Hi, if you want to share your advice you can do it
please feel free to share it.
i will share my little experience here:
1. before you tune a car do a good research in google.
2. before doing a tune check the car, is it in good condition, the oil level,
how the owner takes care of it, before you tune do some logs look is the car having an error
3. look under the seat maybe there is a phone and when you pull the seat you will broke it
its good to check it before reaching the obd port.
4. start with kess v2 and ecm titanium and try to do something on your car. after some learning try to learn winols
5. START WITH DIESEL CAR
6. dont focus just in electronics try to understand how the engine works.
7. first try to learn how to do an egr, dpf off, hardcut...
8. first try to learn and understand in edc15 (1.9tdi is good choice)
9. before you write the car do a checksum correction on winols
10. if you want to do this work after the kess v2 atleast buy a ktag too
11. never use file from databese in clients car
12. if you cant tune the car just say something that you cant do it (dont take big risks)
13. never do a bad tuning on clients car!!! if you cant do it buy file from internet.
14. check what you done after tuning
IF YOU HAVE SOME ADVICES YOU CAN SHARE IT
HAVE GOOD DAY!!!

This looks like really useful advice.

I am trying to work out why my EDC15C2 ECU is not allowing fault codes to be cleared. I read on one post in this forum that immo chip holds the DTC registers but there is no advice on how to clear those registers. My diagnostic tool is a Lexia and it clears intermittent codes on all other Citroen/Peugeot cars but will not clear codes on one particular ECU.

ECU is from car with over 200k miles and I am trying to learn how to correct the immo file to enable it to allow code deletion again. Maybe no-one here actually knows the answer.

I cannot even find any reliable information on checksums so how does someone learn? Do you know of any forum that has this information in a tutorial? It all seems very esoteric like people are trying to protect the knowledge from new initiates until they reach a certain level. I don't understand it. People cannot learn if the information is not presented to them and therefore they cannot pass that knowledge on and when everyone on here has died all the information will be lost in the mists of time...

I have access to Kess3 so today I am going to read a EDC15C2 from a RHZ engine (although that and RHY do not appear in Kess - they have RHW and RHX instead?) and try to write it to an EDC15C2 from a RHY engine. The part numbers are different. One is 0 281 010 251 and the other is 0 281 010 165.

If I brick the RHY ECU, so what? I have at least learned something. If I can successfully write a file to it and it stops pumping out loads of unburnt diesel then great but I guess the question of the checksum will come into it somewhere. I don't know where to get it but I will look at that winols programme people keep talking about and you mention useful in your post here.