Check engine light on - CAUSED BY BAD FUSE

Tiny
SHM777
  • MEMBER
  • 2004 FORD E-SERIES VAN
  • 4.6L
  • V8
  • 2WD
  • AUTOMATIC
  • 178,000 MILES
The check engine light is on. We've had two shops put their scanner on the truck's plug-in receptacle under the dash, and both said they got no reading at all. Both shops said this is because the fuse for the plug-in receptacle is bad. I cannot figure out what fuse it would be because the owner's manual fuse charts do not say what fuse is for this item. We could unplug and check every single fuse under the dash and under the hood (there are a lot!) But maybe you can help before we do that. Thank you.
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Tuesday, July 28th, 2015 AT 3:33 PM

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Tiny
CARADIODOC
  • EXPERT
You're looking at this the wrong way. Each fuse protects multiple parts of multiple circuits today since everything involves numerous computers, so there's no way they can list everything a fuse affects. In the shop, when you're paying us by the hour, we don't have time to fiddle with every fuse to inspect it visually, and we sure don't have time to sit down with a service manual and figure out which three to six fuses are involved with the dead circuit. We grab a test light and poke it into the two small test-point holes on each fuse. In this way, we can test up to two dozen fuses in a minute.

You need the ignition switch turned on, and the circuit with the problem turned on. Both test points on a fuse should have 12 volts. If you find a fuse with 0 volts on both sides, that circuit isn't turned on and a blown fuse would be irrelevant to the story. What you're looking for is a fuse that has 12 volts on one test point and 0 volts on the other.

Fuses often blow from the current surge when connecting the battery. All that's needed is to replace those. If a fuse blows again, either immediately, or intermittently, I have a trick for working with that.
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Tuesday, July 28th, 2015 AT 4:35 PM

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