Check engine light flashes on 99 Ford Expedition, hooked it up to computer and says plug # 3 is bad, I replaced it but the light is still on

Is the check engine light flashing? Have you determined the cylinder is getting fuel and the injector is ok? Does the cylinder have good compression? Is there any intake vacuum leak? Are any other fault codes present when you run a fault code test?

Have you checked the plugs and the rest of the ignition system ?
What are the trouble codes stored in the computer ?

If the check engine light is flashing, there is a serious problem that is affecting the performance of the engine.
You need to get the computer scanned for trouble codes to proceed.

Depend on mileage on them. Personally I don't run plugs more than about 50-60K regardless of what the owner's manual says.

Pull plug three and see what it looks like: fouled (dark oily residue, bent electrode, etc. If electrodes seem worn from use and mileage, replace plug--you'll see scooped out area on outer electrode if excessively worn, clear code and see it it runs ok. If it codes again it could be a bad plug lead as well. If code returns then check out lead to plug or replace it. Plug 3 will be on the left bank of the engine third one from the front. Plug order is:

Back

4 8
3 7
2 6
1 5

Front

Firing order 1-3-7-2-6-5-4-8

A flashing CEL means that you might be causing damage to your CATs. It will flash when the PCM senses a misfire bad enough to cause fuel to dump into the exhaust. When that happens lay off the throttle. Chances are you've got a bad injector or bad plug.

Use a code reader to know the real reason that check engine light come on.

A flashing CEL means that you might be causing damage to your CATs. It will flash when the PCM senses a misfire bad enough to cause fuel to dump into the exhaust. When that happens lay off the throttle. Chances are you've got a bad injector or bad plug.

Use a code reader to know the real reason that check engine light come on.

Check for something draining the battery. It could be something like a switch on an under bonnet light or boot light sticking

you probably got a bad coil. The 99 5.4 has individual coils one could be misfiring, but you will have to hook it up to a computer.

My ford expedition 99 has service engine soon light on. Hooked up to ODB and the equipment showed nothing; no power; checked on different car worked fine. what is cause. Equipment has worked on another ford expedition before by the way.

I just worked on 98 expedition 5.4l had code PO354. This code refer to an open primary coil winding, open wire, or bad PCM.
Check primary winding with ohm meter should be from 0.5 to 1.5 ohms and about 2.5 ohms with a hot engine. check wiring with a noid light, run the engine and shake the harness if light goes out repair harness you should have battery voltage with key on the computer controls the ground side.