Prius P Lock Malfunction – Causes, Troubleshooting & Fixes

The Prius P lock malfunction is a common problem with Prius vehicles. P lock is a central lock option mainly seen with cars with the 12V auxiliary. 

It is a safety option that does not allow your car to shift from park gear without the brake pedal pressed down.

Sometimes this feature is prone to malfunction like many other electrical components in cars. There have been a lot of complaints from owners of the Prius that the P lock will malfunction and they will not be able to shift the vehicle gear away from the park.

The Prius P Lock malfunction is mainly caused by an electrical problem resulting from a damaged wiring harness, a weak auxiliary battery, or a blown fuse. Although not so often, a faulty transmission ECU can also cause this error.

P Lock Malfunction when parking Prius

This article will discuss all the possible causes and explain what you can do to troubleshoot and fix the error.

Causes of the Prius P lock Malfunction

There are a lot of factors that can cause the P lock feature to malfunction. Some of the factors include the following:

1. Damaged wiring harness (e.g. from rodents chewing) 

The general transmission uses a wiring harness that allows the system to communicate with the Powertrain Control Unit (PCU). Proper communication between the various parts of the transmission system, including the Transmission Control Unit (TCU) and the PCM, lets the vehicle know when to shift gears and allows the driver to change from park to drive or neutral. 

There is a disconnection due to damaged wiring, which could be caused by rodents chewing on the wiring. The transmission will not be able to shift from the P lock option. 

How to check & fix it?

Recently, the amount of wiring harnesses in vehicles has been so many, making it difficult to diagnose and fix wiring issues. You will have to check the transmission wiring physically to analyze the problem. 

Consult the manual if you do not know the wiring harness for your vehicle’s transmission. You can disconnect the gearbox, trace the wiring to the fuse box, and ensure that the wiring is not damaged. 

If it is damaged, you must replace the wiring harness.

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2. A blown fuse

A blown fuse can cause a Prius p malfunction. It is an integral part of the vehicle’s electrical components and prevents these components from experiencing an electrical surge. 

Once the fuse gets blown, the electric current will be interrupted, causing the fuse’s electronic component to seize working or malfunction.  

How to check and fix it?

If you want to fix your P lock malfunction and suspect the problem could be due to a blown fuse. You will have to diagnose to ensure the problem is from a blown fuse.

To diagnose, you will first need to consult your vehicle’s manual to locate the fuse or relay related to the transmission system. After finding the fuse or relay, ensure that it is not burnt or damaged. 

If you have a blown fuse, you must replace it immediately to avoid further damage to other parts of the transmission system. Ensure you get a close enough replica of the fuse you will be returning, and confirm it is the same amp rating.

Check the fuse box to ensure that the terminals are working fine. If the terminal is damaged, you may have to replace the entire fuse box.  

3. Weak auxiliary battery (12v battery)

A weak auxiliary battery may be causing the Prius p malfunction. The auxiliary is a backup battery for the principal battery. 

It performs various functions in your Prius, including powering some electronic parts in the engine, radio, electronic power steering actuators, central locking and keyless entry system for unlocking the vehicle, and more.

Although many Pruis users ignore the auxiliary batteries of their vehicle, there are some problems that your vehicle may be experiencing, which may result from a failing or malfunctioning auxiliary. 

The Prius uses a 12v battery, and the central lock system relies on it to get an adequate electricity supply. 

How to check & fix it?

If your P lock malfunction is due to a failing 12v battery, you will have to change the battery. Like the main battery, the 12v auxiliary battery does not work forever. Manufacturers recommend that you replace the battery after six years. 

To properly diagnose this problem, you may need to take your car to a professional mechanic. It is easy to detect that the auxiliary battery is terrible; one symptom is that other electronic components in the vehicle will also start failing.  

4.  Faulty Transmission ECU

The Engine Control Unit is one of the most important electronic components in the vehicle that controls and monitors other vital parts of the car. The Transmission Control Unit (TCU) works with the ECU by communicating with it to ensure the transmission system is working correctly. 

Once the ECU stops working correctly, the transmission system will also lose its function and other significant parts of the vehicle. 

How to check & fix it?

There are so many ways to check for a bad ECU. You can watch for symptoms like engine misfiring and stalling, check engine light, no ignition, engine surge, and failing electronic components. 

You can use scan tools to diagnose failing ECU. You have to look for codes related to a failing ECU. 

Once you are sure that the problem with your transmission is from the ECU, you can try and reset the ECU. You can also upgrade the operating system. If the problem persists, the ECU is damaged, and you should replace it.

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What Else Can You Try?

There are other fast ways to temporarily fix a P lock malfunction when in a difficult situation. However, you should know this may not be a permanent fix. You can fix a Prius P lock malfunction by following these steps: 

1. Disconnect your vehicle’s battery to reset the ECU, and wipe some code and warning messages from the computer. This way, the electricity is wholly drained from those components, and error codes and other computer warnings will subsequently clear. 

Next, reconnect the battery back. Your gear should be able to shift to park now; if not, it could be a more severe problem, as discussed above.

2. Press the power button twice. Please ensure the key fob is in the car and do not press the brake while at it.

3. Shift the car to neutral while your foot is off the brake. Make sure you are parked on a flat surface.

4. Push the park button. It should display that you parked on the dashboard.

5. Step on the brake and firmly push the power button. It should start normally, and the warning message should clear completely.

If everything fails, your last resort is to have a tow truck come to you and get the car to a local repair shop. Alternatively, a mobile mechanic might be cheaper and could have the car up and running in no time.

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Final Thoughts

When your Prius Lock Malfunctions, it is not a time to panic; you need to act fast and get a solution. To do this, you will have to identify the cause, which could be a blown fuse, a wrong wiring harness, and more. Once you find the cause, you will be able to fix your car in no time.