The problem could be something as simple as a bad light in brake lights grounding out, a fuse under the dash or engine compartment ( check all fuses with a test light) or as complex as the turn signal arm wireing rubbing and grounding out.
From the Ford Dealer
On a 2002 Ford Expedition : The " check engine " light is the " SERVICE ENGINE SOON " light just to the left of the fuel gauge
The left brake light works the right brake light doesn't and the bulbs is good
brake light switch is above the brake pedal
On a 2002 Ford Expedition : Turn your low beam headlights on and then pull the headlight switch out towards you to turn on the fog lights
The 2002 Ford Expedition has a V8 engine.
The 2002 Ford Expedition has 16 valves.
According to the 2002 Ford Expedition owners manual : The fuse panel is below and to the left of the steering wheel , by the brake pedal ( it has a cover on it ) Also , the Power Distribution Box ( which is " live " ) is located in the engine compartment , near the brake fluid reservoir
In order to change the brake light on the Ford LTD Expedition 500 series you must remove the light cover from the back of the lighting panel. You can access this from the trunk. Inside you will find the bulbs for the tail lights and brake lights. Remove the light and replace it with a new one.
Remove the tail light housing and replace the bulbs.
There might be a break in the wiring.
The 2002 Ford Expedition has a 4-speed automatic.