Why does a car show a key not detected message?

This blog will explain the causes of the key not detected message in a car and answer the following questions: what are the components in a keyless system? How does a keyless system work? How to fix a key not detected in a car?

Why does a car show a key not detected message?

A car shows a key not detected message due to following reasons:

• Weak key battery
• incorrect/faulty key
• Control unit malfunction
• Keyless start antenna

Let’s take a closer look at each of the causes:

Weak key battery

There is a small battery in the key fob to power the transmitter and internal electrical circuit. Many key fobs come with a low battery indicator. When the button is pressed the low battery indicator will flash and alert the driver.

The key battery lasts very long as the power consumption of the key fob is low but it can drain if the key is frequently used to lock and unlock the vehicle. It is very easy to replace the key battery, simply remove the mechanical key from the key fob and open the battery.

The battery inside the key cannot be charged as there is no charging circuit in the key. If moisture enters the battery try then there can be corrosion which increases the contact resistance of the terminals.

Hence, before replacing the battery check the small contact plates of the key fob for proper contact and signs of corrosion. If the message occurs even after replacing the battery then further diagnosis is needed to rectify the fault.

The key battery can be replaced by the customer and it does not need any special tool. Positive and negative contact points are clearly marked on the key battery and in the battery tray of the key fob. 

The key will not work if the new battery is different from the old battery. The key fobs in old cars have two batteries instead of one because an infrared signal was used to transmit the drive authorization data.

Incorrect key/faulty key

When the key not detected message is shown in the instrument cluster of a vehicle it can simply mean that the wrong key is used to start the vehicle. The keys are assigned to the vehicle and if the drive authorization codes do not match, the vehicle cannot be started.

Similarly, if the key fob of the vehicle is damaged the transmitter antenna is not able to send the data to the electronic ignition lock. The electronic ignition lock will not turn on the ignition of the vehicle until the drive authorization is completed and the key is present in the vehicle.

When an internal malfunction of the key occurs, the vehicle will continue to operate for that particular driving cycle. The engine will not start again after the vehicle is switched off. Usually, if a key is faulty it cannot be repaired, a new key is programmed and it acts as a replacement key for the original key.

Control unit malfunction

The keyless start or keyless go control unit is responsible for searching the key and sending the wake-up signal to the key. It uses a dedicated keyless go antenna to scan for the key. If there is a communication fault in the control unit, it will become inoperative.

A communication fault occurs in the control unit when the power supply is not within 11-14.5 V. the control unit also communicates over the CAN network of the vehicle. if there is a fault in the CAN electrical line, then the control unit can malfunction.

A fault code is stored in the fault memory of the control unit, if the fault code is related to an internal software error; the control unit must be replaced. However, if there is a communication fault then the power supply and CAN signals must be checked for the proper operation of the keyless start systems.

Keyless start antenna

The keyless start systems used up to 4 short-range antennae to scan the key. The antenna is located in the door handles and inside the vehicle. The door keyless go antennas are only capable of transmitting the wake-up signals.

The antennas in the vehicle can transmit as well as receive the drive authorization codes from the key. If there is an open circuit or short circuit in the electrical lines between the antennas and the keyless go control unit, the vehicle will not be able to detect the key.

Similarly, if there is an internal fault in the antenna, it must be replaced. The keyless go control unit can detect which antenna is not working correctly and store a corresponding fault code. Many cars have only a keyless start system and use only one antenna inside to send the key wake-up signal and receive the drive authorization codes. This antenna is located in the dashboard or the center console.

What are the components of a keyless system?

The components of a keyless system are given below:

• Keyless go/start control unit 

The control unit is at the heart of the keyless start/ go system. It is responsible for detecting the key and transferring the data for the drive authorization. The keyless go control unit is always active as it is directly connected to the main battery of the vehicle.  

• Keyless system antenna

There are transmitter and receiver antennas in the keyless system. The antennas are scanning for the correct key. The effective range of the antennas is 2 m, if the vehicle has a keyless entry/exit then it is possible to lock or unlock the car after touching a button or capacitive sensor.

• start/stop switch

The star/stop switch is connected to the keyless go control unit. Some of the cars have a removable switch so that the key can be inserted directly into the barrel. The drive authorization codes are only transmitted from the key when the switch is actuated.

• Key fob

The key fob of a keyless start/ go system consists of the short-range antenna as well as the receiver. Hence, the cost of the spare key is more compared to a vehicle with no keyless system. 

There is also an induction coil in the ignition switch to temporarily power the key and receive the data if the key batteries are dead.

Conclusion

This blog explained the different problems that can cause the key not detected messages to appear in the instrument clusters. The most common cause is the dead key battery but if the message is still present then consult an authorized workshop for further diagnosis.