What does the P0904 code mean?
Today’s transmissions, whether manual, automated manual, adaptive dual-clutch, or automatic, are more monitored and, in some cases, controlled by the engine control module and the transmission control module (ECM and TCM). In some vehicles, the TCM may be integrated into the ECM. In automatic and automated manual transmissions, the ECM and TCM work together to modulate engine output and transmission engagement, even adjusting gear engagement to be as smooth or powerful as desired. Even in manual and adaptive dual-clutch transmissions, the ECM monitors TCM and transmission readings to perform precisely timed throttle cuts and refine engine output.
Depending on the type of transmission and the car manufacturer, the TCM may use various sensors to monitor and control the transmission, such as speed sensors, pressure sensors, and temperature sensors. Many of these transmissions also have gate select position (GSP) sensors, which can be mounted on the transmission, the gear lever, or the shift linkage. The position sensors inform the TCM which gear the driver has commanded for at least two reasons.
In some transmissions, such as automated manual transmissions or automatic transmissions with manual mode, the GSP switch lets the TCM know whether the driver has selected automatic or manual function. In manual transmissions, the GSP switch informs the TCM of the gear the driver is selecting and can act as a neutral safety switch, depending on the system design. The TCM also uses the gate select position switch to monitor transmission operation. For example, if the driver selects manual mode and chooses 3rd gear in an automatic transmission, the TCM can monitor speed sensors to detect clutch and brake slippage.
If the ECM detects a problem with the GSP sensor circuit, it will store a diagnostic trouble code (DTC) in memory and illuminate the MIL (malfunction indicator light). DTC P0904 is defined as “Transmission Gate Select Position Circuit Malfunction” and may be accompanied by other GSP sensor codes, such as P0905, P0906, P0907, or P0908.
What are the common causes of the P0904 code?
Depending on the year, make, and model, DTC P0904 can have several causes. Here are some of the most common ones.
Misalignment – This often occurs after a transmission repair, which may have involved the sensor itself, the shift linkage, or the entire transmission. The technician may have misaligned the sensor or neglected to tighten its mounting bolts properly. If the sensor is misaligned or slips, it may not correctly report the gear select position, causing this issue. Fortunately, alignment is usually a simple adjustment. Use Loctite Blue 242 or similar to prevent bolts from backing out after adjustment.
Faulty sensor – As with all electronic devices, the GSP sensor can fail. Sensors exposed to the elements or spilled coffee in the console can easily lead to corrosion and failure.
What are the symptoms of the P0904 code?
Depending on the system design, you may or may not experience issues beyond the MIL. Some symptoms may include the inability to start your vehicle, irregular shifting, hard or delayed gear engagement, or “missing” gears.
How do you troubleshoot the P0904 code?
Since this DTC refers to a circuit problem, you will need at least a DVOM (digital voltmeter) to diagnose the GSP sensor and the circuit between it and the ECM or TCM. A scan tool with live data streaming capabilities can also be helpful for diagnosis, as it will show you what the ECM or TCM is seeing. Usually, circuit issues, such as loose connections, corrosion, or a damaged wiring harness, cause this DTC to be recorded, but alignment problems can potentially trigger this issue.
Sensor alignment – First, check the sensor alignment. The scan tool can be useful for this step. Adjust as needed.
Rotary sensors usually need to be aligned to a “neutral baseline” to correctly detect the gate select position. If it’s off to one side or the other, GSP voltages may be biased to one side, or the TCM may detect more than one position at a time. Ensure the sensor mounting screws are properly tightened to prevent sensor housing movement.
GSP sensors on the gear lever or shift linkage may rely on physical contact, with tabs or buttons forming the basis of these sensors or switches. Over time, plastic or metal tabs can weaken, bend, or break. At least a few brands use magnetic micro-switches to detect position, and magnets are known to come loose. Repair or replace any obviously damaged parts.
Circuit checks – You will need an EWD (electrical wiring diagram), specific to your year, make, and model, to diagnose the circuit operation between the ECM and the GSP.
Inspect the GSP sensor, connector, wiring harness, and ECM or TCM connector for obvious damage. Check connectors for corrosion or evidence of water entry, and check for aged or loose pins. Ensure all connectors are properly installed. Repair if necessary.
Disconnect the ECM and GSP and check for continuity.
If you measure more than 5 Ω, repair the wiring harness as needed. If you measure less than 5 Ω, suspect a faulty GSP sensor, but do not condemn it until you have completed circuit checks.
Check for short circuits between each line and from each line to ground. If you measure resistance less than 10 kΩ, repair the wiring harness as needed. If you measure more than 10 kΩ, suspect a faulty GSP sensor.
If the diagnostic tool and circuit diagnostics yield no issues, you may have a faulty ECM or TCM, but this is not common.