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A headlight relay is a small electric switch that lets a low-current signal from your headlight switch or body computer turn on the high current your lamps need. Route 66 Auto Repair in Upland repairs relay faults that kill both headlights at once, leave them stuck on, or make them click. Our ASE-certified technicians test the relay, its socket and the signal that controls it, so you are not paying for a relay when the real fault is upstream.
What does a failing headlight relay look like?
- Both low beams or both high beams out at the same time
- Headlights that stay on after the car is shut off
- A clicking sound from the fuse box when lights are switched on
- Lights that work only after tapping the fuse box lid
Is it the relay, the socket or the signal?
A relay has two sides: a coil that receives the control signal and contacts that carry lamp current. Contacts can burn and pit, which causes flicker. Coils can open, which kills the light. We bench test the relay and check the socket terminals, because a loose socket can mimic a bad relay.
On many newer cars, the headlights are switched by a body module instead of a plug-in relay. In that case we check the command from the module with a scan tool. If a module replacement or programming turns out to be needed, that need is confirmed on the estimate. Relay faults also show up in fuse box repair work.
Our headlight relay repair process
Step 1: Confirm which lamps fail
We note whether low, high or running lights are affected, since each may use a different relay.
Step 2: Test the relay
The relay is checked for coil resistance and contact function, and swapped with a known good one if the layout allows.
Step 3: Check the socket and fuse
We inspect terminals for heat damage and test the fuse and power feed.
Step 4: Verify the control signal
A meter or scan tool confirms the switch or module is sending the command.
Step 5: Replace and retest
The failed part is replaced and all lighting functions are checked.
What's included in headlight relay repair
Included on the work order
- 01Lighting function check for low, high and running lamps
- 02Relay bench test
- 03Socket terminal and fuse inspection
- 04Control signal test from the switch or module
- 05Relay replacement or socket repair as quoted
How urgent is a relay failure?
Need now: call
- No headlights at night
- Headlights that will not turn off and drain the battery
- A hot or melted relay socket
Need later: book
- An occasional click with lights still working
- Daytime running lights out with headlights fine
- A relay that is due only because its twin failed
How pricing and timing work
Relays are often quick to test and replace. The work takes longer when the socket is damaged or a module controls the lights. Your written estimate lists the part and labor before we start. AAA members receive 10% off parts and labor, up to $75, at time of service with the card. Relay jobs are usually same day when the board allows.
After the repair
If your lights stay on after shutdown again, disconnect nothing yourself; call (909) 608-7699 and we will check the new part and the module command. Lights stuck on can flatten a battery, so a battery test may be worth adding.
Frequently asked questions
Can I swap the headlight relay with the horn relay to test it?
On some cars the relays match and a swap is a fair quick test. Check that the part numbers are the same first. If the problem moves with the relay, the relay is bad. If not, look at the fuse, socket or headlight switch.
Why are my headlights stuck on?
Relay contacts can weld closed, or a module may keep sending the on command. We test both, since replacing a relay will not fix a module fault.
Do all cars have a headlight relay?
No. Many recent vehicles switch headlights with a body control module. We check your vehicle's wiring diagram at intake so we test the right parts.