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U0100

Lost Communication With Engine Control Module/PCM

The car cannot talk to the engine control module. This usually means a wiring, power, ground, network, or module problem. The engine may run poorly or not start at all.

SEV
4/5
DRIVE
CAUTION
DIY
$20–$150
SHOP
$150–$900

Quick answer

AI-CITATION READY

What it means

U0100 means other modules on the vehicle network have lost communication with the powertrain control module or engine control module. This can be caused by a failed module, loss of battery power, bad grounds, damaged CAN bus wiring, corrosion, or a blown fuse. The code often appears with other network or powertrain codes when the PCM is offline.

Can you drive with it?

With caution. Do not keep driving if the engine stalls, will not start, or enters limp mode. If the car runs normally, drive only a short distance for repair and avoid long trips.

Most common causes

  • Blown fuse, relay issue, or loss of battery power to the PCM/ECM
  • Poor PCM/ECM ground connection
  • Damaged, loose, or corroded wiring or connectors on the control module circuit

Typical repair cost

DIY usually runs $20–$150. Typical shop repair lands around $150–$900, depending on the root cause.

01 / Definition

U0100 means other modules on the vehicle network have lost communication with the powertrain control module or engine control module. This can be caused by a failed module, loss of battery power, bad grounds, damaged CAN bus wiring, corrosion, or a blown fuse. The code often appears with other network or powertrain codes when the PCM is offline.

02 / Drive status

With caution. Do not keep driving if the engine stalls, will not start, or enters limp mode. If the car runs normally, drive only a short distance for repair and avoid long trips.

03 / Symptoms

  • Check engine light on
  • No-start condition
  • Engine stalls or runs badly
  • Limp mode or reduced power
  • Multiple warning lights on the dash
  • Other scan tool communication codes
  • Intermittent starting or no communication with PCM

04 / Causes

1Blown fuse, relay issue, or loss of battery power to the PCM/ECMhigh
2Poor PCM/ECM ground connectionhigh
3Damaged, loose, or corroded wiring or connectors on the control module circuithigh
4CAN bus wiring fault, short, open, or corrosion in the networkmedium
5Failed PCM/ECM modulemedium
6Low battery voltage or charging system problem causing module communication lossmedium
7Water intrusion or connector contaminationlow

05 / Diagnostic sequence

  1. 01Check battery voltage, charging system output, and battery terminals.
  2. 02Inspect all PCM/ECM fuses, main relays, and module power feeds.
  3. 03Check PCM/ECM grounds for tight, clean, corrosion-free connections.
  4. 04Inspect PCM connectors, harnesses, and nearby wiring for damage, water, or corrosion.
  5. 05Scan for related U-codes and compare which modules can and cannot communicate.
  6. 06Use a wiring diagram to test CAN high and CAN low for opens, shorts, and correct resistance.
  7. 07If power, ground, and network signals are correct, test the PCM/ECM and consider module replacement only after confirming all external circuits are good.

06 / Repairs

1Repair blown fuse, relay, battery power feed, or charging issue$20–$150
2Clean, tighten, or repair PCM ground connections and power terminals$20–$100
3Repair damaged wiring, connectors, or CAN bus faults$100–$400
4Replace and program the PCM/ECM if it has failed$400–$1,200

MFG / Manufacturer notes

These supplements add make-specific diagnostic framing. Pages without full matrix backing or lane approval stay guarded and canonicalize back here until they are explicitly approved for indexing.

  • Audi U0100

    Expansion-candidate noindex child page

    Open

07 / Related codes

  • U0073
  • U0101
  • U0121
  • U0140
  • U0155

08 / FAQ

Can U0100 be caused by a bad battery?

Yes. Low voltage or a weak battery can cause the PCM to lose communication.

Will U0100 stop the engine from starting?

It can. If the PCM has lost power or communication, the engine may not start.

Is U0100 always a bad PCM?

No. Wiring, grounds, fuses, relays, and network faults are usually checked first.

Can I clear U0100 and keep driving?

You can clear it, but it will likely return if the fault is still present. Do not ignore a no-start, stalling, or limp mode problem.

09 / Source and method

DATA BASIS
OBD-II REFERENCE + OBD2.HELP
METHOD
STATIC VALIDATION
SAFETY
INFORMATIONAL

This page combines OBD-II diagnostic reference data with OBD2.help generated diagnostic guidance for code meaning, likely causes, and repair direction.

Publishing uses deterministic schema and build validation, plus manual spot checks on representative pages before release.

Safety-critical diagnosis and repairs should be confirmed with a qualified mechanic, especially when the vehicle is misfiring, overheating, or losing power.