A code that signals the Engine Control Module (ECM) or Powertrain Control Module (PCM) has lost communication with another module on the vehicle’s CAN bus network. On Jaguar models, this often points to a wiring fault, a failing module, or a blown fuse. Ignoring it can lead to no-start conditions, erratic behaviour, or complete vehicle shutdown.
U0100 means the vehicle’s Engine Control Module (ECM) or Powertrain Control Module (PCM) has lost communication with another module on the CAN bus. On most Jaguar models - from the Ingenium AJ200D to the AJ133 V8 - this is typically caused by a wiring fault, corroded connector, blown fuse, or a failed module. The fix can range from a £50 fuse or relay replacement to a £500+ module replacement or wiring repair.
Do not ignore U0100. A loss of communication can leave you stranded or cause intermittent faults that are hard to trace. Immediate diagnosis is recommended.
In a modern Jaguar, multiple electronic modules communicate over a Controller Area Network (CAN) bus. The Engine Control Module (ECM) - also called the Powertrain Control Module (PCM) - is the master controller for engine functions. U0100 is a generic OBD-II code that indicates the ECM/PCM has lost communication with another module on the CAN bus. This could be the Transmission Control Module (TCM), Anti-lock Brake System (ABS), Body Control Module (BCM), or any other module. The code is set when the ECM does not receive a message from a specific module for a certain period. The underlying cause is almost always electrical - a wiring break, poor ground, blown fuse, or a failed module. Unlike mechanical fault codes, U0100 is a network issue, and it can affect multiple systems, causing a cascade of warning lights and loss of functions.
If the engine dies while driving, the vehicle may be unsafe to continue. Pull over safely and arrange recovery.
Based on real diagnostic patterns from Jaguar specialists. The order reflects both likelihood and typical repair cost.
| Cause | Likelihood | Typical Cost (Fitted) | Notes |
|---|---|---|---|
| Blown fuse or faulty relay (ECM power supply) | Medium | £20–£50 | Check the ECM fuse in the engine bay or passenger fuse box; a cheap fix |
| Corroded or loose connector at ECM or other modules | High | £50–£150 | Water ingress or poor contact; clean and reseat connectors |
| Damaged CAN bus wiring (chafing, short, or open circuit) | Medium | £150–£500 | Often requires tracing and repairing the twisted-pair CAN wires |
| Failed module (ECM, TCM, BCM, etc.) | Medium | £400–£1,200 | Module replacement and programming; can be expensive |
| Low battery voltage or poor ground connection | Low | £50–£200 | Voltage drop can disrupt CAN communication; check battery and grounds |
| Software glitch or aftermarket accessory interference | Low | £100–£300 | Rare; a dealer software update or removal of non-OEM devices may resolve |
Cost figures [PATTERN DATA] - indicative independent‑specialist pricing, not drawn from our engine replacement dataset.
A competent Jaguar specialist will follow this order to avoid misdiagnosing a costly module replacement when a simple fix will do.
Inspect all fuses and relays related to the ECM and other modules. Replace any blown fuses; a blown fuse may indicate a short.
Visually check the ECM connectors and CAN bus wiring harness for corrosion, chafing, or loose pins. Clean with contact cleaner and reseat.
Measure battery voltage and check main engine and chassis grounds. Low voltage or poor grounds can cause communication errors.
Using a diagnostic tool or oscilloscope, check the CAN bus for proper voltage and signal integrity. Look for shorts or opens.
If the bus is healthy, use a scan tool to identify which module is not responding. Then test that module's power, ground, and communication lines.
Our diagnostic wizard walks the same order, or use the registration lookup to confirm your engine and any outstanding recalls.
Only to get directly to a garage - and only if the engine is running. U0100 indicates a loss of communication between the ECM and another module. If the ECM itself is not receiving power or is faulty, the engine may not start at all. If the engine does start, you may experience stalling, reduced power, or transmission issues. Driving with an intermittent communication fault can be dangerous - the engine could cut out without warning. The safest action is to have the car recovered to a specialist. If you must drive, keep it to a short, low-speed journey and be prepared for sudden loss of power. Do not drive if the engine management light is flashing or the vehicle is in limp mode with reduced power.
It means the Engine Control Module (ECM) has lost communication with another module on the CAN bus network. This could be due to wiring issues, a faulty module, or a blown fuse.
Only to get directly to a garage, and only if the engine runs. Driving with this code risks sudden stalling or loss of power. Have the car recovered if possible.
Yes, it's a serious electrical fault. It can cause the engine to stop running or affect other critical systems like transmission and ABS. Immediate diagnosis is essential.
The cost ranges from about £50 for a fuse or connector repair to £400–£1,200 for a module replacement and programming. Diagnostic time is typically 1–2 hours.
Only temporarily. The code will return if the underlying communication issue persists, and clearing it removes freeze‑frame data that helps diagnosis.
Yes - a low battery or poor ground can cause voltage drops that disrupt CAN bus communication, triggering U0100. Always check battery health first.
Cheapest‑First AlwaysWe rank causes by diagnostic order and cost, never the other way around.
Independent & HonestWe're not a garage. We're an independent resource that routes you to vetted specialists.
15+ Vetted Jaguar SpecialistsDiagnostics and repairs carried out by people who know these engines intimately.
Data‑Backed ContentOur advice is built on real enquiry data from 9,000+ UK owners.
Compare diagnostic and repair quotes from our vetted specialist network.
COMPARE QUOTES NOW → RUN THE DIAGNOSTIC WIZARD