CAN network

U1000 — Class 2 / CAN Communication Malfunction

Severity: medium Drive with care, to a shop

What U1000 means

The control unit detected a failure in data exchange on the vehicle network. Modules normally share messages continuously over a Class 2 or CAN bus, and when those messages are missing or corrupted, U1000 is stored. It is a generic network fault, not a pointer to one specific part, and its exact definition varies by manufacturer.

Symptoms

  • Several systems stop working at once
  • Erratic instrument cluster messages
  • Multiple warning lights on together
  • Some comfort features or modules inoperative
  • Scan tool cannot communicate with some modules
  • Hard starting or odd shifting

Possible causes

  1. Weak battery, low voltage or loose terminals
  2. Open, shorted or damaged CAN/Class 2 wiring
  3. A failed module on the bus disrupting communication
  4. Corroded or loose pins in the diagnostic link connector (DLC)
  5. Poor ground or unstable power supply to a module
  6. Faulty aftermarket accessories or alarm wiring
  7. Module software fault

How to diagnose

  1. Test battery voltage and charging output, and clean and tighten terminals
  2. Scan all modules and note which ones do not respond
  3. Inspect the OBD-II connector for corrosion or spread pins
  4. Check fuses, power and grounds for the silent modules
  5. Measure resistance between the CAN lines (about 60 ohms on a healthy bus) per the factory wiring diagram
  6. Inspect harnesses for damage, moisture, prior repairs and add-on wiring
  7. Unplug suspect modules or accessories one at a time and watch for communication to return
  8. Check for available software updates for the modules

Common fixes

  • Charge or replace the battery and clean the terminals
  • Repair or replace damaged wiring or connectors
  • Clean or replace the diagnostic connector
  • Replace or reflash the faulty control module
  • Remove improperly installed aftermarket equipment

U1000 on different brands

Chevrolet · manuals
A communication fault was detected on the CAN data bus between the vehicle's control modules.
Nissan · manuals
The control module detected improper operation or a signal malfunction on the CAN communication line.
PSA
A CAN communication fault between the automatic transmission ECU (AM6/AT6 on Peugeot) and the other network ECUs; the same units are also used on Citroën models.

Questions and answers

What does U1000 mean?

It is a generic code for lost communication between modules on a Class 2 or CAN network. It points to a network problem rather than one part, so wiring and modules need to be tested.

Can a weak battery cause U1000?

Yes. Low voltage makes modules behave erratically and drop off the network, so check the battery and terminals first.

Is it safe to drive with U1000?

Sometimes, briefly and carefully. If the engine, transmission, brakes or airbags are affected, have it checked right away.

This is general information about the code. The meaning can differ between makes and models: a scan tool and the repair manual for your car will show the exact cause.