The Dying Battery Is Almost Always the Cause
Key fob batteries (typically CR2032, CR2025, or CR1632 coin cells) last 2 to 4 years under normal use. As the battery voltage drops, the fob's radio signal weakens. A healthy fob might work from 50 feet away. A fob with a weak battery may only work from 5 feet, or it may need multiple button presses to register.
The intermittent behavior makes sense once you understand this: the fob works when you are close, the angle is favorable, and there is no interference. It fails when you are farther away, the signal has to pass through walls or other cars, or the battery's voltage sags during a cold morning.
Many people assume the battery is either good or dead. In reality, a fob battery spends weeks or months in a middle state where it works inconsistently before failing completely.
How to Replace the Key Fob Battery
If the New Battery Does Not Fix It
If you have installed a fresh battery and the fob still works intermittently, check these less common causes:
Dirty or corroded contacts. Open the fob and look at the metal contacts that touch the battery. If they are tarnished, corroded, or have a film on them, the electrical connection is intermittent. Clean the contacts with a cotton swab dipped in rubbing alcohol. Let them dry before reassembling.
Worn button contacts. The rubber buttons press against conductive pads on the circuit board. After thousands of presses, the conductive coating on the rubber can wear thin. If you notice the fob only fails with one specific button, this is the likely cause. Replacement rubber button pads are available for many fob models for $5 to $10.
Cracked circuit board. If the fob has been dropped, sat on, or run through the washing machine, the circuit board may have a hairline crack that creates an intermittent connection. A cracked board is usually not repairable. A replacement fob shell with a new board (without the transponder chip) costs $10 to $30 from online retailers. You transfer your existing chip to the new board. Alternatively, a dealer-programmed replacement fob costs $100 to $300 depending on the vehicle.
Radio frequency interference. Certain locations interfere with fob signals. Airport parking garages, areas near cell towers or radio transmitters, and even some LED light installations can create enough RF noise to drown out the fob's weak signal. If the fob works at home but fails consistently in a specific location, interference is the cause. Moving closer to the car or holding the fob against your chin (your skull acts as an amplifier for the radio signal -- this actually works) can help.
The Physical Key Backup
Every key fob with a keyless entry system has a physical emergency key hidden inside it. Usually it slides out from the back or side of the fob when you press a small release button or tab. This key fits a keyhole on the driver's door handle (sometimes hidden under a cap) and lets you unlock the door mechanically when the fob battery is dead.
Once inside, vehicles with push-button start typically have a backup method: hold the fob against the start button or a specific spot on the steering column (marked in the owner's manual). The car's receiver can read the fob's passive RFID chip at very close range, even with a dead battery, and allow you to start the engine.
Knowing where the physical key is and how the backup start method works before you are stranded in a parking lot is worth five minutes of reading the owner's manual.
If your car battery also keeps dying overnight, a fob that is sitting too close to the car can sometimes keep the vehicle's receiver module awake, creating a slow parasitic drain. Try storing the fob farther from the vehicle or in a signal-blocking pouch.
Related: Why Does My Car Battery Keep Dying Overnight? · Check Engine Light Comes On Then Goes Off · Car Idles Rough but Drives Fine
Written by James Chen
James covers technology and gadgets, breaking down complex topics into plain language. He enjoys helping readers get more out of their devices.