U0086 is a network communication code indicating that the controller area network (CAN) bus F is registering a voltage that is lower than the expected threshold. The bus is essentially shorted, held low, or excessively loaded, which prevents normal data exchange between the modules connected to that network segment.
Insulation damage, chafing, or a pinched section of the bus F twisted pair can short the CAN-H or CAN-L line to ground, pulling the bus voltage low and halting communication across that segment.
Corroded, loose, bent, or water-intruded pins in a bus F connector introduce resistance or a partial short that holds the network in a low state and prevents normal transmissions.
An internal failure in one of the modules connected to bus F can pull the communication lines to ground. Identifying the offending module usually requires isolating modules one at a time and watching the bus recover.
CAN bus segments rely on 120-ohm terminating resistors at each end of the bus. A missing, open, or high-resistance terminator alters bus impedance and can cause the network to read as low or unstable.
If a module on bus F loses its supply voltage or ground reference, its transceiver can default to a fault state that drags the bus low and generates U0086 along with other related codes.
Although rarely the root cause, an internal PCM transceiver failure can hold the bus low. This should only be considered after wiring, connectors, terminating resistors, power supplies, and other modules on the bus have all been ruled out.
No. U0086 is almost always a wiring, connector, terminating resistor, or other-module problem. The PCM is one of the least likely causes and should only be suspected after the network hardware and the other modules on the bus have been thoroughly tested.
It depends on which systems are affected. If critical systems such as the transmission, ABS, or power steering lose communication, the vehicle should not be driven. If only non-critical modules are affected, limited driving may be possible, but the fault should be repaired as soon as practical.
When a communication bus fails, every module on that bus loses contact with the others, so each affected module typically sets its own lost-communication code in addition to U0086. Those companion codes are useful clues for identifying the affected bus and the modules on it.
Many vehicles split CAN traffic across multiple segments, commonly labeled A, B, C, D, E, and F, to separate powertrain, chassis, body, and convenience systems. Bus F is simply the segment the manufacturer has designated F, and which modules sit on it varies by vehicle.