Basically the fuel system becomes pressurised whilst the engine is running. This pressure is measured by a sensor in the fuel tank.
If the fuel cap does not sit correctly the pressureescapes, the sensor senses this and illuminates the Check Engine light CEL.
Of course the fuel system has many other places where fuel or fuel vapour can leak from with the same results.
You do not have a liquid fuel leak so the leak must be on the hoses that only have vapour in them i.e. go to the char (charcoal) can which collects vapour and gets purgedeverytime the engine is started.

Of course it could be the sensor that is at fault or the connection to it.

Good luck.

Stuart.

 

Thank a million Ohlord. I took the engine cover out to check the wires. I cleaned the MAF and put it back in. The engine light was not and has not come on yet (sometimes on, sometimes off). I have to clear the codes, I beleive to see if it come back.

I found the climate (outside teprature) sensor, it was loose. I lost one of the smaller black bumper grills (driver side). I think there must bea holder place for the the nibbleon the grill.I could not find any place for it around the fog light. And, probably the teprature sensor is not working good because of the missing grill.HBZ

 

I have a 99 E430 and I was just told that my check engine light is on due to an O2 sensor as well. I am trying to pinpoint whether it is an upstream sensor or downstream sensor. Does anyone have any idea on the complexity of labor this may require? Also i priced them andI was quoted roughly 240 for parts not including labor? Does that sound right for replacement of both sensors?

 

what were the codes?where are you located?if 02's they can be purchased for about 90 ea and installed by yourself.
ohlord