U0037 is a generic OBD-II network communication code that indicates a performance problem has been detected on CAN bus B (a secondary Controller Area Network bus). Bus B typically carries high-speed or medium-speed data between powertrain, chassis, and body control modules, so a fault here can disrupt communication between several ECUs at once. The code itself does not identify the failed component—only that the bus as a whole is not behaving correctly.
Chafed insulation, pinched harness sections, corroded splices, or water intrusion on the CAN B twisted pair are the most common real-world cause. Even a single short between the two bus wires or to power/ground can pull the entire bus down.
Modules on Bus B connect through multi-pin connectors, and any terminal with poor contact (corrosion, spread pins, incomplete mating) can disrupt the shared network. This is especially common at bulkhead and undercar pass-throughs exposed to moisture.
Any ECU connected to Bus B with an internal transceiver failure can clamp the bus low and prevent all other modules from communicating. The offending module is often a non-powertrain one (radio, instrument cluster, body control, telematics, or HVAC module) that shares the bus.
CAN buses require termination resistors (typically 120 ohm at each end) to damp signal reflections. If a resistor opens, the network can show a performance fault and intermittent communication errors even with good wiring.
Modules that lose proper power or ground will drop off the network and may set bus performance codes. Always verify battery health and module supply voltage before chasing the bus itself.
A control module can internally fail in a way that disrupts the bus, but this is statistically uncommon for U0037. It should be considered only after wiring, connectors, and the other modules on the bus have been ruled out.
The definition is universal; the diagnostics are engine-specific. Open your vehicle for the codes documented for it:
Almost never. U0037 is a network performance code, and the most common causes are wiring damage, connector problems, a shorted non-PCM module on the bus, or a termination resistor issue. Replacing the PCM is expensive and should only be done after the wiring, connectors, and the other modules on Bus B have been systematically ruled out.
It depends on which modules have dropped off. If only a non-critical module like the radio is affected, the vehicle may drive normally. If ABS, transmission, or other chassis modules are off-line, the car may enter a reduced-power or limp mode and should be diagnosed promptly because other systems may be functioning in a fallback or disabled state.
These codes refer to different physical buses on the vehicle. U0035 typically relates to bus A (often the high-speed powertrain CAN), U0036 to a different segment, and U0037 specifically to bus B. The general diagnostic approach is the same, but the wiring, modules, and termination resistors are different on each bus.
A basic OBD-II scanner that can read all modules (not just the PCM) is a strong starting point, but an oscilloscope or a lab scope capable of reading CAN signal quality, plus accurate wiring information for the specific vehicle, is usually required to pinpoint wiring faults, termination problems, or a shorted module.