Most Common Reasons a Car Won't Start With a Jumpstart

The most common reasons a car won't start with a jumpstart are that the battery voltage is still too low, the battery terminals are loose or corroded, the starter is broken, the neutral safety switch is bad, the jumper cables are loose or bad, or there is a starting problem that a jumpstart can't fix.

Let's make sure you're jumpstarting correctly then take a look at the most common reasons a car won't start, even with a jumpstart.

Basic jumpstarting procedure

A jumpstart is intended to supply the correct voltage and current to a car that won't start because the battery voltage is currently too low, which is the most common reason that a car won't start. A low battery voltage will prevent the ignition system from working properly.

Double check that you have jumpstart basics covered before you start diagnosing your car:

• You have a car with a good battery that starts and runs on its own (donor), and another car that doesn't start (recipient).
• Make sure both cars are turned off. Open each car's hood and locate each car's battery.
• First, using a jumper cable, connect the positive terminal (+) of the recipient car's battery to the positive terminal (+) on the battery of the donor car. Red cable.
• Second, using a jumper cable, connect the negative terminal (-) of the donor car's battery to the negative terminal (-) on the battery of the recipient car. Black cable.
• With the cables connected, start the donor car.
• With the donor car running and both cars still connected with the jumper cables, let the cars sit for at least 5 minutes before attempting to start the recipient car.
  • When attempting to start the recipient car, keep the the jumper cables connected and the donor car running.

If you've already followed these steps, let's jump into diagnosing.

Most common reasons why your car won’t start with a jumpstart

Let's review the reasons why your car won’t start with a jumpstart:

1. Battery voltage is still too low
2. Battery terminals are loose or corroded
3. The starter is broken
4. Your neutral safety switch is bad
5. Your jumper cables are bad
6. There is a starting problem that a jumpstart can't fix

Let's see what these reasons mean and how you can check them.

Battery voltage is still too low

Be patient! With the jumper cables connected and the donor car running, you should let the recipient car charge for at least 5 minutes to increase the chance that a jumpstart will be successful. If the battery hasn't been given enough time to charge, battery voltage might be low enough to prevent a start even if it is connected to a donor car that is providing power. The battery may need even more time to charge in the cold.

Starting a car requires a short burst of intense energy. If the battery can't provide this, your jumpstart attempt will fail.

Give some more time for the battery to charge and try starting the car again. You can use a voltmeter to check the recipient car's battery voltage. The recipient car will struggle to start if the battery's voltage is below around 12.6 volts.

If you've charged the battery for a while, and the voltage struggles to increase, you may need to replace the battery. All batteries eventually wear out and can no longer hold a charge and will need replacing. Possible causes include a defective cell or a short circuit within the battery. Most roadside technicians, some auto parts stores, and any RepairPal Certified auto repair shops will have the tools to test the capacity of your battery. If this is the case, and the rest of the starting system has been verified to be in good condition, a new battery should solve the problem.

Battery terminals are loose or corroded

A car battery has a positive terminal (typically with a red cable or cover, marked with a + sign) and a negative terminal (typically with a black cable or cover, marked with a - sign). If these connections have corrosion or are not tightly connected, the flow of electricity will be restricted, keeping your car from starting, even with a jump. Tightening the cable clamps or cleaning the corrosion off with a wire brush should restore the full flow of power from your battery.

Note that the battery in some cars can be hidden in the trunk or deep in the engine bay for performance or weight balancing reasons. Typically, cars with these battery configurations will have a separate set of terminals for jumpstarting. Make sure you check the actual terminals on the battery itself.

Your starter is bad

The starter uses energy from the battery to start spinning the engine when the car is off. When your starter stops working, it can't “crank” the engine to get it started. When attempting to start, clicking or grinding sounds are clues that your starter might be the reason your car won’t start with a jump.

Again, make sure you've given enough time for the battery to charge while connected to a running car. A starter will struggle to spin if it doesn't have enough power from the battery.

Your mechanic can determine if your starter is bad when you take your car to the repair shop.

Your neutral safety switch is bad

On cars with automatic transmissions, there is a switch that will keep your car from starting unless the transmission is in either Park or Neutral. If this switch is bad or gets damaged, this information cannot be sent to the engine computer, and your car won’t start with a jumpstart. Make sure your gear shift selector is in Park. This may have prevented your car from starting in the first place.

Your jumper cables are bad

Not all jumper cables are created equal. Inexpensive jumper cables that are not thick enough to carry sufficient power or have been damaged may fail to help your car start. Cables that are too long have more internal resistance, and they may not transfer enough of the donor battery’s power to the recipient car. If you are investing in a set of jumper cables, be sure to get a quality set of a reasonable length.

My car still won't start, what could be wrong?

If you've checked all of the above reasons, it might be time to start considering other systems in your car that can prevent a start.

Can my alternator be bad?

When the car is on, the alternator provides power to the car's systems and charges the battery.

You can normally jumpstart a car even if it has a bad alternator. However, your car won't run for long after the donor car is disconnected as the car will only be using energy from the battery, and since the alternator is no longer charging the battery, the power will eventually run out. In some instances, a bad alternator can prevent a car from starting entirely.

If your car needs to be jumpstarted frequently even with a new battery or one that has tested good, the alternator can be faulty.

You may have a starting issue that a jumpstart can't fix

A jumpstart is meant to provide the correct voltage and current for a car to start. Even if the correct voltage and current is being supplied to your car, there are many systems that still need to work together for a successful start. A RepairPal Certified auto repair shop can diagnose the exact cause of the problem. Issues in other systems that can prevent start include:

Fuel system

• Bad fuel pump
• No fuel in gas tank
• Fuel filter is clogged
• Fuel injection or carburetor problem
• Water in fuel lines (freezes in winter weather)

Air intake or exhaust systems

• Very clogged air filter or obstruction in air intake
• Very clogged catalytic converter or exhaust system
• Significant vacuum leak

Electrical or computer systems

• Worn out spark plugs
• Multiple worn out or dead ignition coils
• Engine or powertrain computer failure
• Bad sensor required for start

Mechanical systems

• Timing belt or chain failure
• Significant head gasket leak

Security systems

• Issue in anti-theft system
• Key or key fob not being recognized by security system

If your car won’t start, can you fix it yourself?

Cleaning or tightening your battery cables can be done by anyone with basic mechanical skills and the ability to follow basic safety precautions.

If you are mechanically inclined, have the right tools, know where and how to buy the right parts, and feel confident in keeping yourself safe when working on a car, replacing the battery, the alternator, or the starter is typically a job you can take on in the comfort of your garage.

However, as with any car repair, proper diagnosis will save you time and money. Remember that a jumpstart will only work if the problem is the battery voltage is too low. But why is the battery voltage too low? Is the battery not able to hold a charge? Is the alternator not charging the battery while the car is running? A battery, starter, alternator, and other starting components can be expensive. Be sure to identify the actual source of the problem, or trust a Repair Certified auto repair shop to do so.

Choosing a mechanic can be hard, but we've made it easy. Schedule an appointment with a RepairPal Certified auto repair shop, where you'll receive quality work at a fair price, backed by a minimum 12,000 mile / 12 month warranty.