Disclaimer - I can only tie the following to my own experience over the last 6-12months with my CAYC engine coded 1.6 common rail diesel Mk2 Octavia. Also, I am not a mechanic, nor overly handy with a spanner. I do however have "The Nack" as Dilbert's paediatrian once put it, so tend to be able to see correlation and decide if causation is linked or not so... here goes!
How to tell early on and as symptoms persist whether your EGR is starting to fail and what will happen as it gets worse, broken down into increasing levels of severity, with a quick description of how the egr system works (broad strokes)
What does the Exhaust Gas Recirculation system do?
Nutshell version - it pumps a certain amount of exhaust gases back into the cylinders via the air intake to cool the burn and help reduce overall emissions.
Ladybird book version - (my understanding)
On cold start and running the EGR inlet valve is closed. When the engine is up to normal running temp and the engine is running at less than full throttle, the valve is opened. at this time a certain amount of the Exhaust gas is diverted into the EGR cooler which on the CAYC is buried down the back of the engine, very inaccessible and the labour time to access is why replacing the system is so expensive. Out of the cooler, the gas is mixed back into the air flow around the inlet manifold, and the volume of gas sent is metered to match the Fuel:Air mix the ECU is expecting. The exhaust gas reduces the volume of combustible mix i. the cylinder which thus results in a slower burn that is meant to be a lower temp and helps burn off more of the exhaust emissions. When you lift off the accelerator and coast in gear or go full throttle, the egr valve is closed - Coasting cuts the fuel flow to the engine, thus no emissions so no gases need to be sent back for reburning. When on full throttle, the ECU doesnt care about emissions, and wants to deliver max power and torque available, so wants the "cleanest" fastest burn, so it doeant want exhaust gases reducing that efficiency.
The above is all great when things are working properly. But, EGRs are notoriously bad at doing their job properly so, below is how detect when yours has had enough.
Level 1 - Early signs
While driving your car along at a consistent speed, such as NSL on a motorway, you have to slow down considerably, or stop very briefly, to pay a toll or similar situation. on accelerating back up to NSL you see the glow plug flashing at you. Cycling the ignition makes this go away and your journey continues uneventful, other than the nagging feeeling in your head.
If you have a fault scanner, there will likely be no stored fault, unless you scan while the GP light is flashing. if you do find a code it will relate to the Mass Air Flow Readings. This is most likely a false error caused by the following - When you coast, the egr should close, but if yours is sticking open, it wont and so after Five Seconds (based on my monitoring over time tracking this) the ecu is going to have unexpected gases flowing into the chamber. When you put your foot down again, the ECU will still be finding unexpected "air volume" in the system as EGR should be closed on full throttle too and so will assume bad readings from the MAF and set the glow plug light flashing.
There will be no limp mode nor will there be any loss or change in power delivery.
Cycling the engine will reset that fault code and it wont come back unless you repeat the above circumstances. If you can't safely cycle the ignition, its absolutely fine, just an annoying little light on the dash.
Level 1 is very intermittent, but will over time (1000s of miles) become more frequent. ignition cycling to clear the code seems to also kick it back to normal operation.
NB - an initial check of your brake lights should be done, as a flashing glow plug light on some cars is also an alert that there is a problem with brake light illumination, probably the brake pedal switch. We will assume the brake lights are all working...
I'll add Level 2 shortly, as ive tyoed all of this on my phone, while drinking brandy and eating crisps on a Sunday night...