The scanner or another module can't communicate with the engine ECU.
U0100 — also written UO1OO, U 0100, 0100 — is the OBD-II code for lost communication with ecm/pcm. Same code, same fix: this page covers every spelling of it.
This is a digital 'handshake' failure where the car's brains have stopped talking to each other, common on high-mileage Transit vans and aging Vauxhalls.
U0100 is a electrical fault code, typically costing £80–£1500 to put right in a UK garage.
What U0100 actually feels like to drive
- Total non-start; the engine won't even crank when you turn the key.
- Dash lights up like a Christmas tree, usually with 'F' or dashes where the gear/temp should be.
- Power steering suddenly goes heavy while driving (limp mode).
- The radiator fan runs at full speed as soon as you turn the ignition on.
- Scanner says 'No Communication' or 'Link Error' when trying to read engine codes.
What causes U0100?
- ECU power/ground fault
- Failed ECU
- CAN wiring fault
UK vehicles that throw U0100 most often
- Vauxhall Corsa D / Astra J (All engines) 2006-2014
- Ford Transit 2.2 / 2.4 TDCi (Duratorq) 2006-2013
- Land Rover Discovery 3 / 4 (TDV6) 2004-2016
- Volkswagen Golf Mk5/6 (PD and CR engines) 2004-2012
- Volvo V70 / XC90 (D5 engines) 2005-2014
How U0100 is fixed
- Test ECU power and grounds
- Check CAN wires
- Reflash or replace ECU
The U0100 misdiagnosis that wastes money
I see it all the time—lads rushing to buy a brand new ECU or a set of injectors because the car won't start and they can't 'see' the engine on their tablet. In reality, on things like the older Corsa D or the Astra, it's almost always a corroded pin in the ABS module plug or a broken wire in the loom near the battery tray. Because the CAN-bus runs through multiple modules, a salty, corroded connector at the ABS pump can literally 'silence' the engine ECU. Don't throw a grand at a new brain until you've checked the plugs for green crusties.
What a UK garage should charge for U0100
For a simple broken wire or corroded plug, you're looking at £80-£150 for an hour's diagnostic and a loom repair. If the ECU is genuinely fried, a refurbished 'cloned' unit from a specialist like ECU Testing Ltd will cost you £250-£450 plus labour. If you're daft enough to go to a main dealer for a brand new Bosch or Denso unit, you won't get much change out of £1,200 to £1,500 once they've finished 'programming' it.
U0100 repair cost in the UK
Most U0100 repairs land between £80–£1500 including parts and labour. The spread is wide because the cheap fix (a sensor, a hose, a leak) and the expensive fix (a major component) throw the same code — which is why a proper diagnosis before parts are bought saves the most money.
Bob's tip on U0100
If your local garage says you need a new ECU, ask them two things: 'Have you checked the CAN-bus resistance?' and 'Have you load-tested the ECU powers and grounds with a bulb, not just a multimeter?' A multimeter can show 12V through a single strand of wire that’s actually too weak to power the module. If they haven't checked the terminating resistance (should be 60 ohms across the bus), they’re guessing with your wallet. Also, insist on a 'clone' service if the ECU is actually dead—it’s hundreds cheaper than a virgin unit from a main dealer.
Can you drive with U0100?
May not start.
Urgency: high.