Faulty shift solenoid G, where the internal coil has shorted or failed; Damaged or chafed wiring in the solenoid control circuit, causing a short to ground; Corroded or loose electrical connector at the solenoid or TCM; Transmission fluid contamination or low fluid level affecting solenoid operation; Failed TCM driver circuit for solenoid G; Internal transmission mechanical damage causing abnormal solenoid load

DTC codes / P09xx · Transmission / P099B

P099BShift Solenoid G Control Circuit Low

Q: What does code P099B mean?

Shift Solenoid G Control Circuit Low. P099B indicates that the transmission control module (TCM) has detected a low-voltage condition on the control circuit for shift solenoid G, which is one of the solenoids that manages hydraulic pressure for gear shifts. The circuit is either shorted to ground, has excessive resistance, or the solenoid itself has failed, causing the TCM to see voltage below its expected threshold.

P099B indicates that the transmission control module (TCM) has detected a low-voltage condition on the control circuit for shift solenoid G, which is one of the solenoids that manages hydraulic pressure for gear shifts. The circuit is either shorted to ground, has excessive resistance, or the solenoid itself has failed, causing the TCM to see voltage below its expected threshold.

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, particularly when the transmission is cold
The transmission may enter limp mode, sticking in a single gear or refusing to shift
Slipping between gears or a flare during upshifts
Poor fuel economy due to improper shift timing
In some cases, the vehicle may fail to engage certain gears entirely

Common causes, most likely first

Faulty shift solenoid G, where the internal coil has shorted or failed
Damaged or chafed wiring in the solenoid control circuit, causing a short to ground
Corroded or loose electrical connector at the solenoid or TCM
Transmission fluid contamination or low fluid level affecting solenoid operation
Failed TCM driver circuit for solenoid G
Internal transmission mechanical damage causing abnormal solenoid load

How to diagnose and fix it

Check transmission fluid level and condition; top off or replace fluid if contaminated
Inspect the solenoid G electrical connector for corrosion, tightness, and damaged pins
Disconnect the solenoid and measure its resistance with a multimeter, comparing to manufacturer specifications
Check the control circuit wiring for continuity and shorts to ground using a wiring diagram
Test the TCM output signal to the solenoid with a oscilloscope or bidirectional scan tool if available
Replace the shift solenoid G if resistance is out of spec or the coil is open/shorted
Clear the code and perform a relearn or adaptation drive cycle to verify the repair

What a low solenoid circuit really means

A low circuit code often results from a short to ground in the wiring harness rather than a failed solenoid, so always inspect the harness before replacing parts.
Transmission fluid that is dark or smells burnt can indicate internal clutch wear that stresses the solenoid, and simply replacing the solenoid may not solve the underlying issue.
Some vehicles use shift solenoid G for multiple functions, so a failure can cause symptoms that mimic other transmission problems like torque converter clutch issues.
A weak battery or charging system fault can cause the TCM to misreport solenoid circuit codes, so verify battery voltage and grounds before diving into transmission diagnostics.
After replacing a solenoid, some TCMs require a adaptive learning reset or a specific drive cycle to clear the code permanently and restore smooth shifting.

Can I keep driving with P099B?

You can usually drive the vehicle to a repair shop, but avoid aggressive acceleration or heavy towing because the transmission may shift erratically or enter limp mode. If the vehicle refuses to shift or slips severely, stop driving and seek professional diagnosis to prevent further transmission damage.

Tools that help

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