Key fobs fail in a handful of predictable ways, and only one of them requires a new fob. Working through them in order takes about ten minutes and quite often ends the problem for the price of a battery.
Start here: does the car still start?
This single question splits the diagnosis in half.
If the car still starts but the buttons do nothing, the immobilizer chip is alive and communicating. That chip is passive — it needs no battery. So a fob that starts the car but will not lock the doors has a working chip and a failed transmitter side, which means a battery, a button pad, or a broken contact. Not a dead fob.
If the car does not detect the fob at all, on a push-start vehicle, the problem could be the fob or the vehicle. Hold the fob directly against the start button and press — most keyless cars have a backup coil there. If it starts that way, the fob's chip is fine and you are looking at a battery or a proximity detection fault in the car.
The checklist, cheapest first
1. Replace the coin cell
This is the cause more often than everything else combined. Fob batteries last two to four years and they fade rather than dying cleanly — you get reduced range for weeks first, which is easy to miss.
Buy the battery from a shop with turnover rather than a discount bin. Coin cells self-discharge on the shelf, and a cheap old battery is a false negative that sends people off buying fobs they did not need.
2. Check the battery contacts
Open the fob and look at the metal tabs touching the battery. They corrode, and they fatigue and lose tension. Clean corrosion off with isopropyl alcohol and gently bend a slack tab back so it grips. A fob that works when squeezed has exactly this problem.
3. Look for a cracked solder joint
Dropped fobs crack the solder where the battery terminals meet the circuit board. Under decent light you may see a hairline ring around a joint. This is repairable in minutes by anyone with a soldering iron, and it is a common cause of intermittent behavior.
4. Assess the button pad
The conductive rubber under each button wears out with use. The lock and unlock buttons go first because they get pressed the most. If one button works and another does not, that is a worn pad, not a dead fob — and replacement pads are inexpensive.
5. Rule out interference
Try the fob away from the car, then close to it. Radio interference near buildings and other transmitters can block the signal. Also try separating the fob from your phone and any second fob — two transmitters in one pocket genuinely interfere.
A note on buying fobs online
It can work and it can save money. It can also waste both money and time, because marketplace fobs are frequently the wrong FCC ID for your vehicle, already locked to another car, refurbished units sold as new, or counterfeit chips that will not accept programming at all.
If you go that route, match the FCC ID exactly — it is printed on the back of your existing fob — and understand that a locksmith charging programming-only labor will still charge it if the fob refuses to program. The time was spent either way.
When it is the car, not the fob
Push-to-start vehicles carry four to six low-frequency antennas that detect where the fob is. When one fails you get a dead zone — the car detects the key from the driver's seat but not the passenger side, or not at the trunk.
That pattern is diagnostic. If detection depends on where you are standing, the fob is fine and an antenna has failed. Replacing the fob will not fix it, and anyone who sells you one without testing this has not diagnosed the fault.
Want this checked on your actual car?
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