What a car diagnostic test can and cannot tell you

A modern car has dozens of control units, and each keeps a record of what it has seen go wrong. A diagnostic test reads those records. It is the right place to start, and it is often mistaken for the whole job.

What the scan reads

• Stored fault codes from every control unit: engine, gearbox, brakes, suspension, body and comfort systems.
• Freeze-frame data: the engine speed, temperature and load at the moment a fault was recorded.
• Live values: what the sensors report while the engine runs, such as pressures, temperatures and fuel corrections.
• Readiness and counters: how many times a fault has come back, and whether the codes were cleared recently.

What a fault code means

A code names the circuit or the value that was outside its limits. It does not name the faulty part. “Mixture too lean” can be an air leak, a weak fuel pump, a dirty sensor or an exhaust leak. “Misfire on cylinder 3” can be a coil, a spark plug, an injector or low compression. The code says where to look, and the rest is testing.

What the scan cannot see

Many faults leave no code at all:

• worn suspension bushes, dampers and ball joints;
• most oil and coolant leaks, until a level runs low;
• brake wear on cars without sensors on every pad;
• noises, vibrations and smells;
• a problem that comes and goes and was not present long enough to be stored.

That is why a scan is paired with a look at the car on a lift and, where the complaint calls for it, a drive.

From code to cause

After the scan come the tests that confirm the cause: a smoke test for air leaks, a pressure test of the cooling system, a compression or leak-down test, measurements on a wiring circuit, a comparison of live values with the manufacturer’s figures. Replacing the part a code seems to point at, without that step, is how a cheap fault becomes an expensive one.

Why several warnings often have one cause

On European cars a weak battery or a poor earth connection can bring up warnings for the gearbox, the stability control and the steering at once, because every unit complains when the voltage drops. A list of ten codes often has one cause, and finding it is the skill.

Clearing codes does not repair anything

A cleared code returns if the cause is still there, sometimes after a few days. Before buying a used car it is worth knowing that a scan also shows whether the codes were cleared shortly before the sale.

When to ask for a test

• A warning light is on, steady or flashing.
• The car has lost power, uses more fuel or changes gear badly.
• Something happens now and then and cannot be shown on demand.
• You are about to buy the car.