Proof isn't always positive

An Astra suffering from judder at large throttle is interrogated to see if the DTC rings true

Published:  11 June, 2015

By Andy Crook

The smoking gun of vehicle diagnostics is the fault code. It is the signpost of component failure except when it isn't; this is when the skills and knowledge of the technician are required. Symptoms and faults are often so strongly linked to the code, it can be difficult to resist the temptation to just order the part that the code is suggesting has failed. Rushing into ordering and fitting parts often costs more than testing the complete circuit.

Figure 1

Fault code descriptions are clues to the nature of the failure or the setting criteria, the line in the sand that needs to be crossed to set the code, but is this sensor circuit range/performance code a mechanical failure, an electrical component (sensor or actuator), wiring or a logic fault? There is no way of being sure from just the DTC. Live data confirmed the fault; with the key on, engine running, the output was 102Kpa. This is higher than expected for idle conditions.

The Astra was driven on a chassis dynamometer to hold a fixed load under controlled conditions, the fault was apparent under boost conditions, wide throttle openings, and once the code was triggered, the ECU went into a safe operating mode to prevent further damage. This 'limp home mode' prevented anything but wastegate pressure from the turbo.

Comparing the output from the ECU using live data, the fault occurred when the ECU reported a voltage output of 0 Volts, a gauge was used to check the manifold pressure which was 138Kpa.

So the DTC description appears to be spot on, it is indeed a range/performance fault, but is it the sensor or the wiring? It can't be the actuator, in this case the boost solenoid, as the gauge has already proved the turbo is capable of providing positive manifold pressure.

Figure 2

This means the test must be carried out at the ECU to ensure there are no wiring faults causing the erroneous data. The voltage at the ECU matched the sensor output exactly. Note the use of voltage, not continuity, to prove the circuit integrity. In 'Through the Keyhole' fashion, let's look at the evidence:

DTC for boost pressure sensor circuit range/performance

Figure 3

Figure 2

Figure 3

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