Yaris check engine light

Hi, check engine light came on in my 04 diesel Yaris so I dutifully brought it to a mechanic. He checked it out and found that it was a sensor fault on one of the pre-heaters. He reset it but it has come back on again. It's been about 5000 miles now so I doubt it's anything serious. I didn't pay anything for the reset but I imagine if I want the problem fixed permanently it will cost quite a bit. There are no issues with the car apart from this but the NCT is coming up and I doubt a car with a check engine light will pass it. Just wondering if anyone knows a way to turn the blasted thing off. Thanks.