P0908 – Transmission gate select position circuit -intermittent circuit malfunction

Reinier

By Reinier (Contact Me)
Last Updated 2017-10-19
Automobile Repair Shop Owner

Trouble CodeFault LocationProbable Cause
P0908 Transmission gate select position circuit -intermittent circuit malfunction Wiring, poor connection, ECM/PCM/TCM

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What Does Code P0908 Mean?

OBD II fault code P0908 is a generic code that is defined as “Transmission gate select position circuit -intermittent circuit malfunction”, and 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, to let the PCM/TCM know which gear the transmission is in. This input data is used to modulate engine power during gearshifts, control torque converter clutch slippage, and to provide a visual indication of the currently selected gear to the driver.

If the signal from the transmission gate selector position switch fails, or fails intermittently, the PCM is unable to control gearshifts effectively, because the control module cannot detect which gear is currently selected. Therefore, when this input data is lost to the PCM, it will set code P0809, and illuminate a warning light. Note that in some cases, the PCM may initiate a failsafe or limp mode both to limit engine power, and to protect the transmission.

Where is the P0908 sensor located?

On some applications, the transmission gate selector position switch is located so that the gear selector or gear selector linkage(s) act on the switch when it is moved, while on other applications, the switch may be located on the transmission casing in such a manner that moving internal components act on the switch as gears are shifted.

Always refer to the manual for the affected application to locate and identify the transmission gate selector switch correctly to avoid a misdiagnosis, since many transmissions are fitted with a variety of unrelated sensors as well.

NOTE: The transmission gate selector switch must not be confused with the transmission safety cut-out switch, whose purpose it is to prevent the engine from starting if the gear selector in a position other than “P” or “N”. Note that in some cases, the safety cutout switch is integrated into the transmission gate selector switch, so always refer to the manual for the affected application to identify the relevant terminals/circuits in the switch connector.

The image below shows a typical transmission gate selector switch, but note that the appearance and location of these switches vary greatly between applications.

What are the common causes of code P0908 ?

Some common causes of code P0908 could include the following-

  • Defective transmission gate selector position switch
  • Misaligned or poorly adjusted transmission gate selector position switch
  • Damaged, burnt, shorted, disconnected, or corroded wiring and or connectors
  • Excessive wear and free play in mechanical gear selector linkages and components
  • In very rare cases, malfunctions in the transmission itself can set this code, or contribute to its setting. Note though that this is unlikely to be the case if no other transmission related codes are present
  • Failed or failing PCM/TCM. Note that this is a rare event, and the fault must be sought elsewhere before any control module is replaced

How expensive is it to fix code P0908 ?

Depending on the application, replacement of the transmission gate selector switch can run from well under $100 on a DIY basis, to more than $1000 if it is replaced by a repair shop. Note that this price estimate does not include fees and taxes that may be payable.

What are the symptoms of code P0908 ?

Some common symptoms of code P0908 could include the following-

  • Stored trouble code, and an illuminated warning light
  • Other transmission related codes may be present as well
  • Depending on the application, a no-start condition may be present
  • Transmission may fail to engage all or some gears
  • Gearshifts may be harsh, erratic, or unpredictable
  • In some cases, the transmission may not shift at all, or may be locked into a failsafe or limp mode until the fault is corrected

What are common solutions to code P0908 ?

Common solutions to code P0908 generally include the following-

  • Inspection, testing, and replacement of the transmission gate selector position switch
  • Inspection and repair or replacement of damaged wiring and/or connectors
  • Inspection and correction of switch alignment/adjustment
  • Inspection and correction of excessive free play in mechanical linkages and/or components

How serious is code P0908 ?

Code P0908 should be considered as serious, since serious driveability issues may develop when the PCM/TCM cannot control the transmission shift pattern effectively. In some cases, serious damage to the transmission may occur if this code is not resolved in a timely manner.

How safe is it to still drive the car with code P0908 ?

Ideally, vehicles that exhibit this code should not be driven, and especially in traffic, since the loss of effective control of the transmission could immobilize the vehicle unexpectedly. This poses an obvious safety risk to both the occupants of the affected vehicle, and other road users.

How difficult is it to repair code P0908 ?

Diagnosing and repairing this code should not present the average non-professional mechanic with undue difficulties, since the repair mostly involves testing of simple circuits. Replacing the transmission gate selector position switch requires only an average mechanical aptitude, and some basic tools.

Note that apart from a code reader and a good quality digital multimeter, a color-coded wiring diagram for the application is required to diagnose this code accurately.

What are the common mistakes when repairing code P0908 ?

Common mistakes when dealing with this code include the following-

  • Failure to check the switch for proper alignment/adjustment
  • Failure to check that the switch is securely attached
  • Failure to correct excessive free play in mechanical gear selector linkages

How do you troubleshoot code P0908 ?

Step 1

Record all fault codes present, as well as all available freeze frame data. This information can be extremely helpful in diagnosing intermittent faults.

Step 2

Note that misalignment of components, poor adjustment of the switch, and/or excessive free play in mechanical linkages is arguably the most common cause of this code. Therefore, it is a good idea at this point to check the condition of the entire gear selector mechanism for evidence of excessive wear/free play, as well as the tightness of all retaining bolts/screws.

Correct all defects found or replace parts/components as required, 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 related wiring. Look for evidence of mechanical damage to the switch, as well as for signs of damaged shorted, disconnected, or corroded wiring and/or connectors. Make repairs or replace wiring as required, clear the code, and operate the vehicle normally to see if the code returns.

Step 4

If the wiring is free of visible damage or defects, refer to the manual for details on the correct procedure to follow to perform resistance, ground integrity, and continuity tests on all relevant circuits.

WARNING: Note that there is no single one-size-fits-all approach to this step. Since the switches and circuits vary greatly between manufacturers, it is imperative to follow the directions in the manual EXCACTLY, since these testing procedures are designed for that specific application. Deviating from prescribed testing methods will almost certainly result in a misdiagnosis and damage to the application’s electrical system.

Compare all obtained readings with the values stated in the manual, and make repairs or replace wiring/connectors as required to ensure that all electrical values fall within the ranges specified by the manufacturer.

NOTE: Be aware that since this code refers to intermittent electrical failures, it is entirely possible that this step might not reveal any issues, and that all electrical values may well fall within specified values. Therefore, it is imperative that the wiring be wiggled about vigorously during continuity/resistance testing in an attempt to induce the fault.

Step 5

If the fault persists despite vigorous wiggle testing, suspect a defective transmission gate selector position switch. Note though that these switches are NOT repairable, which means that replacement of the switch with an OEM replacement is the only reliable, long-term solution.

Note that when replacing the switch, it is important to check that it is properly aligned and adjusted before using a thread-locking agent on the retaining screws/bolts to prevent the switch from moving out of position in the future.

Clear the code after all repairs are complete, and operate the vehicle normally to see if the code returns. If the fault does not immediately return, the repair can be considered complete and successful.

If however, the code does return, repeat all electrical tests until the fault is found and repaired, or refer the vehicle to the dealer or other competent repair facility for professional diagnosis and repair, since some repairs could include testing and replacement of large sections of the vehicle’s wiring.

Codes Related to P0908

  • P0905 – “Gate Select Position Circuit Range/Performance”
  • P0906 – “Gate Select Position Circuit Low”
  • P0907 – “Gate Select Position Circuit High”

 

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