Hi everyone, I have a 2008/9 Mazda 3 Auto and have got the dreaded CEL. First time I got it I scanned for the code and got P21870. So I cleared it and have ran the car for a couple of weeks and the CEL has came back with same code. The car feels a bit sluggish between 3rd and 4th gear but generally is running ok. Is my best course of action to take it to a garage or is there anything I can do to fix the problem? I'm not the most knowledgeable about cars to be honest. The twice that it has happened it has been raining heavily and I had just filled up from my local petrol station, coincidence probably?

Thanks for any help and advice.