A key fob that stopped working is almost always one of three things: a dead coin cell battery, a low car battery, or a fob that has lost its program to the car. Less often it is the receiver in the car itself. Here are the five causes ranked, with the 30-second tests for each.
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These cells last 2-4 years. Symptoms start subtle - works only up close, then only with the button hammered hard, then not at all. Replace it before you assume anything else is wrong. $3 at any drugstore. Cost: $3 - $10. DIY: Easy. Severity: Low.
Get a Free AI Diagnosis →A car battery below about 12.0V can make the receiver fail to hear the fob, even though the car still starts. Check resting voltage and load test the battery if it is over 4 years old. Cost: $0 - $300. DIY: Easy. Severity: Low.
Get a Free AI Diagnosis →After a battery disconnect, ECM swap, or some module faults, the fob can lose its handshake with the immobilizer. Many cars allow a key-cycle re-program; others require a dealer or locksmith with a scan tool. Cost: $0 - $200. DIY: Medium. Severity: Low.
Get a Free AI Diagnosis →Cracked case, water intrusion, or worn button contacts kill the transmitter. If the LED on the fob does not light when you press a button (with a fresh battery), the fob hardware is dead. Cost: $30 - $250. DIY: Easy. Severity: Low.
Get a Free AI Diagnosis →Less common but does happen. If both fobs stopped working at the same time and a new battery in each does nothing, suspect the receiver. Diagnosis usually requires a scan tool. Cost: $150 - $400. DIY: Hard. Severity: Low.
Get a Free AI Diagnosis →If your scanner shows one of these codes along with the symptom, run a free AI diagnosis to confirm the root cause.
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Press any button at night - you should see a faint red LED on the fob. No LED, or a very dim one, means a dead or dying coin cell. Replacements are $3 at any drugstore.
Most fobs split with a small flat screwdriver in a slot on the edge. The valet key (if present) often hides this slot. YouTube your exact fob model - they are all slightly different.
Some cars (older Fords, Chryslers, GMs) allow a key-cycle reprogram you can do in the driveway. Most cars from 2010-onward need a scan tool and a security pin from the dealer.
Unlock uses radio. Start uses an immobilizer transponder inside the fob - a separate chip. The transponder can fail while the radio still works. A locksmith can read the transponder.
Aftermarket: $30-$80 for the fob plus $50-$120 to cut and program. Dealer OEM: $150-$400 all-in. Locksmiths beat dealers on most modern cars.
Yes. The receiver and BCM need at least about 11.5V to work reliably. A weak battery in the car can make the fob seem broken when it is fine.