What does the P0908 code mean?
The OBD II error code P0908 is a generic code defined as “Transmission Gate Select Circuit – Intermittent Circuit Malfunction,” and it is set when the PCM (Powertrain Control Module) or TCM (Transmission Control Module), or both modules, detect an intermittent failure in the signal from the transmission gate selector position switch.
All automatic transmissions or automated manual transmissions use a position sensor to monitor gear selection, allowing the PCM/TCM to know which gear the transmission is in. This input data is used to modulate engine power during gear changes, control torque converter clutch slip, and provide the driver with a visual indication of the currently selected gear.
If the signal from the transmission gate selector position switch fails or becomes intermittent, the PCM cannot effectively control gear shifts because the control module cannot detect the currently selected gear. Therefore, when this input data is lost to the PCM, it sets code P0908 and illuminates a warning light. Note that in some cases, the PCM may trigger a safety or limp mode to both limit engine power and protect the transmission.
Where is the P0908 sensor located?
In some applications, the transmission gate selector position switch is located so that the gear selector or gear selector linkages act on the switch when moved, while in other applications, the switch may be located on the transmission housing in such a way that internal moving components act on the switch when gears are shifted.
Always refer to the relevant application manual to correctly locate and identify the transmission gate selector to avoid misdiagnosis, as many transmissions are also equipped with a variety of independent sensors.
NOTE: The transmission gate selector should not be confused with the transmission safety interlock switch, whose purpose is to prevent the engine from starting if the gear selector is in any position other than “P” or “N.” Note that in some cases, the safety interlock is integrated into the transmission gate selector switch, so always refer to the relevant application manual to identify the appropriate terminals/circuits in the switch connector.
The image below shows a typical transmission gate selector, but note that the appearance and location of these switches vary significantly between applications.
What are the common causes of code P0908?
Some common causes of code P0908 could include the following:
Faulty transmission gate selector position switch
Misaligned or improperly adjusted transmission gate selector position switch
Damaged, burnt, short-circuited, disconnected, or corroded wiring and/or connectors
Excessive wear and free play in mechanical gear selector linkages and components
In very rare cases, transmission malfunctions themselves may set this code or contribute to its setting. However, note that this is unlikely if no other transmission-related codes are present
Failing or malfunctioning PCM/TCM. Note that this is a rare occurrence, and the fault should be sought elsewhere before replacing a control module
How much does it cost to repair code P0908?
Depending on the application, replacing the transmission gate selector can range from well under $100 on a DIY basis to over $1000 if replaced by a repair shop. Please note that this price estimate does not include any applicable fees and taxes.
What are the symptoms of code P0908?
Some common symptoms of code P0908 could include the following:
Stored fault code and illuminated warning light
Other transmission-related codes may also be present
Depending on the application, a no-start condition may be present
The transmission may not engage some or all gears
Gear shifts may be harsh, erratic, or unpredictable
In some cases, the transmission may not shift at all or may be locked in a fail-safe or limp mode until the fault is corrected
What are the common solutions for code P0908?
Common solutions for code P0908 generally include the following:
Inspecting, testing, and replacing the transmission gate selector position switch
Inspecting and repairing or replacing damaged wiring and/or connectors
Inspecting and correcting switch alignment/adjustment
Inspecting and correcting excessive free play in mechanical linkages and/or components
How severe is code P0908?
Code P0908 should be considered severe, as serious driving issues can occur when the PCM/TCM cannot effectively control the transmission shift pattern. In some cases, severe transmission damage may occur if this code is not resolved promptly.
Is it safe to drive the car with code P0908?
Ideally, vehicles displaying this code should not be driven, especially in traffic, because the loss of effective transmission control could unexpectedly immobilize the vehicle. This poses an obvious safety risk to the occupants of the affected vehicle and other road users.
Is it difficult to repair code P0908?
Diagnosing and repairing this code should not present undue difficulties for the average non-professional mechanic, as the repair primarily involves simple circuit testing. Replacing the transmission gate selector position switch requires only average mechanical aptitude and a few basic tools.
Note that aside from a code reader and a good-quality digital multimeter, a color-coded wiring diagram for the application is necessary to accurately diagnose this code.
What are the most common mistakes when repairing code P0908?
Common mistakes when dealing with this code include the following:
Failing to check the proper alignment/adjustment of the switch
Failing to verify that the switch is securely fastened
Neglecting to correct excessive free play in mechanical gear selector linkages
How do you troubleshoot code P0908?
Step 1
Record all present error codes, along with any available freeze frame data. This information can be extremely useful for diagnosing intermittent faults.
Step 2
Note that component misalignment, improper switch adjustment, and/or excessive free play in mechanical linkages are arguably the most common causes of this code. Therefore, it is a good idea at this stage to check the condition of the entire gear selector mechanism for signs of excessive wear/free play, as well as the tightness of all retaining bolts/screws.
Correct any faults found or replace parts/components as needed, clear the code, and operate the vehicle normally to see if the code returns.
Step 3
If the code persists but the gear selector mechanism is in perfect condition, refer to the wiring diagram to determine the function, color coding, and routing of all associated wiring.
Perform a thorough visual inspection of the transmission gate selector position switch and all associated wiring. Look for signs of mechanical damage to the switch, as well as signs of short-circuited, disconnected, or corroded wiring and/or connectors. Make repairs or replace wiring as needed, clear the code, and operate the vehicle normally to see if the code returns.
Step 4
If the wiring shows no visible damage or defects, refer to the manual for detailed procedures on performing resistance, ground integrity, and continuity tests on all relevant circuits.
WARNING: Note that there is no one-size-fits-all approach for this step. Since switches and circuits vary significantly between manufacturers, it is imperative to follow the manual’s instructions EXACTLY, as these test procedures are designed for that specific application. Deviating from prescribed testing methods will almost certainly lead to misdiagnosis and damage to the vehicle’s electrical system.
Compare all obtained readings with the values specified in the manual and perform repairs or replace wiring/connectors as needed to ensure all electrical values fall within the manufacturer-specified ranges.
NOTE: Be aware that since this code refers to intermittent electrical faults, it is entirely possible that this step may reveal no issues and all electrical values may fall within specified ranges. Therefore, it is imperative to vigorously shake the wiring during continuity/resistance tests to attempt to induce the fault.
Step 5
If the fault persists despite vigorous shaking tests, suspect a faulty transmission gate selector position switch. However, note that these switches are NOT repairable, meaning that replacing the switch with an OEM replacement is the only reliable, long-term solution.
Note that when replacing the switch, it is important to verify that it is properly aligned and adjusted before applying thread-locking agent to the retaining screws/bolts to prevent the switch from moving out of position in the future.
Clear the code once all repairs are completed and operate the vehicle normally to see if the code returns. If the fault does not return immediately, the repair can be considered complete and successful.
If, however, the code returns, repeat all electrical tests until the fault is found and repaired, or refer the vehicle to a dealership or other competent repair service for professional diagnosis and repair, as some repairs might involve testing and replacing large sections of the vehicle’s wiring.
