I know what you mean, but, about 10 years ago my son had a rough running vw polo so unable to find fault I bought a cheap code reader off ebay which helped point me in the right direction. It was less than a tenner, and I've used it on every car I've had since. Just for engine codes tho - I just read codes (write them down) google what they might mean, fix what I think, then delete the codes - not failed me yet. On some cars I just accept that it records certain codes (like glow plug relay code on my NV - starts and runs fine with it) My reader's a model u480 but there are tonnes out there.
Disclaimer - some people say they can knacker ECUs so CAUTION. Min'e been OK - I only use it to read and delete the codes, if I haven't fixed it they come back - I don't use it for 'live' monitoring of parameters, which might be where problems start. Every car in the EU since late 1990s I believe has an ODB socket. It's behind the 'card holder' panel under the driver's cup holder on the UK NV. For example, on a petrol car it'll tell you which cylinder is mis-firing so you know which HT lead to wiggle or plug to swap to see if the fault moves with it.
Pin-pointed my wife's cam position sensor failure on her rough running (BMW so not proper ; ) mini. £30 for the part from eurocarparts, 10 mins to change - lovely job, no garage fees.