U0100 🔴 Critique ⚠️ Do not drive

Lost Communication with ECM/PCM

The CAN bus network cannot communicate with the main engine control module. Often an electrical issue (battery, earth) before a failed ECU.

💷

Average repair cost

£0 (battery jump) — £1200 (ECU replacement)

🔍 Symptoms

  • → Multiple warning lights on at once
  • → Engine won't start
  • → Partially dead dashboard
  • → Electrical glitches

⚙️ Common Causes

  1. 1Dead or failing battery (check this first — most common)
  2. 2Poor earth/ground connection
  3. 3Blown main fuse
  4. 4Failed ECU
  5. 5Damaged CAN bus wiring

🔧 Recommended Fixes

  1. 1Test battery (voltage and condition) with a multimeter
  2. 2Check all earth/ground connections (often corroded)
  3. 3Check main fuses
  4. 4CAN network diagnosis by a professional if battery is OK

🛒 Recommended Parts (Amazon UK)

Car battery 60Ah~£55-90
View on Amazon UK →
Battery and alternator tester~£18-35
View on Amazon UK →
Electrical contact/earth cleaner~£6
View on Amazon UK →

Affiliate links — same price for you, we may earn a small commission.

Frequently Asked Questions

Can I clear this code without fixing it?

Yes, but it will return if the underlying issue isn't fixed. Clearing a code only tells you if the problem is permanent or intermittent.

Will this code fail an MOT?

Yes — this code will likely cause an MOT failure. Get it repaired before your test.