Code
P085B
Generic
P — Powertrain
Gear Shift Control Module B Communication Circuit Low
Views:
UK: 21
EN: 22
RU: 21
AI status
Completed
Completed
100%
Causes
- Damaged, chafed, or shorted communication wiring (to ground or battery)
- Corroded, loose, or pushed-back connector terminals at the module or gateway
- Poor or missing ground or power supply to the gear shift control module
- Failed Gear Shift Control Module B
- Faulty gateway/TCM/PCM or network node interfering with bus
- Blown fuse or ignition-switched power loss feeding the module
Symptoms
- Check Engine / MIL or Transmission warning illuminated
- Transmission may go into limp/limited-gear mode
- No gear selection or inability to shift normally
- Erratic shifting or intermittent transmission behavior
- Scan tool shows module B not responding or frequent communication faults
What to check
- Use a scan tool to read freeze-frame data and all related codes; note network errors and which modules are responding
- Visually inspect connectors and wiring for corrosion, damage, moisture, or pushed-out pins at Gear Shift Control Module B and related junctions
- Check fuses and power feeds for the module
- Verify good ground(s) for the module and nearby chassis grounds
- Perform a wiggle test while monitoring communication status with a scanner to reproduce fault
- Measure bus voltages and continuity between module and gateway/TCM (see signal parameters)
Signal parameters
- If on CAN bus: idle voltages ~2.5 V on CAN_H and CAN_L; differential idle ~0 V. Active (dominant) states create a differential (typically ~1–2 V). Termination resistance across CAN_H and CAN_L ~60 ohms (two 120-ohm terminators).
- If on LIN/single-wire bus: idle = battery voltage via pull-up; dominant/active low ~0.5 V. Check for proper pull-up and bus response to LIN frames.
- Expected: module should respond to network requests and appear on module list in a scan tool; loss of response indicates circuit or module fault
Diagnostic algorithm
- Connect a competent scan tool. Verify P085B and note accompanying network or transmission codes and freeze frame data.
- Clear codes, then cycle ignition and attempt to reproduce while monitoring live network/module presence.
- Visually inspect the module and harness for damage, moisture, or corrosion. Repair any obvious damage.
- With ignition ON (engine off), check for proper supply voltage and ground at the module connector. Do not short circuits—use a DMM.
- Measure communication bus signals at the module connector: for CAN measure CAN_H and CAN_L voltages and differential; for LIN check idle voltage and ability to be pulled low during frames.
- Check continuity between the module connector and the gateway/TCM connectors for the communication lines; look for short-to-ground or short-to-battery.
- Check termination resistors across CAN_H and CAN_L and inspect other nodes for wiring issues that could pull the bus low.
- If wiring and power/ground are good, try disconnecting other aftermarket devices or modules on the same bus and retest.
- If fault follows the module and wiring tests good, consider replacing the Gear Shift Control Module B. After replacement, reprogram/configure if required by manufacturer.
- After repairs, clear codes and perform a road test to confirm the issue is resolved and monitor for recurrence.
Likely causes
- Open or shorted communication wire between module B and vehicle network
- Bad connector or poor pin contact at module harness
- Missing module supply voltage or ground
- Module internal failure or ECU network fault
Fault status
Status
Low voltage or loss of communication reported on Gear Shift Control Module B communication circuit.
Repair difficulty: Medium
Diagnostic time: 0.5-2.0 hours
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