In automatic PCH processing mode the compiler indicates that a PCH file is obsolete, and deletes it, under the following circumstances:
If the PCH file is based on at least one out-of-date header file but is otherwise applicable for the current compilation.
If the PCH file has the same base name as the source file being compiled, for example,
xxx.c, but is not applicable for the current compilation, for example, because you have used different command-line options.
These describe some common cases. You must delete other PCH files as required.