U0115: Lost Communication With Engine Control Module B
Do not keep driving
Expect the engine to run very badly or not at all.
If the car has an engine large enough to use two control modules — commonly a V8 or larger — losing one silences an entire bank of cylinders at once. That feels and sounds catastrophic, like a major mechanical failure, and it usually is not: compression, camshafts and timing can all be perfect while one module has simply stopped talking.
The distinguishing feature is that it arrived all at once and affects exactly one side of the engine. Mechanical failures develop, and rarely respect that boundary so precisely. Say so if that is what happened.
These modules are expensive and have to be programmed to the vehicle, so ask what has been checked before one is ordered — its power supply, its earth, its connectors and the network wiring, as at U0100.
What this code means
A second engine control module has stopped being heard from. The general reasoning is at U0100 and applies unchanged.
A vehicle has two engine control modules for one of two reasons, and which applies changes what this code means.
- A large engine managed in halves. Some V-configuration and larger engines use one module per bank, sharing the work and halving the number of long injector and coil circuits each has to drive. Losing one costs half the engine: it runs, badly, on the cylinders the surviving module controls, which presents as a severe misfire across one bank rather than as a no-start.
- A master and a secondary module, where the second handles a defined subsystem. Losing it removes that subsystem while the engine continues.
The first case is the one worth recognising, because a whole bank silent looks like a catastrophic mechanical failure and is not. Compression is fine, the camshafts are fine, the timing is fine — one module has stopped receiving or sending, and every cylinder it commands has gone quiet together. A misfire confined exactly to one bank, arriving all at once rather than developing, is a strong pointer to this rather than to anything mechanical.
The causes are the family's usual ones, and the argument at U0100 about not replacing a module before checking its supply, its earth and the bus applies with more force here, because these modules are expensive and require programming.
Likely causes
Start here
- Failed power or earth supply to the module · moderate
- Damaged data bus wiring · moderate
- Corroded or loose connector · easy
- Blown fuse or failed supply · easy
Less common
- Another module holding the bus down · specialist
- Faulty engine control module · specialist
Symptoms
Components involved
Information, not instruction
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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.
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