My d90 flashes an err sign somtimes when i shoot at manual mode.. but when i change my apperture and shutter speed setting, it will not appear anymore. Even at auto mode sometimes it flashes an err. Does the d90 flashes an error when you made wrong setting in your apperture and shutter speed?

On page 256 of the D-90 user manual it says that an ERR message indicates a camera malfunction and that the malfunction can be cleared by releasing the shutter. I found that the only way I could release the shutter is to turn the camera off and back on. On rare occasions, I have needed to remove and reinstall the battery.

The manual says that if this problem persists or occurs frequently you should contact a Nikon authorized repair center for instructions. I recommend that you contact Nikon tech support at www.nikon.com before you send your camera for repair just in case they may have a solution that won't cost you any money for repairs. If the camera is still under warranty, I would definitely contact Nikon tech support to see what they have to say.

Try shooting in manual mode with lens set at maximum (wide open) aperture. Then try at a smaller f-stop, say 5.6 or 8. If the error message appears when your lens is set to the smaller aperture, you could have a lens problem. Try another lens using the same test. If a known good lens gives an error message, then the problem is in the body. Also try zooming to different focal lengths and see if your 18-105mm gives the error at particular focal lengths. An error here would indicate a lens problem. In any case, your lens or body would have to be repaired by a professional as these repairs require major disassembly and re-calibration.