The Bavarians looked to be in trouble earlier this season, with Carlo Ancelotti losing his job after a poor start to the season.
However, Heynckes returned to the club in October, signing a deal for the rest of the campaign, and he has since guided Bayern to seven wins from as many matches.
Bayern are now four points clear at the top of the Bundesliga table after beating Borussia Dortmund 3-1 last time out but Heynckes insists he has no plans to extend his deal beyond the summer.
When asked if he could sign an extension, he told Welt am Sonntag: "No, that's impossible."
However, he did assure fans that winger Arjen Robben should be offered fresh terms.
He added: "Of course, I will say what I would do, and I also believe that the people in charge value my opinion.
"Arjen is having fun playing football again, and for me he is now better than ever at 33. He is a great role model for young players who want to make a career, because many have talent, but do not know how to behave professionally.
"He is unique, he does everything for his job. Arjen does not need to play anymore to make money, he plays because he enjoys it."