Hi All,
I have the exact problem on my '96 F-150 with the 302cid. I replaced the valve and still have the problem. Is the P0403 code for an electrical malfunction or can it also detect system leaks? (The rear tank doesn't "hiss" when refilling like the front.) The light comes on immediately on the second start after clearing the MIL code so I wouldn't think it performed any kind of leak down test. A friend suggested that it may be the PCM iteself?!?!