1st check for spark by removing one of the spark plugs and plug it back into its coil and rest it on the metal of the engine ( DO NOT hold it on place by hand) then have your assistant turn the engine over whilst you watch the plug. It should give a fat blue spark at regular intervals.
If not then check that there's power to the coil using a meter. If there’s 12V but no spark then check the ECU power supply (fuse and relay) and also the connections to the crankshaft speed sensor.
If there is a spark and its still not starting then its likely that the ECU isn't opening the fuel injectors, if the injectors are working you can hear them click - its difficult to hear them with background noise so use a long screwdriver like a stethoscope, placing the handle end in your ear and the metal tip against the body of the injector whilst your assistant turns the engine over.
If you can't hear anything check the ECU power is present by measuring voltage at the fusebox and on its relay socket if OK then check the connections to the crank speed sensor and consider replacing this sensor if the wiring / connections look OK
If the car will start using a squirt of quick/ easi start into the intake then this confirms the problem is lack of fuel
Also its worth checking that the electric pump that feeds the injectors is working correctly, it should run for a few seconds then switch off when you first turn the ignition on. If you remove the filler cap to the fuel tank and listen inside while your assistant flicks the ignition you should hear it clearly
If the connections to this are corroded or damaged then the pump could stop at any time. Check that its relay switches in and out and the relay contacts are clean and bright – replace the relay if in any doubt and do the same for the fuse, ideally measure the voltage at the pump and check its getting 12V.
You should be getting around 3 bar measured at the injector rail check to see if the plugs are getting wet indicating fuel is flowing – if not and the injectors are opening then there’s a problem with the pump or the filter or even the pressure regulator
Sorry forgot to say its a Seidel.
If the plugs haven't been changed in the last 20K then they may have to be replaced.
You can test the glow-plugs with a multimeter by measuring the resistance across the plug ( between the power connection and the body) it should be around 4-8ohms if its tending to infinity / open circuit the plug is faulty or by using a clip on current clamp, either on the supply wire to glow plugs or onto engine earth, with glow plug light on you should see 15-20amps for each glow plug fitted, 4 cylinders would be approximately 60-80amps.
Also its worth checking that the electric pump that feeds the main fuel pump is working correctly, it should run for a few seconds then switch off when you first turn the ignition on. If the connections to this are corroded or damaged then the pump could stop at any time. Check that its relay switches in and out and the relay contacts are clean and bright – replace the relay if in any doubt and do the same for the fuse, ideally measure the voltage at the pump and check its getting 12V
Also if its not injecting at all then check the connections to the crankshaft sensor, unplug it and check for corroded contacts and broken wiring, and consider fitting a new sensor - do the same for the cam position sensor
If the EGR valve is stuck open then this too can effect starting so try disconnecting it to see if this helps or changes anything
Could also be an injector leaking fuel back through the overspill pipe, so not getting enough fuel pressure to tell ECU to fire the injectors, this is trickier to find and you’ll need the garage to do a ‘run back’ test to identify which one is the issue
Lastly it’s also worth considering fitting a new fuel filter as if only slightly blocked this can lower fuel supply pressure and choke the main pump, the pump will be more sensitive to this when its cold as diesel thickens considerably with low temperatures.