You’re most likely misreading the meter display.

On the EOS 55, the scale shown in Av mode is the exposure compensation scale, not a “live under/overexposure” warning in the same way it works in Manual mode. In Manual mode, that same scale is repurposed to show how far your chosen aperture/shutter settings differ from the camera meter’s recommended exposure.

So the comparison you’re making isn’t apples-to-apples:

• In Av mode, the camera chooses the shutter speed based on its meter, and the scale indicates exposure compensation setting.
• In Manual mode, the scale shows whether your manually selected settings match the meter recommendation.

To check this properly:

1. Set Av mode with exposure compensation at 0.
2. Note the aperture and shutter speed the camera selects.
3. Switch to Manual mode.
4. Dial in the same aperture and shutter speed.

The indicator should then sit at 0 in Manual mode, confirming they match.

So the camera is probably working normally; it’s just using the same scale for two different purposes depending on the exposure mode.