Routine Repair

A Vauxhall Astra with a mystery fault

Published:  24 June, 2014

By Andy Crook

A 2008 Vauxhall Astra had defeated a few local garages, and we agreed to take a look. It appeared in very good condition and looked to have been well looked after. A quick visual offered little evidence other than a new heated exhaust gas oxygen (HEGO) sensor. The symptom was simply a lack of performance and the MIL was illuminated.

Armed with the knowledge that a new sensor had been fitted, it made sense to check the wiring. HEGO sensors require a heater circuit to enable fast response and short warm up cycles.

The codes suggest a fault with both pre and post cat sensor heater circuits. A quick check with a meter showed no voltage present on either of the two heater wires. I disconnected the sensor and tried again, now I had a live feed to one of the wires but it was the wrong one. I needed to check the wiring diagram.

The engine wiring harness appeared to be correct, but it did not correspond to the HEGO sensor wiring. I ordered a new sensor as this would give me the chance to check the wiring with the one fitted. While I waited for it to arrive I looked into the P2182 code this suggested a fault with the coolant temperature sensor. The Scan tool live data Voltage readings agreed with the meter readings.

By now the replacement HEGO sensor had arrived proving that the wiring was indeed different to the one that had been fitted. But the plug was the same.

Now the only code left was the dreaded undocumented code.

With no MIL light on the dash and normal performance restored, the car could be returned with the confidence that the diagnosis and repairs had fixed the faults.

From wiring diagrams: Incorrect oxygen sensor fitted and more than one coolant temperature sensor fitted.

Correct oxygen sensor fitted.

Live data, reporting normal activity.

Codes cleared and no fault codes stored after test drive proved normal performance.

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