Earth straps, connectors full of green powder, and relays that only fail when they are hot.
A customer arrives having been told the car needs a new control module. Nine years of this work and I can tell you what it usually is instead, and it is one of three things, none of which is a computer.
The first is earths. A car does not have a return wire for everything; it earths through the body, and those earths are braided straps and bolted contacts at a handful of places. Salt air turns a bolted contact into a resistor, slowly. What makes it so confusing is that the symptoms have nothing to do with each other: the headlights go dim, the blower runs slowly, the radio resets when you indicate, the starter turns lazily, and a warning light comes on that has no business being related to any of it. Every one of those is the same two bolts. Cleaning them is the cheapest real repair in this workshop and it fixes five complaints at once, and I have watched a customer's face when it does.
The second is connectors with green powder in them. Multiway plugs live in bad places: low in the wheel arches, under the battery tray, behind the front bumper. Water and salt get in, corrosion grows across the pins, and the circuit works when it feels like it, which is the worst kind of fault because it will not be present when anybody is looking. There is no instrument that finds these. You open them one at a time and you look, and that is an hour of somebody's life and it cannot be made shorter. It can be made cheaper by being asked for directly, which is what the Saturday clinic is for.
The third is relays that fail when hot. Perfect at eight in the morning, dead at four in the afternoon, and they test fine on a bench because a bench is not thirty eight degrees inside a dashboard. The tell is always the same: the customer says it happens in the afternoon, and everybody before us ignored that sentence. It is the most useful sentence in the conversation.
Which brings me to the part that is genuinely your job rather than ours. Write down when the fault happens. Hot or cold. Moving or standing. Wet weather or dry. What else was switched on. That note is worth an hour of measuring, and since you are paying for hours, it is worth money to you.
And bring the car while it is broken. I cannot measure a fault that is not happening. If it needs twenty minutes of driving around in the heat before it plays up, do the twenty minutes and arrive hot. If it only happens after the car has sat all night, say so, because then it is not an appointment at all, it is two nights in the yard with a meter on the battery.
When it really is a module, we say so, and we say how we know, and we show you the measurement. It happens. It happens far less than it is blamed.