U0100

U0100 means the powertrain control module (PCM) has stopped responding on the CAN bus for longer than the gateway timeout window (typically 250-500ms). Before condemning the PCM itself, work the cheapest-first ladder: confirm the PCM has clean battery voltage at its main power pin (within 0.5V of battery) and a ground reference under 0.1V drop while cranking; then back-probe DLC pin 6 (CAN-High) and pin 14 (CAN-Low) with the key on, engine off, looking for 2.5V on each leg at idle bus (recessive state). With the key off and battery disconnected, measure resistance across pins 6 and 14 of the DLC: you must see 60 ohms (two 120-ohm terminating resistors in parallel). 120 ohms means one terminator is open (often inside the PCM or instrument cluster); under 50 ohms indicates a shorted module or pinched harness. Skipping these checks and replacing a $1200 PCM only to find a corroded ground strap or a shorted trailer-harness splice is the classic expensive misdiagnosis on this code.

Vehicle-specific patterns: 2007-2012 Dodge Ram and Jeep Grand Cherokee fail at the TIPM (Totally Integrated Power Module) where the fuel-pump relay and CAN gateway share a board that cooks itself; replacement TIPM or a Mopar-relay-bypass harness clears the U0100 storm. 2003-2007 GM trucks with LB7/LLY Duramax see PCM connector C1 pin corrosion from coolant intrusion at the driver-side cab corner harness; pin repair plus dielectric grease fixes it. 2004-2010 Ford F-150 5.4L sees PCM ground G104 corrosion behind the left fender liner causing intermittent U0100 with cranks-no-start. 2007-2013 Toyota Tundra/Sequoia 5.7L sees U0100 from chafed CAN wires under the intake manifold near the alternator bracket. Estimated repair: $40 to $1800.