Symptom guides

Start from the symptom. End with a measurement.

Each guide lists the causes a symptom could have, then the tests that tell them apart, with the tool and the value to expect. No cause is ranked before a reading exists.

Common-rail diesels, roughly 2004 onward
Diesel cranks but will not start
A common-rail diesel that turns over and never fires. The possible causes, then the tests that split them, with the tool and the value to expect.
6 causes · 4 tests →
Turbo diesels, any age
Judders under load with black smoke
A diesel that shudders on a hill and puffs black smoke with no warning light. Possible causes, then the tests that split them.
6 causes · 4 tests →
Any disc-braked car or van
Brake squeal, only when cold
A squeal from one corner on the first few stops that disappears when the brakes warm. What causes it and how to prove which one it is.
5 causes · 4 tests →
Any water-cooled engine
Overheating with no visible leak
Coolant temperature climbing with nothing on the floor. The causes, then the tests that separate a cooling fault from a head gasket.
6 causes · 4 tests →
Petrol engines with port or direct injection
Rough idle when cold, smooth when warm
A petrol engine that shakes for the first two minutes and then runs clean. The causes and the tests that separate fuelling from a mechanical fault.
5 causes · 4 tests →
Petrol engines, port and direct injection
Petrol engine cranks but will not start
A petrol engine that turns over at healthy speed and never fires. The possible causes, then the tests that split them, with the tool and the value to expect.
6 causes · 5 tests →
Any modern vehicle with a CAN network
Battery goes flat overnight
A car that starts fine when driven daily and is dead after a weekend. How to prove it is a drain and not the battery, then find the circuit without guessing.
6 causes · 5 tests →
Petrol engines with pre and post catalyst oxygen sensors
P0420 catalyst efficiency below threshold
The code names the catalyst, but the fault often is not. How to read both oxygen sensors, rule out exhaust leaks and fuelling, and only then condemn the cat.
6 causes · 5 tests →
Any vehicle with four wheel speed sensors
ABS and traction light on together
Both lights on and the speedometer sometimes dropping out. How to isolate a single wheel speed sensor from wiring and a hub, using live data instead of parts.
6 causes · 5 tests →

None of these is your car.

A Case takes the vehicle and the symptom in front of you, asks the two or three questions that narrow it, and names one test at a time with the value to expect.

Open your first Case

Free and complete. No account, no card.