The spark plugs can be checked using a voltage indicator with an LED.
1. With the ignition off, disconnect the glow plug connectors.
2. Connect the indicator wire to the "+" terminal of the battery.

3. Touch the indicator light to the 2 glow plugs 1 one by one. In each case, the indicator LED should light. If the LED on any spark plug does not light up, it means the spark plug is faulty and needs to be replaced.
4. Install a new spark plug and tighten it to 15 Nm.
(Data taken from this portal: SKODABOOK.RU)
