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DTC codes / P09xx · Transmission / P099C

P099CShift Solenoid G Control Circuit High

P099C indicates that the transmission control module (TCM) has detected an abnormally high voltage on the control circuit for shift solenoid G, which is one of the solenoids that manages hydraulic pressure for gear shifts. The TCM expects the circuit to pull the voltage low when the solenoid is commanded on, but instead it sees a voltage that stays near battery positive, indicating an open circuit, a short to power, or a failed solenoid.

SystemPowertrain
Code typeGeneric (all makes)
SeverityMedium
Fix difficultyProfessional
AreaTransmission

Symptoms you may notice

  • Illumination of the check engine light or transmission warning light, often with no other noticeable symptoms
  • Harsh or delayed gear shifts, especially when the transmission is cold
  • The transmission may enter limp mode and stay in a single gear
  • Slipping or flaring during upshifts or downshifts
  • Poor fuel economy or unusual engine RPM at highway speeds
  • In some cases, the vehicle may fail to engage certain gears or may shudder during acceleration

Common causes, most likely first

  1. Faulty shift solenoid G, such as an open coil or internal short
  2. Damaged or corroded wiring harness or connector at the solenoid or TCM
  3. Open circuit in the power or ground circuit to the solenoid
  4. Short to voltage on the control wire between the TCM and solenoid
  5. Failed transmission control module (TCM) or internal driver fault
  6. Low or contaminated transmission fluid causing solenoid malfunction
  7. Loose or damaged pins in the transmission pass-through connector

How to diagnose and fix it

  1. Verify the code with a scan tool and check for any related transmission codes before clearing anything
  2. Inspect the transmission fluid level and condition; top off or service if contaminated or low
  3. Visually inspect the solenoid G wiring and connector for damage, corrosion, or loose pins
  4. Measure resistance across the solenoid G coil and compare to the manufacturer's specification
  5. Check for short to voltage on the control circuit by backprobing the TCM connector with a multimeter
  6. If the circuit tests good, replace the solenoid G and clear the code, then perform a relearn or adaptation drive cycle
  7. If the solenoid and wiring are intact, test the TCM driver output with an oscilloscope or substitute a known-good TCM

Don't replace the solenoid before checking this

  • A high voltage code on a solenoid circuit often results from a simple open circuit, such as a broken wire or unplugged connector, rather than a failed solenoid itself.
  • Corrosion inside the transmission pass-through connector is a frequent hidden cause that can mimic a solenoid failure and is easily overlooked during a quick visual inspection.
  • A short to voltage on the control wire can be caused by chafed wiring near hot exhaust components or sharp metal edges, so always trace the harness carefully.
  • Some vehicles will set P099C only after a specific driving pattern, so a test drive with live data monitoring is more reliable than a static resistance check alone.
  • Replacing a solenoid without clearing the adaptive learning values can lead to harsh shifts that make the repair seem unsuccessful even when the fault is fixed.

Can I keep driving with P099C?

You can usually drive the vehicle to a repair shop if the transmission still shifts, but avoid heavy acceleration or towing and do not continue driving if the transmission slips severely or enters limp mode. Prolonged driving with this code can cause internal transmission damage, so schedule diagnosis soon.

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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