The mustang was sporting a "Service Engine Soon" light for several weeks; I finally decided to take care of it this weekend.
Go to Checker (or whatever they call themselves now) and get the code read. Code = PO401.
Hmmn I don't recognize that one, and the Checker dude knows nothing about cars. Seriously, I had to show him how to use the code reader. I tell the guy "no worries, I'll see what the internet has to say, that is why god invented Google." Took him a second before he realised it was funny.
The interwebs say this is the EGR valve insufficient pressure code. They also say that dispite the name of the code. It has nothing to do with the actual EGR valve, but is either the EGR solenoid, or more likely the DPFE sensor.
Car runs perfectly, so most likely the sensor.
Wait two weeks to get around to going back to the parts store. Buy DPFE sensor. Explain to parts guy what a DPFE sensor is. Tell guy, no not the EGR solenoid, the Differential Pressure Feedback EGR sensor. *sigh*. "just type DPFE into your computer ok?"
I get the sensor and it looks identical to the old one, only twice as big. Hmmn. Well, the plug is right, and the pressure lines are the right diameter... If it doens't work I can always return it.
Install in about 2 minutes (bare hands no tools needed) and a day or so later the light went away.
Though I really wish the auto parts stores would hire someone who knew something about auto parts.