Rear Fog Light Not Working? Causes and How to Fix It
Short answer: the most common reason a rear fog light stops working is a blown bulb. If a new bulb doesn’t fix it, check the bulb holder, the fuse or relay, the switch, and the wiring and connector. On many cars the rear fog light only works when your headlights are on, so also check that it isn’t just a switch setting.
Most of these checks take a few minutes and need no special tools. Here’s how to work through them, in order.
Table of Contents
Quick symptom guide
| What you see | Likely cause | What to try first |
|---|---|---|
| Dashboard symbol comes on, but the light doesn’t | Blown bulb, or a bad bulb holder | Replace the bulb and check the holder |
| Nothing happens, no dashboard symbol | A blown fuse, relay or switch fault | Check the fuse, and try the switch with dipped headlights on |
| It works only sometimes | A loose connector, corroded holder or poor earth | Reseat the plug and clean the contacts |
| It’s dim | Corroded contacts or a poor earth | Clean the holder and earth point |
| Water or fog inside the unit | A failed seal or cracked lens | Find the leak, or replace the unit |
| It won’t switch off | A faulty switch or relay | Have it checked by a garage |
Step 1: Check the basics
Before you take anything apart, make sure it isn’t a setting. Many cars only let you switch on the rear fog light when the dipped headlights or front fog lights are already on. If your headlight switch is in the “Auto” position, try switching the headlights on manually first. Then press the rear fog light button and look for the dashboard symbol.
Ask someone to stand behind the car, or reverse up to a reflective surface such as a shop window, so you can see whether it lights up. Check your owner’s manual if you can’t find the switch.
Step 2: Check the bulb
A blown bulb is the most common cause and the cheapest fix. Many cars keep the rear fog light in the rear light unit, so the bulb is often reached from inside the boot. Take the bulb out and look at the filament. If it’s broken, or the glass looks dark, replace it with the same type. Our guide on how to replace a tail light shows how to reach and swap bulbs in a rear light unit.
Step 3: Check the bulb holder
If a new bulb doesn’t light up, look at the holder. Green or white corrosion, burnt plastic and bent contacts can stop the light working. Clean the contacts gently with a small brush or contact cleaner. If the holder is melted or badly damaged, replace it, or replace the unit if the holder isn’t sold separately.
Step 4: Check the fuse and relay
If the dashboard symbol doesn’t come on either, check the fuse. Your owner’s manual shows which fuse covers the rear fog light and where the fuse box is. Replace a blown fuse with one of the same rating. If it blows again straight away, there’s likely a short circuit in the wiring, and that’s a job for a garage. A faulty relay can also stop the light, and it’s easy for a garage to test.
Step 5: Check the switch
If the fuse is fine and the bulb is new, the switch may be at fault. A switch that feels loose, or only works when you press it in a certain way, is a clue. This is usually best left to a garage.
Step 6: Check the wiring and connector
Look at the plug that connects the light unit to the car. It should be firmly seated with no corrosion or melted plastic. Wires can wear where they run to a boot lid, and a light that works when you wiggle the wiring points to a loose connection. A poor earth connection can also cause dim or flickering lights.
Step 7: Look for moisture inside the unit
Water inside a light can corrode the holder and contacts. A little condensation can clear, but a unit that keeps filling with water has a failed seal or a crack. If the lens or housing is cracked, replacement is usually the sensible fix.
When to replace the whole unit
Replace the unit if the lens is cracked, the housing is damaged, it keeps filling with water, or the holder is burnt and can’t be replaced separately. A matching used unit is often more affordable than a new one. Browse used tail lights or used fog lights on Autolight, and check the fit first with our guide to checking compatibility. If you’d rather have a professional look, you can find a repair shop.
Does a broken rear fog light matter for the NCT?
Lighting faults are among the most common reasons for an NCT failure, and they’re usually cheap to fix. If your car has a rear fog light, it’s sensible to make sure it works before the test. You can see what the NCT checks in the NCTS pre-NCT checklist. This is general information, so check the latest details with NCTS.
Not sure when to use it? Read when to use fog lights in Ireland.
Rear fog light FAQs
Why is my rear fog light not working?
The most common cause is a blown bulb. If a new bulb doesn’t fix it, check the holder, fuse, relay, switch and connector.
Why does the dashboard symbol come on but the light doesn’t?
That usually means the switch and fuse are working, and the fault is at the light: a blown bulb, a corroded holder or a loose connector.
Why won’t my rear fog light switch on?
On many cars it only works with the dipped headlights or front fog lights on. Try switching the headlights on manually first.
Can I replace just the bulb?
If your rear fog light uses a bulb, usually yes. If it has built-in LEDs, you often need to replace the whole unit.
Is it OK to drive with a broken rear fog light?
A rear fog light is only meant for dense fog or falling snow, but if your car has one, it’s best to keep it working. Fix it before your NCT.
Need a replacement light?
Browse used car fog lights in Ireland or used tail lights, and filter by make, model, year, side and county.
