Since diesel engines have no spark plugs or ignition systems, they depend on glow plugs to get the engine going during low temperatures. One or more failed glow plugs will make your engine difficult to start. One failed plug will eventually cause the others to fail; when one plug fails, you should replace them all.
Disconnect the vehicle's battery to prevent any chance of electrical shock. Remove the valve cover if required on your vehicle, and disconnect the electrical connector.
Locate the glow plug in the cylinder head. Using a ratchet wrench with a deep socket placed over the top of the glow plug, unscrew the glow plug from the cylinder head counterclockwise, in the same way you would remove a spark plug.
Clean out the opening for the glow plug by screwing a glow plug reamer all the way into the opening and then all the way out again.
Seat the new glow plug in the opening, and tighten with your fingers until snug. Tighten it down firmly using the socket and ratchet wrench again.
Attach the connector back onto the glow plug terminal. Repeat this process for all the glow plugs; put the valve cover back on if necessary, and you've completed your job.