P0860

P0860 sets when the PCM or BCM cannot communicate with the Gear Shift Module over the CAN bus — the controller is sending a request to the GSM (the electronic shifter module on shift-by-wire vehicles) and either gets no response or gets a malformed response within the protocol timeout window. On a typical 500 kbps CAN bus the dominant state is ~2.5V on CAN-low and ~3.5V on CAN-high, with the recessive state at ~2.5V on both wires; a healthy bus shows steady oscillation between those states under scope. Cheapest-first ladder: (1) Key-on and scope CAN-H and CAN-L at the gear shift module connector — both should show clean differential traffic. A flat-line on either wire means the bus is broken between the module and the rest of the network. (2) Pull the GSM connector and check the module-side resistance between CAN-H and CAN-L; should read ~60 ohms (two 120-ohm terminators in parallel). Anything above ~80 ohms means one terminator is missing or the bus is broken; anything under ~40 ohms means a shorted module is loading the bus. (3) Disconnect the GSM and watch the rest of the bus continue operating — if the network recovers and other modules talk again, the GSM is the failed node. Expensive-misdiagnosis caveat: P0860 with a no-shift complaint will tempt a shop to quote a TCM or valve body, but on shift-by-wire vehicles (Jaguar XF rotary, Lincoln MKZ push-button, Acura ZDX) the actual fault is usually a $300-$500 gear shift module that has lost CAN communication; replacing the transmission won't fix the code.

Vehicle-specific patterns: 2009-2015 Jaguar XF / XJ with the ZF 6HP and the famous rotary shift dial throws P0860 from a failed JaguarDrive Selector module — the rotary lifts out of the console on key-on and the failure mode is no-lift plus no-shift. 2013-2020 Lincoln MKZ / Continental with the push-button shifter throws P0860 from a failed shifter module; Ford issued a TSB on early MKZ shift-by-wire units. 2010-2013 Acura ZDX and 2014+ Acura TLX/MDX with the push-button shifter throws P0860 from a CAN comm fault at the integrated shift module — Acura's symptom is the gear indicator going blank on the dash. 2014-2019 Ram 1500 with the rotary shifter throws P0860 from a failed shift module; the truck strands in Park and refuses to engage gear. Estimated repair: $300 to $1,200.