Signs of a Bad Gear Position Sensor

Your gear position sensor is usually located on your car's automatic transmission. It communicates the gearshift’s position to the vehicle’s electrical system. Once a drive range is selected, the sensor sends that information to the engine computer. The gear position sensor participates in these transmission-related operations:

• Requiring the transmission to be in Park or Neutral before the car can be started
• Requiring the brake pedal to be depressed before the car can be started
• Turning on the backup lights when you shift into reverse

Your gear position sensor may be of the mechanical or electronic type, which uses a cable or a linkage to connect the shift lever to the transmission. Another type of is the “drive by wire” gear position sensor, which has no direct physical connection between the shift lever and transmission. The drive by wire system is computer-controlled, and has the benefit of not allowing transmission-related actions that could damage the vehicle, including:

• Shifting into Park or Reverse while the vehicle is moving forward
• Shifting into a low gear at a high speed

Does your car have a gear position sensor?

If your car has an automatic transmission (and most cars today do), it probably has a gear position sensor.

Signs of a bad gear position sensor

There are several ways you can tell whether you have a bad gear position sensor:

• Your car will not start: A bad gear position sensor can prevent your car from starting if the car’s computer does not “know” whether the car is in Park or Neutral.

• Your car starts, but will not shift into Drive or Reverse: This is a situation where a bad gear position sensor knows that the car is in Park and lets it start, but is not sending the signal that you have shifted into Drive or Reverse. This means that the transmission will not engage those gears, and you will not be going anywhere.

• Your transmission shifts into a different gear than the one selected: This is another sign of a bad gear position sensor. Your transmission is engaging a different gear than the one you need, which can make your car difficult or hazardous to drive.

• Your transmission goes into "limp home" mode: This can happen with computer-controlled transmissions with a bad gear position sensor. To protect the transmission and the car from any further damage, the computer puts the transmission into what is known as “limp home” mode. This allows the car to be driven at low speeds, while locked into one specific gear.

• Your check engine light comes on: A bad gear position sensor can definitely throw off your car’s emissions. This will often cause your engine computer to trigger the check engine light on your dashboard. Trouble codes in the computer may indicate that the source of the problem is a bad gear position sensor.

Should you drive your car with a bad gear position sensor?

Without a functioning transmission position sensor, most cars can’t be driven safely. Even in limp home mode, driving is not a pleasant experience. If you have a bad gear position sensor, you should call your mechanic and have the car repaired, before the problem gets any worse and leaves you stranded!

How do you fix a bad gear position sensor?

A thorough diagnosis of a bad gear position sensor by your mechanic will involve checking all aspects of its operation, analyzing the exact symptoms that your car has produced, as well as taking a close look at any related trouble codes that may be residing in the engine computer. Once that this has been done, your mechanic can repair or replace any defective parts, so that your bad gear position sensor can be returned to normal operation.

At the same time, it is a good idea to check your transmission’s condition and operation, to be sure that the problem does not extend beyond a bad gear position sensor. Any functional issues should be fixed. If a transmission service is due, it should be performed. Your transmission fluid should also be checked, topped up if needed, or replaced if it appears worn out or contaminated. This will assure you that all potential transmission issues have been dealt with, and that this key component of your car has a clean bill of health.

Can you replace a bad gear position sensor?

If you have good auto mechanic skills, and the job of replacing a bad gear position sensor is a simple cable or linkage replacement on the outside of the transmission, you should be able to pull it off. But if the repairs extend to troubleshooting electronics, or opening up and repairing the transmission itself, you should definitely leave the job to a professional.