2. If the pre-heater starts to operate incorrectly, it can be checked with sufficient accuracy only by trial replacement of the components that raise doubts. However, you can perform some preliminary checks.
3. Disconnect the main supply wire from the corresponding spark plug (see photo).
4. Connect a voltmeter between the power wire and ground, making sure that the wire does not touch the engine or the car body. Have an assistant turn on the preheater and check that the voltmeter reads 12 V for a few seconds before the system shuts off. Typically, at an air temperature of +20°C, voltage should be supplied to the spark plugs for 7 seconds. This time decreases at higher air temperatures and increases at lower temperatures. If the voltmeter does not show any voltage at all, the reason for this may be either in the relay or in the wiring. Turn off the ignition.
5. Connect an ammeter between the positive terminal of the battery and the wire connecting the glow plugs. Check that after 20 seconds each spark plug draws 12 A of current (i.e. only 48 A if all 4 spark plugs are working).
6. If one or more spark plugs are not drawing current, disconnect the wire connecting the spark plugs and test them individually, or use an ohmmeter to test the spark plugs for continuity and ensure that they have the same resistance.
7. When finished, connect the main power wire.
Pre-start and post-start heaters (the latest XUD9 engines)
1. On all engines, glow plugs operate for approximately 7 seconds before starting the engine. (The exact operating time of the spark plugs depends on the air temperature under the hood).
2. The after-heater installed on some newer engines ensures spark plug operation for an additional 3 minutes after engine start-up (provided the load lever switch contacts are closed and the engine temperature does not exceed 60°C). This reduces engine noise and exhaust smoke after a cold start.
3. Adjustment of the load lever switch see. in the corresponding section.
4. Temperature control is carried out using a thermal switch located behind the fuel filter housing (the thermal switch can be identified by its pinkish-purple plastic ring).
Source available here [peugeotbook.ru]