Unit-injection - piezo injector (VAG PD): current

The purpose of this test is to check a PD injector current draw under different operating conditions non intrusively.

Connection guidance

Connection for diagnostic work will vary dependent on application.

Technicians should whenever possible gain access to the test circuit without damage to seals and insulation. If this is not possible then make sure appropriate repairs are completed.

General connection advice

PicoScope offers a range of options within the test kits.

Dependent on difficulty of access, choose from:

  1. Breakout leads.
  2. Back-pinning probes.

Testing sensors and actuators (to include relevant circuit/connectors):

  • When testing a sensor, it is desirable to gain access at the control module.
  • When testing an actuator, it is desirable to gain access at the actuator.


Connection for the injectors can be accessed at the round multi-plug on the rear of the cylinder head. It may be necessary to remove the multi-plug and then carefully strip back the loom outer insulation to access the injector cables. Repair the loom insulation once the tests are completed.

How to perform the test

  1. Plug the low amp current clamp into PicoScope Channel A. Set the clamp to the 20 A setting and zero the clamp before connecting to the injector supply circuit.
  2. Start the engine and allow it to idle.
  3. Minimise the help page and with the example waveform on your screen PicoScope has already selected suitable scales for you to capture your waveform.
  4. Start the scope to see live data.
  5. With your live waveform on screen stop the scope.
  6. Turn off the engine.
  7. Use the Waveform Buffer and Zoom tools to examine your waveform.

Note: This waveform has been set up in Auto trigger mode, with a rising-edge trigger. If the waveform does not start with a positive pulse, the current clamp has been connected the wrong way around. Remove it and replace facing the opposite direction.

Waveform notes

These example waveforms show the injector current under different operating conditions.

Example 1: The first injector phase is pre-injection and the second is the main injector phase.

Example 2: As the engine speed is raised, the control module increases the duration of the main injector phase.

Example 3: On overrun, only the pre-injection phase remains.

Waveform Library

There are over 200 injector current waveforms in the Waveform Library. You can refine your search by selecting, make VW, before going to the drop-down window in the lower left corner of the Waveform Library window and selecting injector current.

Example PD-Unit injector


Further guidance

The piezo-mechanical unit injectors are located in the cylinder head and the wiring runs through a circular multi-plug at the rear of the head to the individual injectors. The injectors are unlike common-rail injectors in that the high fuel pressure is generated by an additional rocker-arm running from the camshaft and acting directly onto the injectors. There is no high-pressure pump the injectors can operate across a wide range of pressures from 130 to 2,200 bar.

This system is referred to by the Volkswagen Audi group as Pumpe-Düse (PD).

There may be up to five injection phases:

  • 0 to 2 Pilot phases combat diesel knock by gently starting combustion.
  • 1 Main injection phase for power and torque characteristics
  • 0 to 2 Secondary injection phases to increase combustion temperature during diesel particulate regeneration.

Piezo injectors have a major advantage over conventional electromagnetic injectors, a very quick reaction time up to four times faster.

Piezo injectors contain a stack of about 300 thin ceramic plates. When the switching voltage is applied the plates expand and open the nozzle needle allowing fuel into the cylinder.

Figure 3 shows a piezo element in its two different states. Illustration 1 shows the stack with no supply voltage and illustration 2 shows the increase in height of the stack when the supply voltage is switched on.

Note; Once the stack is in the injector open position, it does not need to have a continuous supply current and remains in the open state until the close command. The open and close commands are the positive and negative current spikes in the waveform. For this reason, never disconnect the injector multi-plug with the engine running. If the injector is in the open state it could hydraulic-lock the engine with diesel fuel.

The engine will cut out if the control module detects any faults with the injectors or driver circuit.

Diagnostic trouble codes

Selection of component-related Diagnostic Trouble Codes (DTCs):

P0251 - Injection Pump Fuel Metering Control "A" Malfunction (Cam/Rotor/Injector)

P0252 - Injection Pump Fuel Metering Control "A" Range/Performance (Cam/Rotor/Injector)

P0253 - Injection Pump Fuel Metering Control "A" Low (Cam/Rotor/Injector)

P0254 - Injection Pump Fuel Metering Control "A" High (Cam/Rotor/Injector)

P0255 - Injection Pump Fuel Metering Control "A" Intermittent (Cam/Rotor/Injector)

P0256 - Injection Pump Fuel Metering Control "B" Malfunction (Cam/Rotor/Injector)

P0257 - Injection Pump Fuel Metering Control "B" Range/Performance (Cam/Rotor/Injector)

P0258 - Injection Pump Fuel Metering Control "B" Low (Cam/Rotor/Injector)

P0259 - Injection Pump Fuel Metering Control "B" High (Cam/Rotor/Injector)

P0260 - Injection Pump Fuel Metering Control "B" Intermittent (Cam/Rotor/Injector)


This help topic is subject to changes without notification. The information within is carefully checked and considered to be correct. This information is an example of our investigations and findings and is not a definitive procedure. Pico Technology accepts no responsibility for inaccuracies. Each vehicle may be different and require unique test settings.

Suitable accessories

  • 30 A (low amps) current clamp


  • 20 A / 60 A DC (low amps) current clamp


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2 comments | Add comment

December 10 2018

Hello Jacob George, firstly apologies for such a delayed reply. We have a guided test for both current and voltage on a VAG pd injector. Go to guided tests, actuators, diesel injectors, VAG PD current voltage and earth. Hope this will answer your question.

Jacob George
March 01 2018

Hi, could you please explain how to test the supply voltage used at the time of expanding the piezo crystal and contract the crystal ?

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Guided test: Unit-injection - piezo injector (VAG PD): current