What causes the check engine light and battery light to come on?

While cars cannot talk, they have unique means of communicating with you when trying to send a certain warning. It can communicate through messages, chimes, and icons.

None of these messages from the car should be ignored as they could signal a dangerous warning that should not be overlooked.

However, not all warnings from cars mean danger; some are meant for preventive maintenance. At some point, you might have noticed an engine check light on and the battery light on the dashboard.

While this is something that you should worry about, you should not panic but try to figure out what might be causing the two lights to come on and find a way of fixing them.

So, what causes the check engine light and battery light to come on?

The main causes why the check engine light and battery light are on at the same time are a bad alternator and a bad or faulty battery. When these two are affected, they are likely to affect the whole function of the car.

Usually, if the battery voltage drops or there are problems with its charging, a battery light will be present on the dashboard. This can also trigger the check engine light to come on, letting you know that there is an issue with the battery or alternator that needs to be checked.

You will see these two error lights come on at the same time, especially in newer cars. An OBD scanner will help you diagnose these codes accurately.

Causes of check engine light and battery light to come on

1. A bad or faulty battery

In most cases, most car problems and warning lights are caused by a faulty or undercharged battery. When the battery is faulty, then it means that almost the entire car’s functionality will be affected, including its engine.

When the car’s check engine light comes on alongside a battery light, it might imply that the battery is faulty, causing a low voltage near the vehicle’s computer.

The ability of the battery to supply the right amount of energy to the car’s computer will be compromised if the battery is undercharged or damaged.

A malfunctioning engine control light might result in the check engine light illuminating. The newer models will display a dashboard warning light for battery problems.

Therefore, both the engine check light and the battery light will come on with a bad battery or undercharged battery.

This is because, besides providing the car’s electrical power, the battery stabilizes and manages the device’s voltage. Electronic and electrical components can be affected by a bad battery.

2. Bad alternator

Besides that, the lights can come on due to a bad alternator. Faulty alternators can turn engine and battery lights on and be dangerous to your safety.

Driving a car with no or faulty alternator can be so dangerous and it will; mean that the interior lights will be dim, difficulties will increase, and there will be no radio.

Nothing remains as all power is lost. Moreover, if the battery suffers from low voltage, the battery warning light if the alternator has a low voltage, an alternator should always start manually.

If faulty, it won’t properly charge the engine. A failing alternator halts the car and can damage the computer controls and electronics.

How to fix the issues and what are the costs?

To fix the issues, you need to inspect the battery and alternator. If they are faulty, they should be replaced. Recharge the battery or the terminals for a car battery and or replace it. Do this first and then move to the alternator.

Likewise, if the alternator is bad, you can replace it or visit a nearby mechanic for immediate action. However, while the car owner can fix some alternator or battery problems, it is important to take the car to an authorized service once you notice the check engine light and battery lights on for diagnostic tests.

The cost of fixing

The cost of fixing an alternator varies between $550 to $1000, including service costs and getting a new one. On the other hand, fixing a battery could cost you between $40 to $300, depending on the battery size, quality, and power.

Can a bad battery real cause the check engine light to come on?

Yes, a bad or faulty battery could cause issues with the car’s electronic and electrical parts and can cause the check engine light.

Can a failing alternator cause the check engine to illuminate?

A failing alternator or an undercharging one can cause a check engine light on newer or modern cars. And can cause problems with computer controls and onboard electronics.


Check engine lights and battery lights illuminating simultaneously is a common issue but could signify some serious safety danger.

The most common cause of these lights is having issues with the battery or the alternator. You should not continue driving once you see these lights but visit the nearest mechanic for diagnostic tests.

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