Two Cheap Tools That Check a Used Car Before You Buy

Most used-car inspection advice is about what to look at. These two tools are about seeing things a walk-round physically cannot — one through the paint, one inside the engine.

A note on this guide: this rests on what is genuinely checkable during a viewing and what is not, not on cars inspected here. See how we test.

The problem, as it appears

Standing in front of a car you might buy, with the seller watching, and perhaps twenty minutes to form a view that will be expensive if it is wrong.

Most inspection advice assumes a lift, a scanner and unlimited time. What is actually needed is the short list of things checkable on a driveway that genuinely change the decision.

A paint gauge reads the car's accident history

Factory paint goes on within a reasonably consistent thickness range. Bodywork repairs do not: a repainted panel carries extra material, and a panel with filler under the paint carries a great deal more.

So a gauge held against each panel in turn produces a set of numbers, and what you are looking for is the outlier. Three doors reading similarly and one reading several times higher is a panel with a history the advert did not mention. Readings that jump around within a single panel are just as telling, because filler is rarely spread evenly.

What it cannot do is tell you why. A repainted wing might be a supermarket car park scrape or the visible corner of structural damage. The gauge tells you where to ask harder questions, not what the answer is.

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Ask the seller before you start. It involves repeatedly touching their car with an unfamiliar device and looks strange unexplained. Most people with nothing to hide do not mind.

A borescope sees where nobody can look

The camera on a flexible cable turns three inaccessible places into things you can actually inspect:

Cylinder bores, through the spark plug holes. Vertical scoring, heavy carbon build-up, or an obviously different-looking cylinder are all visible without removing a single component.

Inside sills, chassis rails and box sections, through existing drain holes. This is where structural rust actually lives, and it is invariably worse inside than the paint outside suggests.

Behind wheel arch liners and trim, where repairs get hidden because nobody looks.

The same tool has a second life afterwards for finding things dropped into an engine bay, which is how most people end up owning one.

If you are serious about the car

A compression test is the most informative measurement available in a driveway, because it reports on the expensive part of the car. It also takes time and means removing every plug, so it belongs at the stage where you are genuinely considering buying rather than on a first look.

What actually matters

Compare, do not judge. Both tools work by contrast — this panel against that panel, this cylinder against that cylinder. A single reading in isolation is close to meaningless.

Bring a scanner too. An OBD2 reader shows stored and pending fault codes, and notably shows whether the readiness monitors have been recently reset — which is what a cleared dashboard warning light looks like from the outside.

Daylight, and a dry car. Wet paint hides swirls and mismatched finish; a dark forecourt hides everything.

The mistake to avoid

Treating a high paint reading as proof the car is damaged goods, or a clean set of readings as proof it is not. A well-executed repair can read close to normal, and an honest respray after vandalism reads high. These tools raise questions efficiently and cheaply. They do not answer them, and they are not a substitute for a professional inspection on a car you are about to spend real money on.