What do the numbers and letters in an NGK glow plug code mean?

Each NGK glow plug part number is structured to describe its design:
- **First letter** (Y / YD) = sheath type (construction and earth method)
- **First number** = thread size (e.g. 2 = 12 mm, 4 = 14 mm)
- **Next digit** = system voltage (0–4 = 12 V, 5–9 = 24 V)
- **Letters** (e.g. V, T, M, QGS) = heating characteristic
- **Suffixes** (S, 1, 2, etc.) = material type or revision

This ensures you choose the exact plug for your engine, not just “one that looks similar".