Sprinter Repair Guides
Glow Plug Testing 101 For Hard-Starting Sprinters
Updated 2026-05-18
Glow plugs are tested individually for resistance and by watching the glow plug module data during a cold start. On a Sprinter, one or two failed plugs cause slow starts and a rough idle until warm. We replace failed plugs and, when needed, the glow plug control module.
How they are tested
Each plug is measured for correct resistance, and we watch the module command current to each cylinder. A dead plug or a failed module shows up clearly in the data.
Why not just replace all of them
Sprinter glow plugs can seize in the head. We use the right heat and technique to remove them safely rather than snapping one off, which turns a small job into a big one.
The module matters
Sometimes the plugs are fine and the control module has failed. Testing first avoids replacing good parts.