Does 9V 400mA adapter work instead of a 9V 1000mA?

Yes, a 9V 400mA adapter can work in place of a 9V 1000mA adapter, but it may not be suitable for all applications. The device using the adapter may not receive enough current, leading to insufficient power or potential malfunction. If the device requires 1000mA, using a lower-rated adapter could cause it to underperform or become damaged. It's best to use an adapter that meets or exceeds the required current rating for optimal performance.