I'm not questioning your specialty. The O2 sensor was just an example. All I'm saying is that the code that appears on the code reader is not the be all and end all of automotive diagnostics as most technicians I have encountered perceive it to be. It is not replacement for sound mechanical judgement and experience. It can only aid you.

And in my O2 sensor example, yes the O2 sensor was fouled when the O2 sensor was bench tested. So they put in a new one. But after a while O2 sensor fouled again. The code pulled only showed O2 sensor failure. Amateur mechanic had to pour over thick books and run the algorithms in the shop manual like crazy. Experienced dude knew what usually caused O2 sensor fouling in particular vehicle. Rectified problem. Job done.

And a loose fuel cap is a VERY common cause of CEL warnings (esp. if you are running OBD II or similar ECU set-up). So why not try it first diba? It does not cost anything and is very easy to rectify. Hindi yung iaasa agad sa code puller. Mapapagastos pa ang client.