Car Electrical Tester Guide: Testing Lights and Wiring Faults at Home

An MOT report that says “nearside brake light inoperative” starts a clock: you usually get a limited free retest window, and a garage can charge by the hour just to trace the fault. A car electrical tester lets you check the obvious causes yourself first — often the difference between a five-minute fix at home and a bill for diagnostic time.

This guide walks through testing a dead light, a suspect fuse and a bad earth in a safe order, so you either fix it outright or walk into the garage knowing exactly what is wrong. Product specs for the Autel PowerScan PS100 come from our live UK product page.

Test in the right order: bulb, fuse, feed, earth

Most lighting failures are one of four things, and checking them in order saves swapping parts that were never at fault.

1. Bulb: inspect the filament or swap in a known-good bulb.
2. Fuse: confirm the fuse for that circuit is intact and has power both sides.
3. Feed: check voltage and polarity are reaching the bulb holder.
4. Earth: if power is present but the lamp is dead, check the earth with a continuity test — a corroded earth is a classic culprit.

A tester that shows voltage and switches to continuity, like the PS100, lets you run that whole sequence with one tool instead of a lamp plus a meter.

Reading the results

Power present, lamp dead

If you have voltage at the holder but no light, suspect the bulb first, then the earth. This is where a continuity check earns its keep — it confirms the return path rather than leaving you guessing.

No power at all

Work back towards the fuse box. No voltage at the holder but a good fuse points to a break in the run or a switch fault. The PS100’s 6-metre lead means you can keep it clipped to the battery and still reach a rear light cluster.

Staying safe and ECU-aware

Conventional 12V lighting, fuse and relay circuits are safe to probe with care. Do not touch the orange high-voltage cabling on hybrids and EVs, and use the PS100’s high-impedance mode around sensitive electronics. If the trail leads to a control module rather than a bulb or fuse, that is the point to involve a specialist. For relays specifically, see how to test a car relay.

Before your MOT retest

Lighting and “electrical” items are among the most common MOT failures, and many are genuinely DIY-fixable. Confirming the cause at home means you either pass the free retest or arrive with a clear description that keeps any paid work short. If the car will not stay charged between attempts, our battery drain guide covers that separately.

The tester that makes home fault-finding realistic

The Autel PowerScan PS100 is a compact car electrical tester that checks voltage, polarity, ground and continuity in one 30 g tool. It reads 0 to 65V and continuity to 200 kOhm on a TFT colour display, has an integrated LED work light for dark footwells, a 10A thermal-trip breaker, and a 6-metre lead that reaches a rear cluster from the battery. It supports 12V and 24V systems, has a high-impedance mode for ECU-sensitive lines, and is powered from the vehicle so it is always ready. It is GBP 212.49 inc. VAT with Free UK Delivery, 30-day returns and a 12-month warranty plus UK support, rated 4.8 out of 5 from 300 reviews.

Frequently asked questions

How do I test a car light that has failed its MOT?

Check in order: bulb, then the fuse for that circuit, then whether voltage and correct polarity reach the holder, then the earth with a continuity check. If power is present but the lamp is dead, the bulb or a corroded earth is the usual cause.

Can I use a 12V circuit tester on hybrid high-voltage systems?

No. High-voltage orange cabling requires insulated tools and qualified technicians. A 12V tester is for conventional low-voltage looms, fuse boxes, relays and lighting only.

What does it mean if there is power at the bulb but no light?

Power present with a dead lamp usually points to the bulb or, very often, a bad earth. Confirm the earth path with a continuity check before replacing other parts, as a high-resistance earth behaves like an intermittent fault.

Will checking it myself save a garage diagnostic fee?

Often, yes. Many lighting faults are a bulb, fuse or earth you can fix at home. Even when they are not, arriving with a clear description of what you tested keeps any paid diagnostic time short.