For many years my 2006 XC90 V8 has frequently shown the "fault in car’s emissions system" warning light (the orange light has the shape of an engine), and it has turned on and off without apparent cause after driving a few miles. Sometimes the warning light does not show up for months and thousands of miles. However, since last week it turns on whenever the engine is on. Does anyone have any idea of what sensor might be causing it?
Thanks.

 

start by having an OBD2 reader to scan for fault codes. Most likely you have an issue with the evap system - typically it can be the purge valve or the J-hose in the rear near the evap cannister but the codes will help you know what is setting off the CEL. Autozone etc will lend you a scanner

 

Thanks mt6127. The OBD reader showed code CCM-0040 Air Quality Sensor internal fault.. I've ordered a new sensor to replace the falty one.