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DTC codes / P08xx · Transmission / P085B

P085BGear Shift Control Module B Communication Circuit Low

P085B indicates that the transmission control module (TCM) has detected a low-voltage condition on the communication circuit dedicated to the gear shift control module B. This low signal on the dedicated serial data line prevents the TCM from reliably commanding or receiving status from the shift control module, which can affect gear selection and shift quality.

SystemPowertrain
Code typeGeneric (all makes)
SeverityMedium
Fix difficultyProfessional
AreaTransmission

Symptoms you may notice

  • Illumination of the check engine light or transmission warning light
  • Transmission stuck in a single gear or limp mode
  • Harsh or delayed gear shifts
  • Inability to select certain gears manually
  • No symptoms besides the warning light in some cases

Common causes, most likely first

  1. Damaged or chafed wiring in the communication circuit between the TCM and gear shift control module B
  2. Corroded or loose electrical connectors at the TCM or shift control module
  3. Failed gear shift control module B
  4. Faulty transmission control module (TCM)
  5. Poor ground connection for the shift control module or TCM
  6. Aftermarket electronic accessories improperly wired into the communication circuit

How to diagnose and fix it

  1. Verify battery voltage and ensure the charging system is functioning properly
  2. Inspect and clean all connectors and grounds related to the TCM and gear shift control module B
  3. Perform a visual inspection of the wiring harness for damage, chafing, or corrosion
  4. Use a multimeter to check for continuity and voltage on the communication circuit between the TCM and shift control module
  5. Check for technical service bulletins (TSBs) or software updates for the TCM
  6. If the circuit tests good, substitute a known-good gear shift control module B to see if the code clears
  7. If the module is not the cause, diagnose or replace the TCM as needed

Common missteps when diagnosing P085B

  • A low voltage on the communication circuit often results from a poor ground rather than a failed module, so always check grounds before replacing parts.
  • The gear shift control module B is sometimes integrated into the shifter assembly, and a faulty shifter can set this code without any obvious mechanical symptoms.
  • Aftermarket remote starters or alarm systems tapped into the data bus can disrupt communication and trigger P085B, so inspect for non-factory wiring.
  • This code can be stored intermittently due to a loose connector that only loses contact during vibration or temperature changes, so wiggle-test connectors while monitoring live data.

Can I keep driving with P085B?

If the transmission is in limp mode or shifting erratically, avoid highway driving and seek service immediately. If the only symptom is a warning light and the vehicle drives normally, you can drive cautiously to a repair shop, but do not ignore the light as the fault may worsen.

Tools that help

Tools we recommend are in the Auto Veteran shop on Amazon. As an Amazon Associate we earn from qualifying purchases.

Guidance is general and written for all OBD-II vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.

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