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Home / DTC / U0100 — A faulty PCM, A problem with the control module circuit, A problem with the CAN bus

U0100 — A faulty PCM, A problem with the control module circuit, A problem with the CAN bus

Detailed page for trouble code U0100.

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Code

U0100

AUDI U — Network/User

A faulty PCM, A problem with the control module circuit, A problem with the CAN bus

Brand: AUDI
Type: U — Network/User
Views: UK: 55 EN: 80 RU: 75
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Page language: EN

Causes

  • Failed or intermittent PCM/ECM
  • Open, shorted or high-resistance CAN bus wiring (CAN_H/CAN_L)
  • Poor power or ground to the PCM (blown fuse, bad ground, corroded connector)
  • Disconnected, corroded or damaged connector pins at the PCM or junctions
  • Missing or incorrect termination (open or shorted 120Ω resistors)
  • Software/firmware mismatch or corrupted PCM software

Symptoms

  • No communication with PCM on an OBD-II scanner (scanner shows 'NO COMM' or times out)
  • Multiple modules show communication-related DTCs
  • Engine may not crank or start, or exhibits limp-home mode
  • Malfunction Indicator Lamp (MIL) or other warning lights
  • Intermittent faults or total loss of drivetrain functions
  • Reduced or no data from PCM on live-data stream (RPM, engine load, sensors)

What to check

  • Verify battery voltage and condition (12.4–12.8 V at rest). Check while cranking if needed.
  • Scan all modules and note additional U-codes; record freeze-frame and freeze DTC times.
  • Visually inspect PCM connectors, wiring harnesses, junction blocks and nearby areas for damage, corrosion or water intrusion.
  • Check relevant fuses and relays powering the PCM and CAN network.
  • Measure power and ground at the PCM (key ON) — good ground and stable 12V supply required.
  • Measure CAN_H and CAN_L with a multimeter and/or scope at the PCM connector and at other network nodes.

Signal parameters

  • CAN bus idle voltages (approx): CAN_H ≈ 2.5 V, CAN_L ≈ 2.5 V (recessive).
  • Dominant state: CAN_H ≈ 3.5 V, CAN_L ≈ 1.5 V (differential ≈ 2 V).
  • Termination resistance: ≈ 60 Ω measured between CAN_H and CAN_L (ignition OFF).
  • Typical CAN data rates: commonly 500 kbps (powertrain) or 250 kbps on some networks — verify OEM spec.
  • PCM message periodicity: many powertrain frames every 20–200 ms (varies by model and signal).
  • If scanning: no response or timeouts from PCM, or bus-off / excessively high error counters.

Diagnostic algorithm

  1. Confirm the code: clear DTCs and attempt to re-create; verify U0100 returns and note any related codes.
  2. Check battery voltage and PCM power/ground circuits. Replace blown fuses and repair poor grounds before further testing.
  3. Scan all modules. Identify other U-codes indicating which part of the network is affected.
  4. Visually inspect PCM connector and nearby harness for corrosion, water damage, broken pins, or chafing. Repair or protect as needed.
  5. Measure CAN_H and CAN_L at PCM connector and at other nodes with ignition ON. Look for stuck dominant (one line held low/high) or missing differential.
  6. With ignition OFF, measure resistance between CAN_H and CAN_L across the network — expect ≈60 Ω. If infinite or very low, isolate sections to find open or short.
  7. Use a scope to verify proper differential signaling while cranking/running. Look for noise, missing frames, or bus-off conditions.
  8. Disconnect suspect aftermarket devices and modules one at a time to see if communication is restored.
  9. If wiring and connectors check good, check for available OEM reflash or programming updates for the PCM. Reprogram PCM per OEM procedure if indicated.
  10. If all else fails and wiring/terminals are verified, consider PCM replacement as last resort; follow OEM procedures for programming and initialization.
  11. After repair, clear codes and verify full network communication and proper operation during a road test or extended monitoring.

Likely causes

  • Blown fuse or loose battery/PCM power/ground
  • Damaged connector at PCM or a nearby splice (water intrusion or corrosion)
  • Short to battery or ground on CAN_H or CAN_L from harness damage
  • Faulty PCM (less common than wiring/connector issues)
  • Aftermarket devices (alarms, chargers) interfering with CAN
  • Missing/failed termination resistor or bus short causing bus-off

Fault status

⚠️ Status
U0100 — Lost Communication with ECM/PCM: CAN bus or PCM communication fault.
🟡 Repair difficulty: Medium
⏱️ Diagnostic time: 1.0-3.0 hours

Similar codes

U0101 U0121 U0073 U0155 P0600 U0422
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