If the PCM detects a transfer case problem it would set a DTC and switch the MIL on. The codes are P1804 and up to P1877 and P1891.
Setting the transfer case light on will usually be done by the GEM, either acting on PCM instructions as above or itself detecting a problem. If the GEM detects a problem it will also set a code, but the GEM must be scanned for this code. Ford dealer can do this, but I am not sure if others can too.
Hope this helps.
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Art
2000 Taurus SEL Wagon 3.0 Vulcan (daily driver)
1993 Taurus GL Wagon 3.8 Essex
1991 Mercury Grand Marquis LS 5.0
1986 Chevrolet 3/4 ton pickup 4x4 350 with turbo400 and 208 transfer case
1978 Chevrolet 3/4 ton pickup 4x4 custom 6.2 na diesel with turbo400 and 203 transfer case
Planning 2008 Taurus X in a year
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