Your Key Starts the Car but the Lock Buttons Are Dead in Plano

This is one of the most useful symptoms a customer can describe, because it rules out more than it leaves in. A key that starts the engine reliably but will not lock or unlock the doors has told us that half of the key is perfectly healthy and half of it is not, and those two halves fail for completely different reasons. Before you buy anything, understand which half quit. For same-day diagnosis anywhere in Plano, call or text (945) 307-3827.

Quick Answer

As of September 2026, a key that starts the car but will not lock or unlock the doors has an intact transponder and a failed remote path. The four common causes are a weak coin cell, a lost rolling-code sync between the fob and the vehicle, a failed remote circuit or button pad inside the fob, and a body control module or door actuator fault on the vehicle side. Expect $10 to $180 for a battery and function service, $70 to $160 for a fob repair, $210 to $430 for a replacement fob programmed on site, and $75 to $140 for diagnosis when the fault turns out to be in the vehicle.

Why the Two Halves Are Independent

Nearly every modern key does two unrelated jobs in one plastic shell.

The transponder is a passive chip that has no battery of its own. The vehicle's antenna, either a ring around the ignition or an antenna in the cabin on push-button cars, energizes it at very short range and reads its identity. That is what permits the engine to start. Because it draws its power from the car, a completely dead coin cell does not stop it.

The remote is an active radio transmitter powered by that coin cell. Pressing lock or unlock sends a coded burst tens of feet to a receiver in the vehicle, which passes the request to the body control module, which then drives the door lock actuators.

So a key that starts the engine and will not lock the doors is a key with a working chip and a broken radio path. That is genuinely good news: the expensive part of a key job is usually the immobilizer work, and the immobilizer is clearly fine.

The Four Causes, In the Order We Find Them

1. A weak or dying coin cell

The single most common answer, and it catches people out because the car still starts. Owners reasonably conclude the battery must be fine. It is not. The transponder does not care about the battery, so a cell that is far too weak to transmit will still let you drive to work every day.

If the range shrank gradually over weeks before it quit entirely, this is almost certainly your answer. Fit a fresh cell of the correct size and make sure the case is fully clipped shut. If you have already tried that and it made things worse, our guide to a fob that died right after a battery change covers what usually goes wrong during the swap.

If a new battery only lasts a few weeks before the buttons go dead again, the battery is not the fault at all. Rapid drain points at the fob's own circuit, which we break down in why a smart key battery dies fast.

2. Lost rolling-code sync

Remotes use a rolling code that advances with every press so the same transmission is never valid twice. If the fob's counter runs far enough ahead of the vehicle's, usually because the buttons were pressed many times out of range, in a pocket or by a curious child, the car will reject the transmission as stale even though the fob is transmitting perfectly.

The fingerprint is a remote that stopped working all at once with no warning while the car continued to behave normally. The fix is a resynchronization procedure, not a new key, and on many vehicles it takes minutes.

3. A failed remote circuit or button pad

Fobs live in pockets, get dropped on concrete, get sat on and, in North Texas, spend July baking in a cabin well over 140 degrees. Three failures come out of that: the conductive pad under a rubber button wears through, a solder joint on the transmitter side cracks, or moisture from a wash cycle or a rainy soccer field corrodes the board.

Two tells worth knowing. If one button works and the others do not, the transmitter is alive and it is a button pad or contact problem, which is a repair. If pressing hard on the case makes it work intermittently, it is a cracked joint, also a repair. If nothing responds at any pressure with a known-good fresh battery, the remote circuit itself is likely gone and the fob needs replacing.

4. The vehicle side, not the key

This is the branch people miss, and it is why we always ask about the spare key first. If both keys stopped locking the doors, the key is almost certainly innocent. Now we are looking at the receiver module, the body control module that drives the locks, a blown fuse on the door lock circuit, or a failed actuator in an individual door.

Two quick discriminators you can run yourself. Does the interior lock switch on the door panel still work? If it does not, the actuator or the module driving it is the suspect, not any key. Does one door refuse to lock while the others behave? A single failed actuator behaves exactly like a broken fob for that one door and nothing else.

When the trail leads into the body control module, the work moves from key programming into module territory, and we cover what that actually involves in BCM body control module programming. That family of work spans the powertrain and transmission side too, from PCM replacement and VIN programming to transmission control module programming, and it is a different discipline from cutting a key.

Fifteen-Minute Driveway Checklist

1. Test the spare fob. If the spare locks the doors, the vehicle is fine and the fault is inside the first fob. If neither works, it is the vehicle.
2. Fit a known-good fresh cell of the correct size, printed face oriented the way the old one sat, case fully clipped shut.
3. Press each button separately and watch for the small indicator LED on the fob if it has one. One dead button and three live ones is a button pad problem.
4. Squeeze the case while pressing. Intermittent response means a cracked joint.
5. Try the interior door lock switch. No response there removes the key from suspicion entirely.
6. Stand at different distances and locations. Garage door openers, LED shop lighting and dense apartment RF can swamp a remote at close range while it works fine at the end of the driveway. That pattern is interference, covered in our proximity key interference notes.
7. Check the physical key blade. If the fob is dead and the blade will not turn the door cylinder either, you are one bad afternoon from being locked out. Fix it now rather than in a parking garage.

What Each Outcome Costs in Plano

Finding	What gets done	Typical Plano range (Sep 2026)	On-site time
Weak or wrong coin cell	Correct cell fitted, both halves tested	$10 - $180	10 - 30 min
Rolling-code sync lost	Fob resynchronized to the vehicle	$10 - $180	15 - 30 min
Button pad or cracked solder joint	Contact and pad repair, board reused	$70 - $160	15 - 35 min
Cracked shell, worn buttons	Shell and blade transfer, board reused	$70 - $160	15 - 35 min
Remote circuit dead, transponder alive	Replacement fob cut and programmed	$210 - $430	30 - 60 min
Vehicle-side fault suspected	Diagnosis, applied to the repair if you proceed	$75 - $140	20 - 35 min
Body module or actuator work	Quoted after diagnosis	Varies	Varies

The row worth arguing about is the fifth. A working transponder does not entitle anyone to sell you an all-keys-lost session. Since your car still starts, whatever new fob gets fitted is an add-a-key job, which is the cheaper end of every key procedure. If a shop quotes you an all-keys-lost price for a car that currently starts, ask them to explain why.

Why It Is Worth Fixing Rather Than Living With It

Plenty of people shrug this off and use the blade for months. Two reasons not to.

First, the remote is often the early warning. A fob whose radio side has failed from heat or moisture frequently has a transponder living on borrowed time in the same housing. When that side goes, the car does not start, and it usually does not pick a convenient moment.

Second, on many push-to-start vehicles the passive entry and passive start functions run through the same battery-powered path as the buttons. A fob that cannot transmit may still start the car when held against the start button, but it will stop being detected from the driver's seat, and that is a much worse day than a dead unlock button.

If your car has reached the point where it only recognizes the key when the fob is touching the start button, treat it as urgent and read push-button start will not recognize the smart key.

When to Call

Call if the spare key has the same problem, if the interior lock switch is also dead, if only one door refuses to respond, or if the car has started intermittently failing to detect the key. Call sooner if this is your only key, because a car with one key is a single misplaced fob away from a job that costs several times more than a spare made today.

For fob repair, resync, replacement or a proper vehicle-side diagnosis across Plano, Frisco, McKinney, Allen, Richardson and Garland, call or text (945) 307-3827. Most of this is finished in a driveway in under an hour.

Frequently Asked Questions

Why does my key start the car but not lock the doors? Because the two functions are separate systems. The transponder chip that permits starting is powered by the vehicle and needs no battery, while the lock and unlock buttons are a battery-powered radio transmitter. A dead coin cell, a lost rolling-code sync, or a failed remote circuit stops the buttons while the engine keeps starting normally.

Do I need a whole new key fob for this? Often not. A weak battery, a lost sync, a worn button pad or a cracked solder joint are all repairs in the $10 to $160 range in Plano as of September 2026. Replacement is only necessary when the remote circuit board itself has failed.

Both of my key fobs stopped locking the doors. What now? Two fobs do not fail at the same time by coincidence. That points to the vehicle: the receiver, a fuse on the lock circuit, the body control module, or the door actuators. Get a diagnosis rather than buying keys.

Can a locksmith resync a key fob to my car? Yes. Rolling-code resynchronization is a standard procedure and it is far cheaper than a new key. It is one of the first things a technician should try when the buttons stopped working suddenly and the car still starts.

How much does this cost to fix in Plano? Battery and function service runs $10 to $180, fob repair $70 to $160, and a replacement fob cut and programmed $210 to $430 as of September 2026. Because your car still starts, this is an add-a-key job rather than the more expensive all-keys-lost procedure.

Is it safe to keep driving like this? It is drivable, but treat it as a warning. The remote side often fails first on a fob whose transponder side is also aging, and on push-button cars the same battery-powered path handles passive detection, so the next failure can leave the car unable to see the key at all.

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Written by the Plano Locksmith Automotive Locksmith Team. Prices reflect typical Plano-area ranges as of September 2026 and are confirmed against your specific vehicle before any work begins.