Why the car doesn't start after replacing the battery?

September 18, 2022
If the car was running before, but doesn't start right after the battery has been replaced, there are three common reasons:

1. The first reason is more common in high-mileage cars and trucks. When the battery is disconnected, the learned settings in the engine computer are reset. If the part that is called throttle body was dirty, the car may not start because the learned throttle angle that was compensated for dirty throttle body is now reset. Read what you can do below.

2. This could also happen if the battery is not connected correctly or if while changing the battery something accidentally got shorted. It happens often.

3. Your car may be equipped with some aftermarket alarm system that needs to be reset after the battery has been disconnected or replaced.

1. The car doesn't start because the throttle body learned angle reset

The symptom of an issue with the throttle body is when the car starts when the accelerator pedal is depressed slightly, but once the pedal is released it stalls and does not stay running. In some cases, it may start, but the idle speed will fluctuate up and down.
Dirty throttle body Dirty throttle body.
In this case, your mechanic may suggest cleaning the throttle body first. Read more: When does the throttle body need to be serviced?

Another step is to perform the idle speed relearn procedure. The idle relearn procedure or it could be also called throttle angle learn procedure or throttle body calibration is different for different cars.

What you can do is to search for idle relearn procedure for your vehicle year make and model. It could include steps like turn all the loads and headlight off, leave the ignition on for some time and let the engine idle for some time. Once the vehicle relearn the throttle angle, it will run properly and the idle speed will be stable.

2. The car doesn't start due to incorrect installation or blown fuses

Assuming the new battery is good and fully charged, check if the terminals are connected correctly, negative to the ground and positive to the positive (red) terminal and are not loose.

Check if the engine icon illuminates on the instrument panel when the ignition is on. The illuminated engine icon also known as Check Engine light, indicates that the main engine computer called Powertrain Control Module or PCM is powered.

Check if the security warning light or key-shaped icon illuminates shortly and then turns off, indicating that the immobilizer system is working and recognizing the key.

Check if the radio, wipers or power windows work. If something else doesn't work or the engine icon doesn't illuminate when you turn the ignition on, or the immobilizer doesn't recognize the key, it's possible that some of the fuses have been blown.

Main fuses Main fuses.
Check the fuses, often it's one of the main fuses that you will find blown if something was accidentally shorted at the battery terminals. It happens quite often and the fuses are not too expensive to replace.

The main (large) fuses could be attached to the positive battery terminal as in this photo or located in the under-hood fuse box. You can usually see a blown fuse visually or you check them with a multimeter.

Set the multimeter to DC Volt, connect the black "COM" probe to the ground connection and check the voltage with the red probe. Good fuses will show 12 Volts at both ends (pins) of the fuse with the ignition on. If one pin of the fuse shows 12 Volt and the other shows 0, the fuse is blown.

3. The car doesn't start because of an aftermarket alarm system

How can you tell that your car is equipped with an aftermarket alarm or remote start system? Check your remote, does it look like an original equipment or it's an aftermarket unit? Does your car have some switch or button that doesn't look like an original equipment?
Aftermarket alarm switch and button Switch and button of the aftermarket alarm system.
Typically you can find this type of a switch or button at the driver's kick panel or at the lower portion of the dash on the driver's side. The switch or button wires could be connected to a module that is held by zip ties somewhere under the dash. That's how you know it's not an original equipment.

If you can find the model of the alarm system on the remote or on the alarm module, search for reset procedure. It might be as simple as to press that button while starting the car. Check your glove box, the alarm system manual could be left in there.