Electrical Faults and Modern Car Electronics: What to Check

By Márton Kovács, Lead vehicle inspector · Published 2026-06-09 · 5 min read

The short version

A modern car runs dozens of networked control units over a CAN-bus backbone.
Intermittent faults from corroded connectors and earths are the hardest type to trace.
A parasitic drain that flattens the battery overnight usually means a module that will not sleep.
Water ingress is a leading cause of electrical gremlins; we look for flood and damp signs.

A modern car is as much a computer network as a machine, and that has changed what can go wrong with it. Dozens of electronic control units manage everything from the engine to the door locks, and they talk to each other over a shared digital backbone. When that system misbehaves, the symptoms can be baffling — a warning light that comes and goes, an accessory that works until it is hot, a battery flat after a weekend — and they are among the hardest faults to pin down without the right tools.

This guide explains how a car's electronics are organised, the symptoms of electrical trouble, the particular problem of a parasitic battery drain, why water ingress causes so many electrical faults, and what an inspection does to expose them. For a used buyer, a thorough electrical check is one of the most valuable parts of an inspection, because these faults are easy to hide and expensive to chase later.

How a modern car's electronics work

Rather than wiring every component directly to a switch, a modern car uses a 12-volt system feeding dozens of networked control units — for the engine, transmission, ABS, body functions, infotainment, airbags and more. These modules share data over a CAN-bus, a two-wire digital backbone, broadcasting and reading messages rather than being individually wired together. It is an elegant system that saves weight and adds features, but it also means a single fault can have wide-ranging effects.

When the car is switched off, the modules are designed to drop into a low-power sleep state within a few minutes, leaving only a tiny standby current. Understanding this matters, because a module that fails to sleep is the most common cause of the flat-battery complaint, and recognising the normal behaviour is the first step to spotting the abnormal.

The symptoms — and the hardest type to trace

Electrical faults show up as warning lights, sometimes several at once, and as dead or erratic accessories — windows, mirrors, lights, infotainment, central locking. The hardest type to trace is the intermittent fault: a system that works fine until heat, humidity or vibration changes the quality of a connection, then fails unpredictably. This is the classic signature of a corroded pin or a poor earth, and it is why a fault that is not present during a brief look can still be real.

Corroded connectors and earth points are often the underlying cause, especially after exposure to road salt or water. Corrosion shows as a fuzzy green or blue crust on copper pins, a white powdery deposit on aluminium, or blackened terminals, and it creates the high resistance and signal distortion behind so many odd electrical behaviours.

Parasitic drain and water ingress

A parasitic drain — a battery flat overnight or after a few days standing — usually means a module that will not go to sleep, such as the body control, infotainment or telematics unit, which keeps drawing near its operating current. Other causes are aftermarket accessories wired to a permanent live, stuck relays and wiring shorts. A normal standby draw is small; a sustained high draw can flatten a battery within a day, so a seller's car that needs a jump or has a recently fitted battery is worth questioning.

Water ingress is a leading cause of electrical gremlins, and it is one we look for specifically. Water reaches the electronics through blocked scuttle or sunroof drains, failed door or windscreen seals, or a blocked air-conditioning drain soaking the footwells, and it corrodes connector pins, earth points and circuit boards. The damage is often intermittent and worsens with age, which is exactly why a car can run fine until conditions change.

What an inspector checks

The foundation of the electrical check is a full diagnostic scan of every module, not just the engine, reading stored and pending fault codes plus live data. A cluster of communication codes points to the CAN-bus wiring or corroded connectors rather than a single failed module, so the network is inspected before any expensive unit is condemned, and the live data catches faults that are not currently triggering a hard code.

Alongside the scan we functionally test every electrical feature — lights, windows, mirrors, wipers, heating and air-conditioning, infotainment and central locking — and we look hard for the signs of prior water damage. A musty or wet-carpet smell, white mineral deposits on plastic edges, tide marks on module cases and brackets, corrosion on low-mounted connectors and earth straps, and damp carpet underlay all point to a history that explains intermittent electrical faults.

What an inspector checks — UsedCarCheck pre-purchase inspection.

Where to read next

A modern car's electronics fail in subtle, intermittent ways that are easy to miss in a brief viewing and expensive to chase after a sale. Our on-site inspection scans every module, tests every electrical function and looks for the signs of water damage, then reports it all in plain English before you buy.

For background on the digital network that modern car modules communicate over, see the CAN bus that links a car's electronics.

Frequently asked questions

Why are intermittent electrical faults so hard to find?

Because they are usually caused by corroded connectors or poor earths that only fail when heat, humidity or vibration changes the contact quality, so the fault may not be present during a brief look. We scan for stored and pending codes and inspect the wiring and earths rather than relying on what shows at the moment. Everything is geared toward one outcome: a confident, well-informed purchase.

What causes a car battery to go flat overnight?

Usually a parasitic drain from a module that will not go to sleep, such as the infotainment, body-control or telematics unit, though aftermarket accessories and stuck relays can also cause it. A normal standby draw is tiny, so a car that needs jump-starts or has a freshly fitted battery is worth questioning. Either way, you end up knowing exactly what you are buying before any money changes hands.

Can water damage cause electrical problems?

Yes — water ingress from blocked drains, failed seals or a soaked footwell corrodes connector pins, earths and circuit boards, and is a leading cause of intermittent electrical faults. We look for musty smells, mineral deposits, tide marks and damp carpet that reveal a flood or leak history. Because we work only for you, there is no hidden agenda anywhere in the report.

Do you run a full electrical scan on site in Budapest?

Yes. We scan every module for stored and pending codes plus live data, functionally test every electrical feature and look for water-damage signs, as part of every inspection across Budapest, with an English-speaking inspector and an English report. We would rather you walked away from the wrong car than regretted a good-looking one.

Would a comprehensive used car inspection onsite catch the problems described above?

Yes — this is exactly the kind of risk a comprehensive used car inspection onsite is designed to catch. Our inspector goes over the car on site, documents what they find with photos, and explains every point to you in plain English. You then decide with the evidence in front of you rather than the seller's word.

Márton Kovács · Lead vehicle inspector

Márton has spent over 18 years assessing used cars, from auction lanes to private driveways. He leads UsedCarCheck's on-site inspections in Budapest and writes the technical checklists the team works from.

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