C0300 Error Code: Rear Speed Sensor Malfunction

Error codes also known as diagnostic or engine codes are codes triggered by a malfunction in one or more parts of your car. The onboard diagnostic (OBD) system is an internal computer that monitors car performance and detects any malfunction in these areas.

This article explores the C0300 error code which is a common error code that signifies a rear speed sensor malfunction. Reading further, you will get a better understanding of this error code, the symptoms to look out for, how to diagnose this code appropriately, and how to fix it. 

Overview of the C0300 error code

The ‘C’ in this error code stands for the chassis. The chassis is the underneath supporting structure of a vehicle and it is responsible for housing the transmission, engine, and wheeling system. The C0300 code represents a specific malfunction in one of the chassis components which is the rear speed sensor. 

The rear speed sensor is a part of the braking system that receives output from the electronic control module (ECM) to monitor the rotation of each tire. This is to ensure that a signal is sent when a tire becomes locked during braking or when the tires slip during acceleration. 

Possible causes of C0300 error code

Some possible problems that may trigger the appearance of this code in your car include:

Faulty rear propshaft speed sensor

When the rear propshaft sensor is faulty, it means that your car will not receive signals about any abnormalities which can lead to not being aware of your tires locking up when braking or slipping during acceleration. This triggers the C0300 error code on your OBS scanner and other symptoms of this code as well.

Open or shorted rear propshaft speed sensor harness

The wiring harness of a vehicle is responsible for the flow of energy and information from the car’s primary systems: ECM and anti-lock braking system (ABS). in shorting, current jumps from the speed sensor wire harness to take a different route, and when the harness wire is open, the wire harness has lost its insulating cover.

Rear propshaft speed sensor circuit for poor electrical connection

The electrical connection of the speed sensor circuit enables it to deliver signals when needed. When this connection fails or is poorly done, the speed sensor will fail to carry out its duty.

Symptoms of the C0300 error code

The presence of this code affects the normal functioning of your vehicle, and this is noticeable by the presence of the following symptoms or unusual behaviors in your car.

ABS warning light on


The anti-lock braking system (ABS) is a safety feature present in most modern cars and its responsibility is to prevent the tires of cars from locking. This allows drivers to steer safely when an emergency happens. 

Engine light on

The check engine light is another display warning you may get on your dashboard. This kind of warning indicates a problem in your car but does not specify the location. It is also displayed in yellow light. 

Engine stalling or misfiring

This is a situation when your car’s engine suddenly stops for a while causing your car to slow down or come to a complete halt. Engine stalls can be caused by several car malfunctions, so you shouldn’t rely on just this symptom to assume that this code is present. 

Misfires occur when one or more cylinders in your car don’t produce power which can lead to your car slowing down even when attempting to accelerate. This can also feel like your driving experience is rough. 

Troubleshooting the C0300 error code

To properly troubleshoot this error code, you need to diagnose it first using an on-board diagnostics (OBD) scanner or reader and then you can proceed to fix the problem. 

Diagnosing this error code


An OBD2 scanner is a diagnostic tool implemented in 1996. Collects information about your car’s performance using a chain of connected sensors and also helps to regulate this performance. If your car model was manufactured after 1996, then you have a diagnostic link connector (DLC) for an OBD 2 scanner.

Here are the steps you can take to use an OBD scanner.

Step one: Locate the OBD connector port

On the driver's side of your car just below the steering wheel, your connector port lies in that area, but it may be hiding behind a small door or flap.

Step two: Connect your OBD scanner

Using the cord that comes with your OBD scanner, connect this device to the DLC. Some scanners don’t require external power so the scanner may tell you to leave your car on, idle, or off. 

The scanner automatically reads your vehicle information when connected, otherwise, you may need to input your car’s information (model, manufacture year, and vehicle identification number).

Step three: Initiate a scan

For some scanners, you may just need to check the menu and navigate your way to discover the error codes present in your car while others may require you to manually scan for these codes.

You may get an active code (triggers engine light) which should be attended to immediately or pending codes which indicate minor issues. However, pending codes can become active codes if left unaddressed for a while.  

Step four: Note the code

Take note of the error code displayed on your OBD scanner. If it is the C0300 error code, then you can keep reading this article to get an idea of how to fix this issue.

How to fix the C0300 diagnostic error code

Here are some possible steps you can take to get rid of this code from your vehicle.

  1. Replace the propshaft speed sensor: If the rear speed sensor is faulty, you need to purchase a new rear propsahft speed sensor and replace the old one. You can get a professional or auto expert to help with the installation.
  2. Insulate the propshaft speed sensor harness: If the wiring has lost its insulation, you can get a replacement. However, if further issues are involved with this sensor, you may need to get a new one ready for installation. 
  3. Check the connection: When your rear speed sensor is malfunctioning may be a result of a wrong connection. Cross-check the harness connection and ensure that it is done appropriately. 
Using the ABS

Using the ABS, you want to apply some pressure on your car's brake pedal and then hold this pressure before steering to safety if your car ABS is not working. Before the installation of ABS in modern cars, drivers were advised to pump their brakes when they start to skid. This technique is still applied to cars without ABS.

For your ABS to function properly during these kinds of situations, ensure that your tire is inflated, has its lining in place, and that your brake fluid is not contaminated. 

Since the rear speed sensor is related o the ABS it triggers the warning display light on your dashboard and it is visible as a yellow circle with ‘ABS’ written inside it. You may also see a yellow traction control warning sign appear as well.


The C0300 error code is one of the most popular diagnostic codes drivers get, however, it is important to seek necessary repair for this code immediately. After reading this article, you should have a clear understanding of what the C0300 code represents, what can cause it, the symptoms associated with it, how you can diagnose it with an OBD scanner, and how you can get it fixed.