We were recently asked to investigate a fault on a 2015 Corsa E 1.4 petrol, the customer reported that when idling the ESP light would illuminate. Upon driving the vehicle into the workshop I immediately noticed the engine was a little erratic at idle but became much smoother when part throttle was applied.
The ESP light was indeed illuminated but not the ABS lamp so the first thing to do was to interrogate the brake module which had an error code set U0401 (invalid data received from PCM). This code is set as a result of an engine management issue so the next step was to check this system. The PCM had logged an error code P0171 (system lean) this code can be set for a number of reasons but as I noticed earlier the engine was erratic at idle suggesting a possible air leak.
I then monitored the oxygen sensor signal whilst idling which was stuck firmly on zero volts, this type of sensor (zirconia) should fluctuate between zero to one volt if the the air/fuel ratio is correct (14.7:1). Once I applied part throttle the sensor began to switch indicating it was functional.
The checks so far would appear to indicate an air leak predominantly at idle, my initial inspection did not reveal any obvious external air leaks so I began to isolate potential causes of an internal leak, I.E. brake servo, breather system etc. I eventually traced the fault to a leaking fuel tank vent solenoid (see picture). A new valve was installed and a fuel trim reset carried out, the engine now ran smoothly at idle with the oxygen sensor switching correctly.