The used car electrical system check: how to test battery, alternator, lights and infotainment before you buy

You can forgive a used car with a scraped bumper or a tired clutch. What you can’t forgive is an electrical fault that leaves you stranded on a wet Tuesday. Most expensive electrical problems — a dying battery, a tired alternator, a wiring gremlin — show themselves in the first 15 minutes of a viewing. All you need is a cheap multimeter, a torch, your phone and ideally a friend to press pedals while you watch.

Before you turn a wheel

Ask the seller to leave the car cold. A warm engine can mask a battery that only just cranks when chilly, and it hides charging faults that only show at idle. Bring a basic multimeter (£8–£20), a torch and your phone. Ask permission before prodding, and don’t dismantle anything — you’re inspecting, not repairing. If the seller won’t let you test the electrics, that’s your first red flag.

The battery: resting voltage and drop test

Resting voltage

Switch everything off and wait ten minutes if the car has just been run. Set the multimeter to 20V DC. Red probe to positive, black to negative. A healthy battery reads 12.6V. At 12.4V it’s about three-quarters charged; 12.2V is half; under 12.0V is weak. If it’s just been charged, switch on the headlights for ten seconds to clear the surface charge, wait a minute, then retest.

Cranking drop test

Keep the multimeter connected and ask your helper to crank the engine. Healthy batteries shouldn’t drop below about 9.6V while cranking — ideally above 10V. If it plunges to 8V and the dash lights dim, the battery is failing, the starter is drawing too much or there’s an earth fault. Slow, laboured cranking or rapid clicking is a classic warning sign.

Physical clues

Check the battery date code (a small sticker with month and year), terminals for white or green fluffy corrosion, and the case for swelling. Here’s the subtle one: a recently replaced battery. Sellers often fit a fresh one to get a tired car started, but if the alternator isn’t charging, you’ll be buying the same fault again within weeks. A new battery is only good news if the charging voltage checks out.

The alternator: charging voltage under load

With the engine idling, measure across the battery. You want 13.8–14.6V. Below 13.2V means it isn’t charging enough; above 14.8V means it’s overcharging, which cooks the battery. Now add load: headlights, blower on full, heated rear window, radio. Voltage should stay above 13.2V. Ask your helper to hold about 2,000rpm and check again. Flickering headlights at idle, or a whine that rises with engine speed, points to a tired alternator, failing regulator or slipping belt. A replacement alternator typically costs £150–£400 fitted, plus £50–£150 for a belt.

Lights: every single one

Get your helper to work the switches while you walk around. Ignition on, engine off for most of it. Check dipped beam, main beam and flash, sidelights, daytime running lights, front and rear fog lights. Indicators both sides, then hazards. Brake lights — helper presses the pedal while you watch, or reverse up to a shop window. Reverse lights need the engine running, gearbox in reverse and handbrake on. Don’t forget number plate, interior, boot and glovebox lights. Check lenses for cracks and condensation, too: a failed LED headlight can cost £300–£1,000, not £5 for a bulb.

Infotainment and cabin electrics

Sit in and work through everything. Is the touchscreen responsive, or are there dead spots and cracks? Pair your phone — test Bluetooth, Apple CarPlay or Android Auto, and the USB ports. Check the sat nav SD card is present (they often go missing) and DAB radio works. Try heated seats, heated mirrors, heated rear window and air conditioning. Then the windows: all four from the driver’s switch and each door’s own switch. Grinding or slow movement suggests a tired regulator (£150–£350). Test central locking from both fobs, boot release and fuel flap. If fitted, check the electric tailgate, reversing camera and parking sensors.

Warning lights: which should go out, and which shouldn’t

Turn the key to ignition without starting. Every warning light should illuminate as a bulb check. If some don’t come on, someone may have removed the bulb to hide a fault — a proper red flag. Now start the engine. Within a few seconds, the battery, oil, engine management, ABS, ESP, airbag, tyre pressure, DPF and AdBlue lights should go out. A battery light that stays on means the alternator isn’t charging. An engine management light could be a loose sensor or something serious — don’t accept “it just needs a reset”. A basic code reader costs £20–£60, or a garage will scan it for £40–£80. Some lights only clear once you drive, so ask for a test drive.

Red flags that should make you walk

• Slow cranking or clicking when you turn the key
• Interior or exterior lights that flicker at idle
• A recently replaced battery with no explanation
• Warning lights that don’t illuminate at all
• Tape or stickers over the dashboard warning area
• A burning smell, hot battery or aftermarket alarm wiring

Your 15-minute pre-purchase checklist

• Car left cold if possible
• Battery resting voltage 12.6V or higher
• Cranking voltage stays above 9.6V
• Charging 13.8–14.6V, and above 13.2V with lights and blower on
• Every exterior light works, including brake, reverse and fogs
• All windows, locks and both fobs work
• Infotainment pairs with your phone and cabin electrics function
• No warning lights remain on after start-up
• Battery date code and terminals look healthy
• Diagnostic scan if anything seems off

Rough costs if something is wrong

Battery £80–£250. Alternator £150–£400 fitted. Bulb £5–£30; LED headlight unit £300–£1,000. Window regulator £150–£350. Infotainment unit £300–£1,500. Body control module or ECU £100–£600 plus coding. Diagnostic scan £40–£80.

What to do if you find a fault

Use it as a negotiating tool. Get a quote from a local garage before agreeing a price, and knock that amount off — plus a bit for inconvenience. If the seller insists it’s nothing, or won’t let you get it checked, walk away. There are plenty of used cars out there, and electrical gremlins are the ones that keep costing money long after the shine has worn off.

When you’re ready to make a change, it’s worth looking at UpCars.uk, where every car is checked over properly before it’s listed — so you can spend less time worrying about someone else’s electrical shortcuts and more time enjoying your new motor.