If you get a "warning" light (not a Check engine light) accompanied by a "check gas cap" light and -- let's pretend -- that the problem with the evap system IS a loose gas cap, will tightening the cap make the warning light go away or do you have to go through the whole code-clearing ritual and start over? Thanks

 

From what i remember if you catch it as a pending code and you fix the cause it will reset on its own after a few ignition run cycles. If it sets as hard code then yes you will have to do the entire clear codes sceario and have them all reset again. Are you getting the small leak detected evap code?

 

Yes, exactly what I'm getting.

 

that one is 9 out of 10 the loose or worn out gas cap. Check the cap o ring real good and if ok re install cap nice and tight. Even put a little pressure when tightening

 

It depends. If it's an OBD code (typically P0xxx or P2xxx) and you fix the cause it will self-clear after about 4 warm ups. You do NOT need to clear it and if you are in a smog-test region do not (as you reset all the monitors so will fail test)! The code stays harmlessly stored (logged) for 40 or 80 warm ups but will not affect any testing (it's kept in case you take the car to a tech and say the MIL was on but not now to give the tech a chance to see why).

Always bear in mind a codes clear can be a really bad idea and think whether you need to do one. Clearing the monitors also suppresses many codes until when/if the monitors set again.

 

Yes, it's a P0456. A "warning" light in dash instruments but not a CEL.

 

It should self-clear (after 4 or so warm ups) so long as the fault is fixed. Jaguar put it in their DTC doc, too.