This 'free' plug-in device can save drivers thousands on garage bills

It's now very easy to pick up a cheap car diagnostic tool. They've been making regular appearances in the middle aisle of Aldi, and they're less than £20 on Amazon.

These tools usually save you having to pay a garage fee if a warning light flashes up, because you can find out for yourself what caused it, rather than have it professionally scanned. So they're worth having

But if you plug one of these scanners into your car, it gives you a fault code. And that code could mean pretty much anything to the uninitiated. Would you know what P0130 meant? Exactly.

OBD scanners that pair up to smart apps are an alternative, because the app can digest the code and suggest some possible reasons why it has been stored.

One of the most comprehensive packages out there has always been Carly, an OBD scanner developed by a German car tech company that can give you the deepest possible insights into your car's inner workings.

It's such a comprehensive device, when paired with the Carly app, that it can even "code" certain vehicles, unlocking hidden features.

For example, I used a Carly with my wife's BMW to turn on a hidden digital speedometer, which we didn't even know was available on the instrument display. It's very clever.

Its biggest strength, however, is in its diagnostic capabilities. Carly can get deep into the engine's "brain" and hunt around for fault codes, and then you have access to not only an online library telling you what the faults mean, but also a "Smart Mechanic" feature, which talks you through what might be wrong and how serious it could be.

Giving you access to this data, and interpreting it in a smart way is what can save you money on your garage bills.

The Smart Mechanic system will give you guidance on what might have caused the fault, and whether it can be easily fixed. Meanwhile, Carly's software will warn you if you're about to do more damage driving the car, or whether you're OK to sort the fault at your convenience.

The app can even suggest repair tips, in case you're brave enough to roll up your sleeves and fix something.

Sorting a faulty sensor, for example, can cost hundreds of pounds in garage labour, but swapping it at home could cost a few pounds. Carly will tell you whether it's worth braving the job yourself, or whether you should call in the experts.

There is, of course, a cost to this vast database of knowledge. And pricing up a Carly scanner is far from simple. It depends on the type of car you want to use it with, and the type of package you go for.

If you go for the premium packages, which usually include the Smart Mechanic service, you're likely to get the scanner for free. And then you'll pay a certain amount for the full access to the app features.

This could be around £50, or it could be a little more, but because it's Carly's Birthday Sale this week, you will save 20% using the discount code BDAY20.

Enter the code at the checkout and the price will drop. To find out how much it will cost to keep your car in fine fettle, click here.