Hello Rene
I am sorry to hear that you are still having issues.
Please ensure that you have done the SMC exactly as per Apple's instructions. Do not do anything other than what Apple recommends for your model. If you've done this there is no point doing it over and over again. To be brutally honest, Apple recommends this be one of the last steps to try actually.
If the AHT (Apple hardware test) has come up negative, let it run in loop mode overnight. To run in loop mode use control-L and let it run overnight in another room, it may pick something up. If I am not mistaken, a sensor error usually has an SNS in its string.
A bit of background, all macs have temperature sensors that connect as it were, to the SMC, which is a small computer of it's own in many ways. The temperature readings are passed on to the SMC and in turn the SMC regulates the fans according to temperature readings, of course the SMC has to have settings or thresholds set their defaults. If those thesholds are incorrect it will trigger fans incorrectly. This is only one of the SMC's functions by the way. It is very important that you run software update to ensure your mac is not missing any firmware updates which if absent may be responsible for the issue.
So, the SMC may be damaged or a sensor is damaged. Either of these are hardware related. Obviously there will be an associated cost for repair but this will depend on the components/labor needed to repair this.
As noted by a previous poster, it is true that AHT does not pick up everything, this can be expected. AASPs and Retail Stores have more robust service diagnostic tools to help diagnose issues.
Please note, the importance of testing against an external HD with a clean copy of the OS is to rule out any software you may have installed but forgot about. I know that there are software packages, which I would never recommend, to help control fans at the user level. This should be avoided. If you installed anything like this, remove it of course.
If you leave your mac off overnight and it turns on and fans just pick up at ful rate just after starting up then the SMC does not have it's setting to default or a sensor is delivering the wrong information to the SMC and the SMC reacts accordingly but incorrectly, blasting the fans.
In conclusion, if the above yields not positive results I would recommend the mac be evaluated for a potential hardware issue, whether to go forward with a full repair beyond the evaluation is an economic question that only you can best answer.
Do backup your data before anything else.
PS don't worry about asking questions, that's what this place is for and I for one am delighted to offer my help.