Key Not Detected? Understanding the Two Ways Your Car Reads Your Key

One of the most frustrating problems car owners can experience is when your vehicle doesn’t read your key. Errors such as “key not detected” or “key not recognised” can be a headache to see, but there are a few ways to overcome this problem without a massive cost or hassle.

A common issue that occurs in this scenario is when a keyless vehicle cannot pick up the fob in the car and start as normal with the button press. However, if the physical key is inserted into the steering column, it then starts. This is a sign there is a module issue somewhere in the car. So, one can ascertain that the part of the car that reads an inserted key is fine, but the part that listens for the key from a distance is not.

The ECU Pro offers an affordable mail-in testing service to inspect your car access system (CAS), ECU, and key fob, along with repair and replacement options.

Understanding fobs: two conversations, two radios

Modern cars use more sophisticated fobs that essentially have two different frequencies (or signals) with the car: one of which allows for unlocking and push-button start functionality (the radio signal), and the other which acts as a handshake with the ignition (security signal) to deactivate the immobilizer. Both can fail separately without affecting the other.

How keyless cars work with fobs (radio signal)

The fob transmits on a licensed radio band, 315 MHz in North America and 433 MHz in most other markets, and the car answers through an antenna and a receiver module. The receiver is rarely at the ignition. On many models, it lives in the rear of the car, in the trunk area or behind the rear seat, and hands-free cars add antennas in the doors, bumpers and cabin so the car can tell whether the fob is outside, inside, or in the trunk.

This side needs a healthy fob battery, and it needs that receiver to be powered and talking to the rest of the car. When it fails, you lose remote locking, remote start and walk-up entry, and the car may report "key not detected" while the key is sitting in the cup holder.

The importance of the security features (security signal)

The second part of this communication between your fob and car is the security element. This is the handshake that disarms the immobilizer, which then decides whether the engine is ‘allowed’ to run. The key carries a transponder that is powered by induction from a coil in the car, at 125 kHz, with a reach of a couple of inches. On BMW and MINI that coil sits behind the marked spot on the steering column, or around the key slot on older cars. On Mercedes the key is read when it is inserted into the EIS, the Electronic Ignition Switch, and Mercedes keys of that era also talk to the EIS by infrared through the tip of the key.

Because the coil powers the transponder, this conversation works with a completely dead fob battery. That is the "emergency start" you may already have used: hold the fob against the column, or push it into the switch, and the car starts.

The modules that handle this side of the communication is the EWS, CAS or FEM, depending on the BMW and MINI generation. If the car doesn’t recognise the key, this ‘handshake’ fails and you get a no crank, crank-no-start, or a "key not recognized" type of message.

Due to the nature of these issues, it’s always best to test the various modules in order to narrow down and resolve the problem.

You can simply mail your EWS, CAS, or FEM to The ECU Pro for a thorough inspection and replacement or repair service.

Five-minute checks before you ship anything

1. Test your vehicle with the spare key (if possible): If your spare key works fine, it points to a problem with the original key. Two keys failing the same way points to an issue with the car.

2. Try the emergency start: Hold the fob to the column, or insert it. If the car starts, the security side is alive, and you are chasing the radio side.

3. Check the car's battery voltage: Low voltage produces a huge range of key and module complaints that may be resolved with a charged battery.

4. Don't buy a used module before you know which side failed: A used CAS, FEM or EIS is paired to the original car's keys and engine computer. It cannot simply be plugged in and requires programming.

What a bench test proves, and what it can't

We’ll isolate the issue with the vehicle by testing your fob, module, and engine computer to see which parts are causing the issues. If all of the tests on the module and keys don’t reproduce the problem, then we can narrow down the issue to your car itself, and not the modules and key.

When the security side is the problem

That is the part we do every day:

• BMW and MINI CAS: testing, repair or replacement, with keys programmed to match.

• BMW and MINI FEM: testing and repair for the F-series.

• BMW and MINI EWS: testing and repair for the earlier generation.

• Mercedes-Benz EIS: testing, repair or replacement on the generations we support.

• Mercedes-Benz keys and BMW and MINI keys: cut, programmed and bench-tested against your module.

What we need from you:

Send the security module, the engine computer and one key together. The set is tested as a set. If the module comes back fine, you have a clean answer and can go straight to the other side of the conversation, which on a lot of cars means the back of the vehicle.