What Does P099B Mean?
The TCM has detected that the shift solenoid 'G' control circuit voltage is below the expected threshold, indicating a short to ground. This prevents proper modulation of the solenoid, which may cause the associated clutch pack to engage at full pressure or remain disengaged depending on solenoid design (normally open vs. normally closed).
Common Causes
35%
Shift solenoid G coil internally shorted to ground
30%
Control wire shorted to ground due to harness chafing inside the transmission
20%
Corroded connector allowing moisture ingress and short circuit
15%
TCM solenoid driver shorted internally
Diagnostic Steps
1
Step 1: Disconnect the transmission case connector and measure solenoid G resistance. A reading below specification (typically under 5 ohms when spec is 10-25 ohms) indicates an internal short.
2
Step 2: Check for continuity between the solenoid G control wire and chassis ground on the harness side. Any continuity indicates a short to ground in the wiring.
3
Step 3: Visually inspect the transmission internal harness for chafing against the case, solenoids, or valve body. Internal harness damage is common from heat and vibration.
4
Step 4: Verify no ATF is wicking through the harness into the TCM connector. Transmission fluid migration through wiring can cause shorts and corrosion at the TCM.
5
Step 5: If harness and solenoid are good, test the TCM driver by measuring output voltage. A stuck-low output with no external faults suggests a failed TCM driver.
Estimated Repair Cost
$200 - $850
Parts + labor, varies by vehicle and location
The TCM has detected that the shift solenoid 'G' control circuit voltage is below the expected threshold, indicating a short to ground. This prevents proper modulation of the solenoid, which may cause the associated clutch pack to engage at full pressure or remain disengaged depending on solenoid de...
The most common cause of P099B (Shift Solenoid "G" Control Circuit Low) is: Shift solenoid G coil internally shorted to ground
Typical repair costs for P099B range from $200 to $850, depending on the vehicle, location, and whether you do it yourself or go to a shop.
Transmission will likely enter limp mode. Specific gear engagement may be lost. Drive only short distances to a repair facility.
Start by connecting an OBD2 scanner to read the code and any freeze frame data. Then follow the diagnostic steps specific to P099B to identify the root cause.
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Quick Info
Category
Powertrain
System
Transmission Shift Control
Difficulty
Type
Generic (SAE)
Recommended Tools
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