OBD-II Code · Network
U0100
Lost Communication with ECM/PCM
Severity
critical
Highest on our scale. Expect major repair cost if it is ignored.
Can you drive it
Not for long
Driving on this one risks further damage. Get it to a shop rather than carrying on with it.
Indicative cost (USD)
$100–$2,000
Varies by vehicle, diagnosis, parts, and local labor. This is not a repair quote.
A module can't communicate with the engine control module.
How a shop works this
Work from evidence to part, not the other way round. A code names a circuit or a symptom; it does not name the part that failed.
Note every code that is set, not just this one.
Codes that appear together usually narrow the cause faster than any one of them read alone.
Read the freeze frame — the conditions the fault was recorded under.
Cold or warm, idle or load, road speed and engine speed. A fault that only appears cold is a different fault from one that only appears under load, even with the same code on the scanner.
Confirm the fault is current rather than history.
Clear the code and see whether it returns under those same conditions. A code that does not come back may have been logged during a repair, a flat battery, or a bad connection that has since been disturbed.
Then work the likely-causes list in the order shown.
That order is what a shop checks first for this code — wiring and connections before the component they run to, because a connector costs nothing to inspect and is a common answer.
If the code comes back after a repair, the underlying fault is still there. That is the point to bring in a scanner with live data and freeze-frame, or a shop that has one.
Common symptoms
- Multiple warning lights
- Engine may not start
- Gauges inoperative
Likely causes
- Failed PCMMost common
- CAN bus wiring issue
- Short circuit
Read the full diagnostic procedureU0100 means the powertrain control module (PCM) has stopped responding on the CAN bus for longer than the gateway timeout window (typically 250-500ms…
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 patterns2007-2012 Dodge Ram and Jeep Grand Cherokee fail at the TIPM (Totally Integrated Power Module) where the fuel-pump relay and CAN gateway share a boar…
- 2007-2012Dodge 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.
- 2003-2007GM 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.
- 2004-2010Ford F-150 5.4L sees PCM ground G104 corrosion behind the left fender liner causing intermittent U0100 with cranks-no-start.
- 2007-2013Toyota Tundra/Sequoia 5.7L sees U0100 from chafed CAN wires under the intake manifold near the alternator bracket.
Estimated repair$40 to $1800.
Look up another code
U0100 — frequently asked
What does a U0100 code mean?
A module can't communicate with the engine control module.
What causes a U0100 code?
Common causes: Failed PCM; CAN bus wiring issue; Short circuit.
What are the symptoms of a U0100 code?
Typical symptoms: Multiple warning lights; Engine may not start; Gauges inoperative.
Is it safe to drive with a U0100 code?
Not for long. Driving on this one risks further damage. Get it to a shop rather than carrying on with it. Estimated repair: $100-$2,000.
How do I diagnose a U0100 code?
Work from evidence to part, not the other way round. A code names a circuit or a symptom; it does not name the part that failed. 1. Note every code that is set, not just this one. 2. Read the freeze frame — the conditions the fault was recorded under. 3. Confirm the fault is current rather than history. 4. Then work the likely-causes list in the order shown.
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