Meaning of bad earth connection in cars?

Answer 1

A bad "earth" connection means a poor ground connection to the earth which is not technically how it works in cars. This would be the appropriate way to say something for homes or systems connected physically to the ground.

A bad ground connection in cars simply means that there is one or more grounding wires that have a loose, corroded, or otherwise faulty connection.

This can adversely affect vehicle performance depending on what circuits use the ground. When there is a poor ground connection there is more resistance in the circuit that needs it (not good). There can also be an intermittent connection which will also cause issues.

Ground connections may come loose from age if they are bolted in place or become rusted.

A shop manual for the specific vehicle should have detailed information on where every single ground connection is on the car.