My car shows a gear sign on dashboard, it does not show any trouble codes on diagnosis

It may be just a temporary issue. Try this,

Put shifter in park, put the park brake and turn the engine off. Wait a few seconds and restart the engine. If the gear sign still shows, then it's a more serious problem than it looks.

 If such is the case then your transmission is probably slipping.

Drive the vehicle while trying to maintain a constant throttle position as it accelerates upwards thru all the gears. If the transmission is shifting properly, it should be in 4th gear by the time you reach 60 to 80 kph on level ground. Repeat the procedure 3 to 5 times Rescan the PCM for trouble codes, if nothing appears again then the problem could have been a onetime occurrence. If a trouble code appears you take it from there and fix as necessary.

Even if it resultantly appears to be a onetime thing,… that is not an incentive enough for you to trust that your transmission is okay. 

It owes partly to bearing, clutch or hard part failure, and this will come with a grinding, whirring noise but again, not every time.

What are the Effects of a Faulty Speed Sensor on a Modern Car?

"An engine speed sensor retrieves and sends data to your car’s computer system."

Transmission speed sensors are used to calculate the actual gear ratio of the transmission while in use. There are generally 2 of these VSSes that work in conjunction to provide accurate transmission data to the vehicle's powertrain control module.(PCM)

The 1st is the input shaft speed (ISS) sensor. This sensor is used to monitor the speed of the transmission' input shaft. The other is the output shaft speed (OSS) sensor.

If either of these two sensors falls out of alignment or experience electrical issues, it impacts the operation of the entire transmission.

After registering data, the two transmission speed sensors, send data to the PCM,which compares these two inputs and calculates which gear the transmission should engage for efficient driving. The actual gear ratio is then compared to the desired gear ratio.

If the desired gear and the actual gear do not match, then the PCM will set a diagnostic trouble code (DTC) and illuminate the Check Engine Light.

A defective car speed sensor may cause different problems and just below are the most common ::


1. Harsh or improper shifting
When there are symptoms of a bad transmission speed sensor, the powertrain control module cannot control the shifting of gears within the transmission properly.  This may cause transmission revs higher before shifting gear or late transmission overdrive and inability to go into that top gear. Harsh shifting may damage internal components such as valve bodies, hydraulic lines, and even mechanical gears in some cases.

2. Inactive Cruise Control
The transmission speed sensor does not only control the input and output shaft speed. Meanwhile, it also plays a role in monitoring the cruise control. When there are some engine speed sensor problems, an error code will be sent to the car’s Engine Control Unit (ECU). Then the cruise control will be shut down and does not engage when you press the button. The speedometer also may work frantically or sometimes stop working altogether.


3. Check Engine Light comes on
If your car has some engine speed sensor problems, the Powertrain Control Module (PCM) will set a diagnostic trouble code and illuminate the Check Engine Light on the vehicle dashboard. Then the check engine light will come on sporadically and intermittently when accelerating. The overdrive light also may go ‘On’ or ‘Off’ for no apparent reason.