Quick answer
Most trailer light faults come down to four things: a poor earth, corroded plug or socket pins, a blown globe, or a blown fuse or faulty module on the vehicle side. Start by testing the vehicle’s socket with a trailer tester. If the socket works, the fault is on the trailer, and the earth is the first thing to check. Lights that glow dimly, flash together or work the “wrong way” almost always point to a bad earth.
Is this a DIY job?
- Difficulty: Easy
- Time: 15 to 60 minutes
- Tools: trailer circuit tester (or a helper), multimeter or test light, wire brush, contact cleaner, screwdriver, spare globes
- Skills: basic electrical checks
Step by step
- Test the vehicle side first. Plug a trailer circuit tester into the vehicle’s socket, or use a test light between each pin and pin 3 (earth). Check tail lights, left and right indicators, stop lights and reversing lights. This tells you which half has the fault.
- If the vehicle socket fails: – Check the towbar or trailer fuses (many vehicles have dedicated ones; see the owner’s manual or the towbar harness instructions). – Look inside the socket for green corrosion or bent pins. Clean with contact cleaner and a small brush. – Many modern vehicles feed the socket through a trailer module. If the module has no power or has been damaged, the socket won’t work. (see Do I need a vehicle-specific towbar wiring harness?)
- If the vehicle socket works, check the trailer plug. Open it and check every wire is tight and there’s no corrosion. Cut back and re-terminate any wire that’s green or brittle. (see How do I wire a 7-pin flat or 7-pin round trailer plug?)
- Check the trailer earth. Follow the white earth wire from the plug to where it bolts to the trailer chassis. Remove the bolt, clean the terminal and the chassis back to bare metal, refit and protect it. Many trailers also rely on each lamp’s mounting for earth: check those too.
- Check the globes. Swap in new globes or test them. A blackened or broken filament means it’s blown. LED lamps can’t usually be repaired; replace the lamp.
- Check the lamp sockets. Corrosion in a lamp holder causes flickering. Clean it, and use a little dielectric grease to keep moisture out.
- Look for damaged cable. Check the cable along the drawbar and under the trailer for cuts, crushing or rubbing, especially where it passes through holes without grommets.
- Retest everything before you tow, including brakes if fitted.
Specs table
Symptom and likely cause
| Symptom | Likely cause |
|---|---|
| Nothing works | Vehicle fuse or trailer module, plug not fully in, broken earth |
| Lights dim, or glow when other lights are on | Poor earth (most common) |
| Indicator lights up the tail lights, or both sides flash | Poor earth |
| One function missing on both sides | That pin or wire at the plug or socket |
| One lamp out | Globe, lamp socket, or that lamp’s earth |
| Works when wiggling the plug | Corroded pins or loose wire in plug or socket |
| Indicators flash fast with trailer on | Trailer globe out, or LED trailer lamps on an older flasher |
| Trailer lights fine, brakes weak | Earth, pin 5 connection, or brake controller setting |
Pin reference (7-pin flat and 7-pin large round): 1 left indicator (yellow), 2 reversing (black), 3 earth (white), 4 right indicator (green), 5 electric brakes (blue), 6 stop (red), 7 tail (brown).
What you’ll need
- trailer circuit tester
- 12V test light
- electrical contact cleaner
- dielectric grease
- ring terminals and adhesive heat shrink
- LED trailer lamps or trailer globes
- 7 pin flat trailer plug (if the plug is badly corroded)
When to call an auto electrician
- The vehicle socket doesn’t work and the fuses are fine.
- The trailer module or towbar harness has failed, or the vehicle shows trailer or lamp warnings.
- The trailer cable needs replacing along its length, or the trailer has electric brakes that aren’t working properly.
- You can’t find the fault after checking the earth and plug.
Need an auto electrician? Find local auto electricians and mobile services in your city.
Related questions
Q: Why do my trailer lights flicker? A: Usually a poor earth or corroded pins. Clean the earth point back to bare metal and clean the plug and socket.
Q: Why does my car’s indicator flash fast with the trailer connected? A: A trailer indicator globe is likely blown. If the trailer has LED lamps and the vehicle is older, the flasher may not recognise the low load.
Q: Can I use the towball for the trailer earth? A: No. The towball and coupling aren’t a reliable electrical path, and the trailer rules require a dedicated earth return wire between the trailer and the vehicle. Use the white earth wire through the plug.
Q: Do LED trailer lights need special wiring? A: Usually not, but they’re sensitive to poor earths, and some vehicles need a trailer module to recognise them.
Sources
- ExplorOz, “Trailer wiring diagrams” – pin functions and colours. https://www.exploroz.com/vehicle/electrics/trailerwiringdiagrams.aspx (checked October 2026)
- Department of Infrastructure, Vehicle Standards Bulletin 1 (Revision 6, August 2026) – trailers up to 3,500 kg ATM use a 7-pin or 12-pin plug to AS 4177.5; 7-pin functions and colours; an earth return wire between trailer and tow vehicle must be provided; required trailer lamps. https://www.infrastructure.gov.au/sites/default/files/documents/vsb1-revision-6-trailers-with-an-aggregate-trailer-mass-of-4.5-tonnes-or-less-august-2026.pdf (checked October 2026)