4 Symptoms of a Bad Headlight Relay
When your headlights mysteriously stop working, a faulty relay is often to blame. This small but important component controls the flow of current to the headlight circuit.
Learn the warning signs of a bad headlight relay, the most common causes of failure, and how much it costs to replace the relay. Replace it yourself without a service appointment.
Symptoms of a Bad Headlight Relay
Headlight failure is often accompanied by a number of secondary symptoms, some of which are quite obvious. Recognizing these symptoms can be extremely helpful when it comes to quickly diagnosing and resolving the problem at hand.
#1 – No low or high beams
The most common symptom of a bad headlight relay is that the high or low beams won’t turn on. However, often the opposite headlight setting will work perfectly. This is because most vehicles have a separate relay for each separate light setting.
#2 – Headlights work intermittently
If the headlight relay fails first, it’s not uncommon for one headlight adjustment to stop working one day, but work fine the next. However, in most cases, such issues will worsen over time, eventually resulting in a loss of headlight functionality.
#3 – Headlights that stay on
A less common issue, a failed headlight relay can actually get stuck in the closed position, causing your vehicle’s headlights to be constantly on in high or low beam.
#4 – Erratic Pop-Up Headlight Behavior
Although it’s becoming increasingly rare, many vehicles on the road today still feature covered (pop-up) headlights, where the covers retract when the headlights themselves are turned on. While the 2004 Corvette may be the last vehicle with hidden headlights, other cars with this feature include the Mitsubishi Eclipse, BMW 8 Series, Mazda RX-7, and Nissan 300ZX.
Because this light and lens combination often uses a single relay, a failed headlight relay can cause erratic headlight cover behavior.
What does a headlight relay do? The headlight relay acts as a type of switching device, regulating the power distribution of the headlight circuits in your vehicle.
1. When closed, the headlight relay provides fusion power to the headlight itself.
2. When the headlight relay opens, it cuts off the flow of power on the affected circuit, de-energizing both headlights and preventing them from turning on.
This prevents premature failure of the headlight switch, since the switch itself only needs to control the control wiring to the coil side of the relay itself. The relay also acts as a valuable circuit protection device of sorts.
Where is the relay located?
In most cases, a vehicle’s headlight relay is located under the hood in the power distribution box (also known as the fuse box). Additional fuse boxes may be located in the driver’s side footwell, glove compartment, or other locations in the vehicle; however, these locations are not usually where the headlight relay is located.
The engine compartment panel is usually located at the far left or right side of the engine compartment and is often rectangular with a snap-on or twist-lock cover. Removing the distribution panel cover reveals many fuses and relays.
Most distribution panel lids have simple printing on the underside that identifies the location of the individual fuses and relays. This will help with diagnosis.
What Causes a Headlight Relay Switch to Fail?
There are many different reasons why a headlight relay can fail, but it is often due to normal wear and tear. Because relays function similarly to switches, repeatedly breaking and re-opening a circuit, their internal contacts are prone to some degree of arcing.
Over time, this arcing can erode the contacts and leave behind a flux-like substance that can prevent the contacts from functioning properly. The internal coil portion of the relay can also weaken or fail over time.
Another common cause of headlight relay failure is water ingress into the vehicle’s power distribution panel. This moisture can cause a short circuit between the relay’s poles or corrode the individual pins of the relay.