engine light on,car runs well,sometimes light goes off for days.

yes there is a scan tool that will read out the fault. Does this vehicle have a year/make and model?

Glenn Beasley Chrysler Tech

Many Chrysler cars, particularly 80s/90s allow you to read the check codes by:

-Turn the ignition switch on three times, leaving it on the third time.

-read the codes in two parts.

-the ending code is "5 5".

-the code explanations are in the maintenance book and also on the web.

could be something as simple as an oxegen sensor going bad or a leaky gass cap both will cause this problem and are easy fixes. could also be an air filter issue. try replaceing the oxegen sensor first very cheap fix along with a new fuel filter and that may do it

Also, most Advance Auto parts and AutoZones have the code scanners and will read the code out at no charge....