October 2021

BY Damien Coleman, Product Manager/ EBI Specialist at Snap-on A 2010 Volkswagen Polo with a 1.6 litre common rail diesel injection engine (engine code – CAYC) was reported to have an engine management light and glow plug warning light both illuminated. A diagnostic scantool was connected to the vehicle’s data link connector and the following error code was stored in the electronic control module: P2455 – Diesel particulate filter pressure sensor, short to positive or open circuit. This fault code can be caused by a number of factors: Short circuit in the wiring between the sensor supply voltage (5 volts) and the signal wire Open circuit on the ground circuit of the sensor Faulty exhaust gas differential pressure sensor. A volt meter was connected to the signal wire of the sensor and a voltage of 4.9 Volts was measured. The simplified diagram as seen in Fig.1 shows the circuit layout. The signal wire (pin 3) has a voltage of ≈5 volts applied to the sensor signal circuit from the engine control 30 AFTERMARKET OCTOBER 2021 TECHNICAL/SNAP-ON www.aftermarketonline.net VOLKSWAGEN POLO: ENGINE MANAGEMENT LIGHT ILLUMINATED AND GLOW PLUG LIGHT FLASHING module. The sensor is a piezo-resistive measuring cell with integrated evaluation electronics. Pinpoint When the differential pressure is low, the voltage is ‘pulled’ closer to ground potential as the differential pressure across the filter increases the output voltage increases proportionally. The engine control module uses this voltage signal to determine the soot loading of the diesel particulate filter. Increased soot loading will cause an increase in exhaust gas pressure upstream of the filter. This is referred to as closed loop monitoring. As can be seen from the fault code a, number of electrical tests must be conducted to pinpoint the actual fault. After the initial voltage test at the sensor signal wire (4.9 volts), the next test was to validate the ground circuit. The ground for the differential pressure sensor is shared with the turbocharger boost pressure sensor and the transmission neutral position sensor, so a fault here was highly unlikely as there were no other fault codes stored in the Engine ECM, however the circuit was tested and proved Alerted by the dreaded EML, Damien gets to work on a problematic Polo