Mercedes-Benz A-Class is equipped with remote keyless entry system that allows the owners to conveniently lock and unlock the vehicle with the press of a button on the key fob. There are many reasons why the remote control of your A-Class may not be working which we will discuss in this article.
The most common cause of key fob not locking or unlocking the doors in Mercedes-Benz A-Class is dead coin battery inside the key fob. But the remote control can also stop working due to worn buttons, poor battery contact, water damage, receiver module issues, signal interference, dead 12 volt battery, unpaired key requiring reprogramming, or defective electronic chip.
1. Dead coin battery inside key fob
Dead coin battery is the leading cause of key fob not locking or unlocking the doors in A-Class. Usually there are signs of weakening battery before it gives up. If the remote was working previously, but the range of the remote was gradually decreasing until it ceased to function, this indicates that the battery was wearing out.
Replace the battery
If the button cell battery in the key fob of A-Class is replaced incorrectly or a battery is unsuitable, it can damage the vehicle key. Only replace drained batteries with new batteries in the same voltage, size, and specification. Make sure the battery is facing in the right direction when inserting it.
Key fob not working after replacing the battery
When purchasing a new battery for the key fob of your A-Class, keep in mind that the battery may be sitting on the shelf for a long time. Therefore after replacement it may not work, or may not last that long. Try purchasing the battery from a store where stock moves fast. If you still can’t get the key fob to work after verifying that the battery is healthy, your key fob issues may not be due to bad battery.
How long does the key fob battery last?
Depending on how often you use the key fob or remote control of your A-Class, the battery should last between two and four years. This is assuming the new battery you installed is healthy and its use by date is many years away.
Tip: You can try to lock/unlock your A-Class with its spare key. It will rule out any other problem with the first key, for example, water damage.
2. Poor battery connection
The battery inside the key fob of A-Class has metal retaining clips that hold the battery in position and completes the circuit. If the clips don’t have any tension, it can cause contact issues and the remote may not receive any power. Make sure the battery is firmly placed and does not move around when you shake the remote. Also make sure all the battery contacts are corrosion and dust free.
3. Water damage
If your Mercedes-Benz A-Class key fob has taken a bath in the soapy water of the washing machine after you forgot to take it out of your pants pocket, water damage is very likely.
Water resistant, not waterproof
Your A-Class key has rubber seals which prevents the water from getting to the electronic chip inside. A splash of water or brief exposure to light rain should be fine, but exposure to excessive amount of water like submerging the key in the pool can cause the water to penetrate the seals and damage the chip. Submerging the key in ocean water at the beach can be much more damaging to key fob electronics due to highly conductive nature of salt water.
But my key fob survived water exposure?
If your A-Class key fob survived a wash cycle, or a deep bath in the ocean or pool water, it doesn’t make the key fob waterproof, it means you got lucky. Exposure to excessive water increases the risk of water damage, so you shouldn’t make this a habit.
What to do in case of water damage?
If your A-Class key fob stopping working after exposure to clean tap water or rain, immediately remove the battery and clean the electronic component with paper towel and let it dry fully before putting back the battery. In case of exposure to salt water, or soapy or dirty water, it is recommended to clean the chip with isopropyl alcohol or electronic cleaner. If the key fob still doesn’t work after cleaning and drying, the chip is probably fried and you will have to buy a new key.
4. Dead 12 volt battery
The remote keyless system and the central locking system in A-Class require the 12 volt battery to be in healthy state to function properly. If the 12 volt battery is dead or if the electric current has been interrupted due to corrosion on the battery terminals or the ground connection, all on-board electronics will stop working.
In this case, you will have to manually unlock the door of your A-Class with physical key. Also, the electrical accessories (e.g. headlights, fan, and infotainment system) will not function or may function poorly, and the engine will not start. Check the 12v battery and its connections.
5. Signal interference
A-Class remote control signal can be disrupted by objects, poor weather conditions or transmitters on the same frequency band close to the vehicle. The remote key may work when you’re near the vehicle, but may not work from a distance. In case of severe interference, the remote may not work at all.
Interference signals can be caused not only by neighboring radio masts and towers, but also by unprofessionally installed satellite systems. A large part of the radio interference is caused by defects and aging effects in electrical devices and radio systems, which lead to unwanted emissions, radiation and repercussions on power and communication networks. Your A-Class key fob should start working again when you leave the area of interference.
6. Reprogramming required
In rare cases, the remote key of A-Class may stop working due to pairing issues. The key can be reprogrammed with an OBDII scanner. You may also be able to program a key by following a few steps, an original key is typically required for successful pairing. The instructions should be mentioned in the owners manual. If you fail to do it yourself, you may have to visit an authorized dealer for reprogramming of the key.
7. Defective key fob
If your A-Class key fob stopped working after you dropped it on the floor, you may have damaged the internal chip. Or, if your key fob just stopped working one day out of the blue, in rare cases this could indicate a faulty key fob – assuming you have tried everything to fix the issue e.g. battery replacement, reprogramming and the spare key fob works.
8. Faulty receiver module
Mercedes-Benz A-Class has a remote keyless system receiver that receives radio frequency signals from the key fob. If none of your key fobs work, and and you can’t reprogram the keys, it is possible the receiver module has developed a fault.
You may have to perform further diagnostics to identify the faulty. The keyless entry antenna could be damaged, or the module is just not communicating with other modules on the vehicle. In these cases an OBDII scan tool can be used to verify if the commands are being transmitted and acted on by the security module.
Reset the system
If the key fob is not working due to a glitch in the keyless entry system, it may be possible to restore the functionality by resetting the keyless entry system in your A-Class.
There isn’t any button or option to reset the keyless entry system in A-Class, but you can reboot all the on-board computers by disconnecting the 12 volt battery for a few minutes. Remove the cable from the negative terminal of the battery first, then from the positive terminal. With the battery disconnected, press the horn button a few times and turn on the headlights to drain out all the residual electricity from the system. After about 15 minutes, reconnect the battery in reverse order: positive cable first, then negative cable.
Use OBD2 scanner for diagnosis
Since Mercedes-Benz A-Class is equipped with on-board diagnostics (OBD), a fault diagnosis can provide initial indications of where the malfunction is located. This method will be especially useful when diagnosing a faulty remote keyless system receiver module.
To begin troubleshooting, you must first connect the diagnostic tool to your A-Class. The OBDII connector is usually located under the dashboard. With the tool connected, turn on the ignition. Most diagnostic devices then ask for some information about the vehicle. It is important that you enter this 100% correctly, otherwise the result of the search may be inaccurate. In addition to the vehicle make, model, and engine type, you usually also have to type in the Vehicle Identification Number (VIN). Since some OBD codes are manufacturer-specific, the scanner will be able to give you more accurate information if you enter more details about your A-Class.
There are many reasons why your Mercedes-Benz A-Class key fob is not locking or unlocking the doors. When looking for the reason, you should start with the most obvious cause that is easy to diagnose: dead coin battery, which you can replace in a few minutes.