeed seem low for the ambient temperature recorded of 34°C. Slightly puzzled by why the engine coolant temperature is showing as 11° higher still. Was the front of the car facing South? @J.R. We should both compare VCDS and high side gauge readings to put to bed this question about whether VCDS talks absolute pressure or gauge; I'm almost sure it's the latter, due to empty systems showing a 0.0 bar pressure, not 1.0 bar.

The remaining possible causes of resistor failure if pollen filter isn't blocked seem to be fan taking too much current due to physical restriction such as PipH suggests above, short-circuit in motor windings, or because supply voltage is above spec. The latter seems unlikely, but check voltage across battery terminals with engine running just to rule out alternator misbehaviour.

The pollen filter or the resistor?

Has the pollen/cabin filter been changed lately? Blockage can help the resistor overheat. Resistor can fail in several places; one way just affects speed 1, another takes out 1 and 2, and finally 1, 2 and 3. Speed 4 works even in the absence of the resistor as it has a direct link from the switch. Ignore the big arrows, they were for something else. N24 is the resistor pack, V2 the fan motor. Diagram is for an 06 Mk1 so ignore the fuse number too, that'll probably be different.

The brake pedal switch is actually two separate switches, one does the brake lights (non-conducting until pedal pressed), the other tells the engine ECU when the brake pedal is pressed (conducting until pedal pressed). The latter is fed by cabin fuse 28, so check or replace that. The ECU switch is the thinner pair of wires, middle two on the connector, brake light switch is the outer, thicker wires. Genuine brake pedal switch is cheap enough from Skoda, but a bit of a faff to fit due to the lower dash trim having to come off to get to it. Guide in the Fabia Guides section, I think. I don't know what impact the ECU part of the switch will have on starting behaviour in automatic cars, I've never got involved with one.

OK, battery light behaviour is correct, it was that initial illumination I was wanting to check. Brake servo is the big black round thing on the back wall of the engine Bay. Vacuum hose connects it to the inlet manifold. Cracks/leaks most often found at junctions of hard plastic sections onto non-return valve near engine end, Feel underside with fingertips as well as visible parts.

Have you checked that the brake light bulbs are working? Where are you getting brake light switches from? (Genuine only £11 or so last time I bought one). Does the battery light symbol come on when you turn on the ignition? Have you inspected the brake servo vacuum hose yet? Fault codes may shed some light.

😲

Presumably an aftermarket/pattern part rather than genuine replacement? If so, that generally goes with the territory. Thinner walled construction for cost reasons.

Are you sure it wasn't round the outside edge, outside all the screws, sealing the back cover of the ECU where the two halves of the case meet? In which case, it probably isn't replaceable other than by dribbling some new silicone round there, like the edging round your bath/wall junction? Speculating wildly here.

Dunno, can't spot it in the parts catalogue, and none of the ECUs on ebay that I've just been looking at seem to have anything there. What's it seal against?

Only in relation to interior lighting I think. The terminal 30 (permanent 12V) supply to the engine ECU (and a bunch of other stuff) all comes direct from battery positive through the relay in the ebox, the only one in the photo on page 1; which is the one that's been changed I believe. Good idea though.