Central locking and key fobs: why the car stops locking

Central locking is a system where the symptom almost never identifies the culprit. “It will not lock from the fob” can mean five different faults, and four of them are not in the fob.

First separate two situations

The most important step, and it is free.

Only the fob fails, while the key or the interior button works. Then the problem is the control channel: fob, receiver, synchronisation.

Nothing works at all, or specific doors fail. Then the problem is the lock, the actuator, or the wiring.

When the fob is at fault

A flat battery — the single most common cause. The sign is range falling to a few metres, then to nothing. It happens gradually, which is exactly why the moment of failure feels sudden.

A worn button — the contact inside abrades while the case still looks perfect.

Lost synchronisation. Receiver and fob both count a code sequence. Pressing buttons repeatedly far from the car pushes the counters apart, and a relearn procedure is needed.

Radio interference. In car parks by shopping centres and airports this is real: jammers and strong transmitters. The sign is failure in one specific place only. The check: walk up to the glass and press.

A practical rule: if one fob failed abruptly and completely while the spare works, it is the fob. If neither works, it is the car.

When the door actuator is at fault

One door will not lock — that is almost always the actuator. It contains a motor and a mechanism, and both wear.

The signs: buzzing with no result, working intermittently, working only in warm weather.

Freezing in winter — moisture inside the actuator; below zero the mechanism will not move.

A door that will not open from inside or outside while the lock works is a rod or handle fault, cheaper than an actuator.

The role of the door switch

The door switch tells the module whether a door is open. It is also the component that most often lies.

Symptoms of a faulty switch:

• The car will not lock because it “believes” a door is open.
• A door-open lamp with every door shut.
• The interior light never goes out.
• The battery goes flat overnight — the module never sleeps; covered in the parasitic drain article.

That last point is the main reason a door switch is worth fixing rather than ignoring.

What diagnostics shows

Body electronics keep their own codes, and this is where a scanner saves the most time.

Body module codes — per door: actuator, switch, button. That identifies the guilty door immediately, without stripping any door cards.

Switch states in live data. Open each door in turn and watch the parameter. The door whose state never changes is the one.

Receiver and immobiliser codes — if no fob works at all; covered in the immobiliser article.

System voltage. During a sag, body modules behave strangely and log codes for faults that do not exist; mechanics in the ghost codes article.

The checking order

1. Replace the fob battery — five minutes and the cheapest hypothesis.
2. Try the spare fob.
3. Try the interior button — working there means it is the fob channel.
4. Lock each door individually — find which ones fail.
5. Watch the door-open lamp — it points at the switch.
6. Read the body module codes.

In short

• First separate a fob-only failure from a total one.
• Range falling gradually is a battery, not a fault.
• One door out of five is an actuator, not the system.
• A lying door switch brings a flat battery overnight with it.
• The body module names the specific door and the specific component.

Download the app — body electronics codes and door switch states are read with an ELM327 adapter over Bluetooth SPP on Android.