3 Symptoms of a Bad Main Relay (and Replacement Cost)

Modern vehicles are electrical marvels. And while that electrical ingenuity has led to a wide range of functionality that wouldn’t otherwise be possible, it can start to kick in and create a bit of a headache.

One of the main features of all these electrical marvels is the main relay. Although these relays rarely fail and last the life of your vehicle, it is possible for a relay to break from time to time. The good news is that if you suspect a faulty main relay, it’s fairly easy to rule out and inexpensive to replace.

Below we highlight the most common signs of a faulty main relay before delving into what it does and where you can find it. Let’s start with the signs to look for.

The most common symptom of a bad master relay is that you won’t be able to start your car and nothing will happen when you turn the key. It can also cause the engine to stall while driving or warning lights on your dashboard.

Below we highlight a more detailed list of the three most common causes of a faulty main relay.

Faulty main relay symptoms

1. Your vehicle cannot be started

Starting the engine

If you try to start your vehicle and it won’t start, you may have a faulty main relay. You should always rule out other, more likely problems, such as a dead battery or bad starter, before jumping straight to a faulty main relay.

If you have verified that your battery has enough charge, you can turn the key to different ignition points and listen to the fuse box click. If you hear a clicking sound, your main relay is not the problem; If not, you will need to investigate further. But remember that your vehicle needs enough battery power for the relay to activate.

2. Check engine light

check engine light

If you are able to start your vehicle, you will notice a check engine light on the dashboard if you have a faulty main relay. To verify that the check engine light matches the main relay, you will need to connect an OBD II scan tool and read what the code is for.

3. Your vehicle will not continue to run

stall the engine

If you manage to start your vehicle, if it has a faulty main relay, you will not be able to operate it for a long time. Whether it’s a matter of seconds or minutes depends on the specific fault, but it won’t run your engine with a faulty main relay.

If your engine runs and stays that way, the problem you are having is not related to the main relay.

Main relay function

starter relay

Your vehicle’s main relay powers most of your car’s electrical components, such as the engine control unit and other control modules. When you start your vehicle, it moves the relay to a new position that allows it to start your vehicle by sending power to the necessary components.

Main relay location

Relay fuse box

Your vehicle’s main relay is usually located in your vehicle’s fuse box. There are two common locations for this fuse box. First, it may be under the passenger-side dash. Secondly, it may be in the engine compartment itself.

Either way, you should have a fuse box cover which you will need to access the main relay. From there, you will need to determine which relay is the main relay in the fuse box. Some vehicles will have all fuses and relays labeled inside the fuse box cover; others require you to do a little more work to find it.

You will have to look on the web or in your owner’s manual for a fuse box diagram if it is not inside the fuse box cover.

Main Relay Replacement Cost

The average main relay replacement cost is between $70 and $120, depending on the type of vehicle you drive and where you take it for repair. Most of this cost will come from troubleshooting costs and the cost of the relay itself.

In fact, while diagnosing your main relay can be a bit complicated if you don’t know what you’re doing, replacing it is easy. The relay itself costs between $30 and $70, while labor typically costs an additional $40 to $50.

So if you are sure that the relay is the problem, you can remove the fuse box cover and replace it yourself. But even better, if you think the relay is the problem, try swapping it for another relay in the fuse box. If the problem “moves”, you have a faulty relay and need to replace it.

Diagnose your main relay

Diagnose the car

Although a faulty main relay is rare, it is neither impossible nor unheard of. Although correctly diagnosing the main relay requires a little technical knowledge, there are some solutions you can use to identify the problem.

First, you need to find the main relay. From there, have someone stand next to the fuse box while you start the engine. You should hear a clicking sound, if not you could have a faulty main relay. If they do, the relay most likely is not the problem.

Anyway, if you can determine which relay is the main relay, try replacing it with another relay in the system. Although technically both could be faulty, the chances of this happening are extremely low. Once you have changed the relays, try starting your vehicle.

If the vehicle starts but the system you moved the main relay to now works, you have a bad relay and need to replace it. However, if you don’t notice any difference and the vehicle still won’t start or continue to run, the problem is probably not the main relay.

Keep in mind that you may have an electrical problem in your main relay circuit, meaning the main relay will be seen as the problem, but replacing it will never help. If so, you’ll need a little technical knowledge to figure out what’s going on.


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