Spark plug testers will only help you figure out issues related to spark plugs. Does that make them useless, or is there a point to having them around?

For most cars and trucks, gasoline and diesel engines follow the same four basic steps to make power: intake, to get a mixture of fuel and air into the cylinders; compression, to squeeze that mixture ...

Diesel engines require on heated and compressed air to run efficiently. Glow plugs and intake heaters help make that happen, but how do they work?