My 2003 Expedition EB (5.4L Triton - 155,000 miles) began flashing "Service Engine Soon" today. Any ideas?

 

This light can come on for many reasons.

You need to get an OBD tester to identify the exact reason. Many auto stores will loan these out to test for specifics.

 

Flashing means you have a misfire that is sever enough to destroy the catalytic converters.

 

I think flashing means there is a code pending. Mine did this last month. I could feel that it was a misfire so I drove it for another few miles until the light stayed on, meaning the code was hard set. Then I had it read so I knew which cylinder was misfiring. I didn't have time to trouble shoot parts so I replaced both the spark plug and COP on the misfiring cylinder...fixed.

 

Nope. When the light stopped flashing and stayed on steady, it meant that the misfire was no longer sever enough to cause CAT damage.

 

OBD-II Background Information Engine Light

Tell me about that "Check Engine Light".

The service industry calls the Check Engine light on your dash an "MIL" or Malfunction Indicator Light. It shows three different types of signals. Occasional flashes show momentary malfunctions. It stays on if the problem is of a more serious nature, affecting the emissions output or safety of the vehicle. A constantly flashing MIL is a sign of a major problem which can cause serious damage if the engine is not stopped immediately. In all cases a "freeze frame" of all sensor readings at the time is recorded in the central computer of the vehicle.

 

Thanks, informative. I guess in my situation it was an "occasional flash", indicating a momentary malfunction. Reading from your link, the next paragraph after your quote talks about hard failures causing the light to stay on. I guess I always interpreted this as a "hard set" code.