A simple question, on its face, but more interesting after a bit of research. My first instinct was to simply say "in good condition" is the better choice -- it is what we say here in the U.S.-- but I decided to google each one, just out of curiosity. (This method, i.e. "googling", does not always yield valid results, but in this case I think it did.) "in good condition" brought up over 300 million web hits. "in a good condition" brought up about 74 million web hits. (Switch the search from Web to News and the difference is even greater.) However, scan through 3 pages of the "in a good condition" hits and we find they are almost all from sources outside the U.S. So as with any answer you get here on italki, check the source. In the U.S. we say "in good condition", regardless of the circumstance. ~"That used car is in good condition for its age." ~"She is still in the hospital, but she is in good condition." Outside the U.S., "in a good condition" may be equally common.

In my opinion, without having any given sentence/s, both "in good condition" and "in a good condition" are right and can be used either way. As the word "condition" is singular / non- countable noun, inserting or omitting the article "a" will not change the meaning and effect of the entire clause. :)

The former means you are in good circumstance or atmosphere情况良好.The latter is seldomly used.in good conditionsmeans you are in good health状态良好