, especially those made since 1996 up to the present. Specifications on the definition, troubleshooting, and repairs, of course, vary from one make and model to another.

Today, most modern vehicles, whether manual or automatic, automated manual, or adaptive dual-clutch, are monitored and controlled by the Engine Control Module (ECM) and Transmission Control Module (TCM). For some, the TCM is integrated into the ECM. Automatic and automated-manual transmission vehicles have ECM and TCM that work together to modulate engine output and transmission engagement. They can tweak gear engagement for either smooth or powerful performance. For manual and adaptive dual-clutch transmission vehicles, the ECM monitors the TCM and the readings to affect precise throttle blips and fine-tune the engine output.

Depending on the maker and type of transmission, the TCM may use various sensors to monitor and control the transmission, such as temperature sensors, pressure sensors, and speed sensors. Many of these transmissions also have Gate Select Position (GSP) sensors, which may be mounted on the shift linkage, shift lever, or transmission. The GSP sensors tell the TCM which gear position the driver has positioned for at least a couple of reasons.

For vehicles with automated manual transmission or automatic transmissions with manual mode, the GSP switch tells the TCM whether the driver has chosen an automatic or manual function. For manual transmissions, the GSP informs the TCM which gear the driver sets the vehicle, and depending on the design, may work as a neutral safety switch. Subsequently, the TCM can also use the GSP switch to monitor the transmission operation. If the driver chooses manual mode and sets the vehicle to third gear in an automatic transmission, the TCM may monitor the speed sensor to detect any slippage in the brake or clutch.

If the ECM determines a problem with the GSP sensor circuit, it will automatically store the error code and activate the Check Engine light. Error Code P0906 means the Transmission Gate Select Position Circuit Low.

This code may be accompanied by related GSP sensor codes, such as:

  • Error Code P0904
  • Error Code P0905
  • Error Code P0907
  • Error Code P0908

Common Symptoms

This code activates the Check Engine light. Depending on the design of the system, the vehicle may not exhibit drivability problems. Common symptoms, however, include:

  • Vehicle unable to start
  • Erratic shifting pattern
  • Delayed or harsh gear engagement
  • Missing gears

Possible Causes

  • Misalignment of sensors (usually happens after transmission repair)
  • Faulty GSP sensor (exposed to elements, corrosion, etc.)

How to Check

Since this error code refers to a circuit problem, you will need a digital volt-ohm meter (DVOM) to diagnose the GSP sensor and the circuit in the ECM or TCM. A scan tool with live data streaming capabilities can help diagnose the problem since it can show you what happens in the ECM and TCM.

The usual suspects for this code are circuit problems, such as corrosion, damaged wire harness, loose connections. However, alignment problems can also cause this problem.

First, check the sensor alignment using the scan too. Rotation sensors must be aligned to a “neutral baseline” to be able to detect the GSP. If it is off to one side or another, the GSP voltage may be skewered, or the TCM may detect more than one position at a time. Also, make sure the screws that mount the sensors are properly torqued to keep the sensor case from moving.

GSP sensors on the shift lever or shift linkage may depend on physical contact, buttons, or tabs forming the base of the sensors or switches. Over time, the metal or plastic tabs may bend, weaken, or break off. A couple of brands that use magnetic micro-switches to sense position have been known to come loose. Repair or replace the damaged parts.

Next, you will need an Electrical Wiring Diagram (EWD) to check the circuits that run between the ECM and GSP, specific to the year, make, and vehicle model.

Inspect the GSP sensor; look for signs of damage in the wire harness, connector, ECM, and TCM connector. Check the connections and look for signs of corrosion. This could be a sign of water entry. Also, look for damaged or loose pins. Make sure the connections are secured and seated correctly—repair as needed.

Check for continuity between the ECM and GSP. If you get more than 5 ohms, you may need to fix the wire harness. If it’s less than 5 ohms, then you can suspect a failed GSP sensor. However, do not condemn the sensor until you finish checking the circuit.

Next, look for short circuits between each and from each line to ground. If you get more than 10 kiloohms of resistance, then you may need to fix the wire harness. If you get more than 10 kiloohms, then you can suspect a failed GSP sensor.

How to Fix

  • Realignment of sensors that were in the wrong position
  • Repair or replacement of damaged electrical components
  • Replacement of the faulty sensor

If both scan tool and circuit diagnostics show no problem, you may have a failed ECM or TCM. This is rare but not entirely impossible.