A critical communication fault code indicating that the Engine Control Module (ECM) or Powertrain Control Module (PCM) is not responding on the vehicle's network. Usually a wiring, power, or ground issue — but it can leave you stranded if ignored.
U0100 means the ECU has lost communication with the engine control module (ECM) or powertrain control module (PCM) on the vehicle's CAN bus network. This is almost always an electrical issue — a wiring fault, a blown fuse, a poor ground, or a failed module. On Land Rover and Range Rover models, it can also be caused by a battery voltage issue or a faulty CAN bus termination resistor.
Do not let anyone sell you a new ECM before the fuses, relays, wiring, and battery voltage have been ruled out. Full diagnostic order below.
The Engine Control Module (ECM) and Powertrain Control Module (PCM) are the central computers that manage engine operation. They communicate with other modules — such as the transmission control module, ABS module, and instrument cluster — via a controller area network (CAN) bus. U0100 is a network diagnostic trouble code (DTC) that is logged when the ECM/PCM does not respond to a request for communication from another module on the network. This code indicates a communication failure, not a mechanical engine failure. The ECM/PCM itself could be faulty, but equally the issue could be a loss of power or ground to the module, a damaged CAN bus wire, a faulty CAN bus termination resistor, or even a dead battery or poor battery connection. This distinction is the difference between a £50 fix and a £1,200 module replacement. This code appears across the full range of JLR vehicles, from the 204DTD Ingenium to the 306DT V6, and the diagnostic approach is essentially identical regardless of which engine triggers it.
Ordered by how often each turns out to be the real culprit — with typical fitted cost against each, so you can sanity-check any quote you've been given.
| Cause | Likelihood | Typical Cost (fitted) | Notes |
|---|---|---|---|
| Blown fuse or faulty relay | Very High | £50–£150 | Cheapest fix — check ECM/PCM power supply fuses and relays first |
| Low battery voltage or poor battery connection | High | £50–£200 | Low voltage can prevent modules from communicating; clean terminals and test battery |
| Wiring issue (CAN bus or power/ground) | High | £120–£400 | Chafed or broken wires, corrosion, or poor earth points — often intermittent |
| Faulty CAN bus termination resistor | Medium | £150–£350 | Termination resistors at the ends of the CAN bus — if missing, communication fails |
| Failed ECM/PCM module | Low | £600–£1,200 | Genuine module failure — least common but most expensive |
Cost figures [PATTERN DATA] — indicative independent-specialist pricing, not drawn from our engine replacement dataset.
Locate the ECM/PCM power supply fuses and relays in the fuse box (often under the bonnet or in the passenger footwell). Replace any blown fuses or suspect relays. This is the cheapest and quickest fix.
Check battery voltage — it should be at least 12.4V with the ignition off. Clean and tighten battery terminals and earth straps. A weak battery can cause module communication issues.
Visually inspect the CAN bus wiring (twisted pair, usually yellow/green or orange/green) for chafing, corrosion, or damage. Check continuity between modules.
Using a multimeter, check the resistance between CAN high and CAN low on the diagnostic connector (OBD-II port). Should read 60 ohms. If it's 120 ohms, one termination resistor is missing or faulty.
If all above check out and the fault persists, the ECM/PCM itself may be faulty. This should only be done by a specialist with the correct diagnostic equipment.
Our diagnostic wizard walks the same order, or confirm your engine first with the registration lookup.
Probably not. U0100 indicates that the engine control module is not communicating — which means the engine may not start at all, or if it does, it may be in limp mode with very limited power. In many cases, the vehicle will not move under its own power. If the engine is running and the car is drivable, you may be able to drive a short distance to a garage, but the risk of the engine cutting out is high. In most cases, the vehicle will need to be recovered. Do not attempt to drive if the engine is running poorly, if warning lights are flashing, or if the car is not responding properly to the throttle.
Probably not. The engine may not start, or if it does, it will likely be in limp mode or cut out. The vehicle should be recovered to a garage — do not drive it if the engine is running poorly or the car is not responding properly.
No. In most cases, a blown fuse, a dead battery, poor battery connection, or damaged CAN bus wiring is the culprit. The ECM itself is one of the least common causes of U0100. Always check the simple things first.
Yes. Low battery voltage can prevent modules from communicating on the CAN bus. If the battery is weak or discharged, charge or replace it first before condemning any module.
Expect one to two hours of diagnostic time at an independent JLR specialist — around £80–£160. Many will offset this against the repair if you proceed.
Only if the fault was intermittent and caused by a temporary glitch. In most cases, the code will return immediately or on the next ignition cycle. Clearing it without fixing the root cause is not a repair.
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