| Trailer lights not working | - Blown bulb or fuse
- Corroded plug or socket
- Bad earth connection
- Corroded or chafed wiring
| - Test with a known-good car. If it fails on two cars, the fault is on the trailer.
- Open the plug: green corrosion, loose or broken wires.
- Check the earth wire’s connection to the chassis and at each lamp.
- Follow the cable along the chassis for chafing and crushed sections.
| - Which functions fail.
- Whether it worked on the seller’s car recently.
- Plug type.
| Basic to Intermediate, depending on the cause Diagnosis time: Under an hour Safety-relevant | Bulbs, a plug or a complete light set are modest parts. Rewiring a long or boat trailer takes time. The job ends with every function tested on a car. | - Wiring damage inside a welded or sealed chassis you cannot reach.
- Lighting that was never compliant on a home-built trailer.
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| Old tyres or noisy, loose wheel bearings | - Tyres perished with age
- Worn wheel bearing
| - Four-digit date code on each tyre; cracks in the sidewall and between tread blocks.
- Load index on the tyre against the trailer’s maximum mass.
- Jack up each wheel: play when rocked, roughness or noise when spun, heat after towing.
| - Tyre age and when bearings were last serviced.
- Whether it has been used for launching boats — water and bearings do not mix.
| Intermediate to Professional, depending on the cause Diagnosis time: Under an hour Safety-critical | Tyres by size and load rating. Bearings on unbraked hubs are a moderate job; on many braked axles they are sealed units pressed in and secured with a one-use nut at a high torque — workshop work. | - Bearing play on a braked axle without a workshop quote.
- Tyres that do not match the required load rating.
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| Coupling, overrun brake or breakaway cable problem | - Worn coupling head
- Seized overrun brake
- Breakaway cable missing or damaged
| - Coupling on a ball: wear indicator position, any vertical play.
- Push the overrun shaft in: it should resist smoothly and return.
- Apply the handbrake and try to push the trailer.
- Breakaway cable present, not frayed, with its clip.
| - Make and type of the coupling and axle.
- Last brake service.
| Professional Diagnosis time: Under an hour Safety-critical | Couplings, dampers, brake shoes and cables are specialist-fitted parts with set torques and adjustment procedures. | - Coupling worn past its indicator.
- Seized overrun or brakes with no quote.
- Missing breakaway cable on a braked trailer you would tow home.
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| Rotten floor, rusted chassis or bent axle | - Rotten floor
- Cracked or badly rusted chassis
- Bent axle or drawbar
| - Sight along the drawbar and chassis rails; look for cracks at welds and spring or axle mounts.
- Stand back: do the wheels lean, does the trailer sit level?
- Probe the floor from below for soft, delaminated plywood.
| - Any overload or accident history.
- What it carried.
| Intermediate to Professional, depending on the cause Diagnosis time: Under an hour Safety-critical | A floor panel is carpentry if the frame is sound and the correct board is used. Cracked chassis members, bent drawbars and bent axles are not repair candidates. | - Any crack in chassis or drawbar.
- Bent axle.
- Home-made welds on load-bearing parts.
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| Papers or identification plate missing | - Ownership or registration documents missing
- Identification plate missing
- Carrying capacity cannot be verified
| - Plate present and legible; number matches the chassis and the documents.
- Check what your country requires for a trailer of this weight before viewing.
| - Photo of plate and documents together.
- Proof of ownership.
| Professional Diagnosis time: Under an hour Safety-critical | Re-plating or individual approval, where possible at all, is a process with fees and uncertain outcome. | - No plate.
- No documents where they are required.
- Home-built with no approval.
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