My 2003 Ford Expedition runs rough at idle and the service light started blinking.

Sounds like a tune up is due.

run a teat. Remove the vacuum line from the brake booster and plug off. Start engine and see if it idles normally. . Put your foot on the brake pedal and apply the brakes . If the idle is still normal replace the brake booster as there is an air leak in the diaphram.. The service light is probably recording misfiring and air fuel mixture problems.. run fault codes any way just to check it out .

Google the codes you have in the pcm
& then you will know what system has a
possible failure

Same thing everyone has been doing for
decades every time a check light comes on

The flashing and rough running is to to a misfire or misfires cause by arcing due to moisture on ignition components. Eventually these misfires can lead to other problems such as damage catalytic converter. Start by identifying which cylinder(s) is/are misfiring. The arching can be reproduced with a spray bottle filled with soapy water. Spray the ignition components (wires, coils, distributor cap...)and look for the vehicle to idle rough. Replace any faulty component.

If the check engine light is on I would check for trouble codes first.
If not, look for a vac leak or low fuel pressure.

There could be many issues. These codes read your engine is running too lean. Check : all your vacuum hoses for leaks, PCV valve and hose, MAF sensor. This may require an experienced technician to figure this out. Hopefully I help'd!!

running rough means it is misfiring.does it run smoothly without load or at idle?If it does then better check you fuel pressure as the engine needs more fuel under load.

idle air control motor?

Had same problem: cutting out at low rpm, jerking, a huge clunk or shudder at higher speed (like transmission was going out), and rough idle. All these symptoms were intermittent.

CHECK THE FUEL FILTER!! Changed the fuel filter and thing runs fine!!

PS mine 2003 expedition has 105k