U0120: Lost Communication With Starter/Generator Control Module
What to do
Drive it somewhere useful now rather than making one more short trip, and do not switch off at a filling station assuming it will start again.
Many recent cars have a small hybrid system built around one machine that both starts the engine and charges the battery — there is no separate starter motor and no separate alternator. This code means the computer controlling it has stopped talking to the rest of the car.
Two things follow. If the charging has stopped, the car is running on the battery alone and will keep going until it does not. A battery warning light here is a countdown, not a notification. And if the same machine is what starts the engine, the car may not restart once you switch it off, even though it drove perfectly to where it is parked.
You may also notice stop-start no longer working, a rougher restart if it does, a flat spot pulling away, and worse fuel consumption.
Two cheap things to ask for first. These cars have a second battery — usually under a seat or in the boot — and their own separate fuses. Either can be the whole problem, and both are far cheaper than the machine itself, which is expensive and awkwardly placed.
What this code means
The starter/generator control module has stopped being heard from. The general reasoning is at U0100, and U0469 explains what this machine does and why most owners do not know their vehicle has one.
Silence has one consequence bad data does not, and it is the one that strands people: the vehicle may lose its charging system.
On a mild hybrid the starter/generator is the alternator. There is no second one. So a module that has gone quiet can leave the 12-volt system running on the battery alone — which works perfectly for a while, then does not. A battery warning lamp alongside this code should be read as a countdown rather than a notification.
And there is a second-order effect worth knowing. On many of these vehicles the engine is started by the same machine, so a system that has taken itself offline may leave the vehicle unable to restart once it is switched off — even though it drove to where it is parked without complaint. Where there is a conventional starter as a backup it will crank, less smoothly than usual; where there is not, the vehicle stays where it is.
So the practical instruction is to drive it somewhere useful now rather than making one more short trip, and not to switch off at a filling station on the assumption it will start again.
Where the fault is: this machine carries heavy cable and sits on the engine, so its connections see vibration and heat. On a 48-volt system there is also a separate fuse and a separate battery, usually under a seat or in the boot, and either can be the whole story — worth checking before the machine is condemned, since it is an expensive part in an awkward place.
Likely causes
Start here
- Blown fuse or failed supply · easy
- Corroded or loose connector · easy
- Low system voltage · moderate
- Failed power or earth supply to the module · moderate
Less common
- Heat damage to sensor or wiring · moderate
- Damaged data bus wiring · moderate
Symptoms
Components involved
Information, not instruction
CarGeek is a reference, not a service manual. Its pages are written from public sources and from what other people have found, never from a manufacturer's own service information, so they can be wrong or incomplete for your particular vehicle. Some of what they describe carries real risk: high voltage, fuel under pressure, stored energy, hot and moving parts.
Have anything you intend to act on checked by a trained person against the manufacturer's own procedure for your vehicle. Nothing here replaces that procedure.
Community experience
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What makes one worth reading:
- The car — make, model, year and engine. The same fault behaves differently on each.
- What you measured, not only what you replaced.
- What actually fixed it — or that nothing did, which is worth knowing too.
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