Car Relay Problems? 9 Problems & Solutions

Your car uses relays to power many electrical circuits that require high voltage and/or high amperage. Relays use a low-power signal to turn a circuit on or off, which makes them very useful and in high demand for your car.

When a relay goes bad or starts to show signs of failing, it can cause your car to act strangely.

We’ve gathered the most common signs you have a failing relay in your car and how to fix each issue.

#1 – Your Warning Lights Indicate a Problem

Newer cars have a very robust diagnostic system that continuously monitors every circuit in your car.

When it detects a problem, it will illuminate a warning light in your gauges to notify you that there is a problem.

Some unique warning lights may indicate a problem with an individual system such as the charging system, while otherwise, you may just have a simple warning light indicating a general problem.

Any light indicating a problem should be diagnosed as soon as possible to prevent further problems.

Car relay closeup

How to Fix It

Cars since 1995 have an onboard diagnostic system that will identify what is causing a problem. You can use a hand-held scan tool that will read the information and help you identify what is causing a problem.

From there you may need to use a multimeter to test the relay causing the problem to verify it is failing. Once you have found the bad relay, it should be replaced.

#2 – Your Electrical Accessories Stop Working

Your car has many smaller electrical circuits that won’t illuminate a warning light when they have a problem.

Accessories such as the radio, your headlights, or cooling fans can stop working and you won’t have an extra indication from your car.

How to Fix It

When you find something has stopped working, the first item to check is the fuse for that specific circuit.

Your car may have multiple fuse boxes, which can be located under the dash or in the engine bay. If the fuse for the failed item is still good, the relay that powers the circuit may be the problem.

Most relays are also located in the fuse box, which makes them easy to diagnose and replace. You can use a multimeter and service manual to find the specific relay that appears to cause the problem and replace it with a new one.

#3 – Your Air Conditioning or Heat Stops Working

Your car has a main relay and multiple smaller relays that control the air conditioning and heating systems.

A common problem with air conditioning is that it doesn’t blow cold air all the time when you want it to. It may only circulate warm air because the relay that turns the compressor on and off is bad.

The heat can have a similar problem. You want warmth, and it blows cold air because the blower motor relay or resistor has gone bad.

How to Fix It

The key to restoring your heating and air conditioning is to find the relay that is causing the problem. The main relay may also affect other systems in the car, which can make it easier to diagnose.

The individual heat and air conditioning systems can have separate relays in the distribution module under the hood. Use a service manual to find the relay location for the system affected and replace it with a new one.

#4 – Your Engine Won’t Start

The main relay in the car distributes power to other relays such as the starter relay, the fuel pump, and the ignition. One of those relays could prevent your engine from starting but allow others to attempt to start the engine.

How to Fix It

You can use a multimeter to check for voltage, amperage, and resistance at the main relay and the individual relays for each system.

If the car won’t start, but the engine cranks, the starter relay would be in working order. That leaves the ignition and fuel pump relays as potential problems.

You may be able to hear the fuel pump pumping fuel to indicate that this specific relay is good, and so on. Use simple diagnostic methods to find the problematic relay and replace it with a new one.

#5 – Your Engine Runs Rough or Continually Stalls Out

Many of the relays in your car’s electrical system control how well the engine runs. The main relay sends power to each relay for the ignition, fuel pump, and engine control unit (ECU).

An intermittent power signal to any one relay may cause a rough running engine or it may stall out and leave you stuck in traffic.

How to Fix It

A bad relay is easy to find because it has stopped working. The system it is related to doesn’t operate, and it points directly to the relay controlling the flow of power.

An intermittent condition where it works sometimes and then stops may be harder to diagnose. It may only occur under certain conditions like driving in stop-and-go traffic, wet weather, hot days that could overheat the engine, etc.

You may need to have diagnostic tools ready to start finding the problem when it occurs, even if that means on the side of the road. Once you find the bad relay, replace it and enjoy driving again.

#6 – You Hear Weird Noises Coming from the Engine

Relays can prevent your car from starting, and they can also plague your engine with problems when they fail.

If your car utilizes relays to control motors and solenoids, a running engine may start to send you mixed signals with weird noises. Solenoids may activate at the wrong time to reduce engine performance or cause other components to significantly wear.

How to Fix It

Some engines are naturally noisy by design. Their lifters may tick more than other engines you have listened to, but if you hear weird noises from your car it’s time to find out why.

Simple tools such as a stethoscope can help pinpoint where weird noises are coming from while the engine is running.

Finding the noise will help diagnose which relay is related to the noise and it can be further researched if the relay should be replaced.

#7 – Your Transmission Refuses to Go Into Gear

Automatic transmissions are likely to not go into gear if a relay has failed.

The brake pedal is connected to a switch that detects when you have applied the brakes, and that information is sent to a relay that unlocks the shifter mechanism. If the relay has failed, the shifter may be locked in Park.

Other transmissions use control modules to shift between gears and the shifts are controlled by relays. One bad relay and your car may not shift to third gear and be limited to a slow speed.

How to Fix It

The shifter mechanism usually has an override that is accessible inside the car. Some require the ignition key to turn a lock on the shifter housing, while others may just have a push-button combination to unlock the shifting mechanism.

These relays may not be easily replaced and it may be more economical to find a new or used replacement and change the whole unit.

Transmissions may be harder to diagnose, and it may be a better option to take your car to a transmission mechanic.

Document the situation when the transmission won’t shift between gears, which can help the mechanic pinpoint the relay or control module causing the problem. They have the tools and skills to quickly diagnose the exact cause and to replace the problematic component.

#8 – Your Engine Overheats

Your engine should stay in a consistent temperature range while it’s running. The cooling system should circulate liquid coolant between the radiator and the engine to keep the engine cool, but if a relay fails it can cause the engine to overheat.

How to Fix It

The cooling system interacts with different relays to control the coolant flow and the cooling fans.

Your air conditioning system typically will engage the relay to start the cooling fans and prevent a buildup of heat in the engine. Some cars use an electric water pump, which is also controlled by one or more relays.

The diagnosis of what is occurring will indicate which relay to investigate and replace if it has failed.

#9 – Your Fuel Economy Degrades Over Time

Many relays contribute to your engine performance and fuel economy. One under-performing relay can degrade that performance and slowly reduce your fuel economy.

The main relay of the car can hinder performance, while smaller relays can degrade the performance of the fuel system.

How to Fix It

This problem can be hard to diagnose and determine that a specific relay is the cause. Many things can contribute to reduced performance and fuel economy, so the diagnostic process needs to rule out the basics first.

Check the filters for each system and replace them if necessary. A mechanic with a diagnostic scanner may need to data-log how well your car runs to pinpoint a relay that is causing a problem.

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