Auto zone for error check?

[gracefilledfool]

Has anyone used Auto Zone for an error check on Check Engine light?  I hear they do it for free.  

Here is why I am asking... I was returning from a 2100 mile trip and about 150 miles out it when the engine light came on.  I checked the fuel cap... still no luck... it burned on.  I suspect that it might be the air mass meter on the intake... car has been hard to start on occasion.
What do you all think?

[vhyman]

My experience is that once the check engine / lamda light is triggered - say by a loose gas cap - it will remain lit until reset, even if the problem - retightening of the gas cap - has been solved.

[P Hartmann]

I just today had a code read at autozone using their scanner.
My error was P1603 and I had them delete. Using data from alldatadiy.com, I found this is the 91A7 volvo code: throttle mechanics, high friction fault.  I suppose this means the trottle body needs cleaning.  99V70XC at 51K.