Key fob range suddenly short: battery, aerial or interference

A key fob that worked from across a car park and now needs to be pressed against the door has lost transmission power or the car has lost reception sensitivity.

Related: central locking and key fob faults.

The fob battery, first

The obvious answer, and it is right most of the time.

Reduced range is the classic symptom of a fob battery approaching the end of its life. It does not fail suddenly — transmission power falls gradually, so range shrinks over weeks.

Notes:

• A weak battery still shows a working LED. The LED needs far less power than the transmitter, so an illuminated light proves very little.
• Cold weather makes it worse, which is why this often appears in winter.
• Buy quality cells. Cheap coin cells lose capacity quickly and you will repeat this in months.
• Two fobs, two batteries. If both have declined and you only replaced one, the spare will surprise you later.

Try the spare key. If the spare works from a normal distance, the problem is the first fob rather than the car.

When it is the car

If both fobs are equally poor, the vehicle’s receiver is the suspect.

Interference

Genuinely common and location-specific.

The tell: the fob works everywhere except one place — usually a particular car park, a spot near a building, or an area with radio equipment.

That is external interference, not a fault. Nothing in the car is wrong.

Sources include nearby transmitters, LED lighting installations, some electric vehicle chargers, and dense electronic equipment.

An aftermarket device inside the car

Dashcams, chargers, trackers and LED bulbs all emit some radio noise. A cheap one emits a lot.

If the range dropped after installing something, unplug it and test again. This is a common and easily proven cause.

Aftermarket accessories electrical faults, dashcam installation done right and phone mount and USB charging issues.

The receiver aerial

Often integrated into a window, the rear screen, or a module. Damage during window replacement or trim work affects reception.

Low system voltage

A marginal battery affects reception modules along with everything else, and comfort functions are shed first: P0560 to P0563 system voltage faults.

Water

In the receiver module or its connector. If the range dropped after heavy rain or a car wash, look there: car wash and electronics damage.

Keyless entry is different

Proximity systems work at much shorter range by design — the car must detect the key within a metre or two.

Reduced range on a keyless system is usually the fob battery, and most have an emergency method: holding the fob against a marked spot on the door handle or the steering column, using near-field coupling that works with a nearly dead battery.

Find out where that spot is on your car now, not in a car park at night.

Car won’t start: immobiliser troubleshooting.

The security note

Keyless systems are vulnerable to relay attacks: two people, two devices, one near your key inside the house and one at the car, extending the key’s signal.

Practical defences:

• Store keys away from external walls, doors and windows. Relay attacks work through walls.
• Use a signal-blocking pouch.
• Some vehicles allow the fob to be put to sleep — often a button sequence. Check the handbook.
• A physical steering lock remains effective precisely because it cannot be defeated electronically.

Car cybersecurity and CAN injection theft and UN R155 and R156 vehicle cybersecurity.

Sequence

1. Try the spare key — this splits the diagnosis immediately.
2. Replace the fob battery with a quality cell.
3. Note whether it fails only in specific locations — that is interference.
4. Unplug recently added accessories and test.
5. Check system voltage.
6. Locate the emergency start method for a keyless system.

Check vehicle battery voltage before suspecting a receiver — an ELM327 adapter and the free tier of the app are all it takes. Comfort functions weakening together is usually a supply symptom.