Problem with ETTL on 430EX

When I setup my 30D in P or totally automatic mode , the ETTL on 430EX works fine. When I switched to Av , Tv or M mode and used the same Aperture, ISO, and Shutter Speed when it was in P mode , the 430EX still in ETTL mode, the picture came out way over-exposed (same subject, under same lighting condition). According to the Flash manual , ETTL would work with any mode of camera setting. Obviously, I did something wrong.
Any pointer will be greatly appreciated.

Did this just happen once, or is it a behavior you can duplicate whenever you try?

I spent about 5 hours and took about 50 shots trying to correct the problem with no success. All came out too white (over-exposed)

This may not be what you are seeing because you state that you are also seeing the problem going from P-mode to Av, Tv or M ... but check anyway because maybe you meant "green-box mode" (which is a Basic Zone mode) and not "P-mode"

In the Basic Zone modes flash exposure compensation isn't used. So, if you have a bunch of "+" FEC dialed in you could see a big difference in flash output going from the Basic Zone modes to Av, Tv or M. FEC is also used in P-mode though.

Example: If you accidentally left FEC at +1 1/2 stops it won't be used in the Basic Zone modes and your images will be exposed more or less correctly. Once you switch to a Creative Zone mode the FEC you left dialed in will now be applied and there will be 1 1/2 stop brighter exposure.

I borrowed a 430Ex from a friend and it worked nicely with all camera setup modes. The problems was my 430 . I'll have Canon looks into it.
Thank you very much Bob and Curtis . I have learned a great deal from this forum .
Neil B.

Not so fast!

1. Check that the Wireless selector at the bottom of the flash is set to "Off"
2. Ensure that "ETTL" is showing on the display panel. If it isn't, press the mode button until it does.
3. The 430EX allows you to specify FEC (+/- icon) on the flash the same way you can set it on the camera, however the flash setting overrides the camera setting. Make sure it's set to zero (page 14 of you 430EX manual).
4. Try a brand new set of AA alkaline batteries.
5. If all fails press the "Clear" button to revert everything except for custom functions back to the default, then try again.

If none of the above works it sounds like either the contacts on the flash or dirty or it's not seating in the hotshoe properly.