(1) On certain vehicles, the bolt that secures the head idler gear, which drives the engine camshaft, is not properly tightened, which may cause the gear to be unstable. If this problem is left unattended, the bolt may loosen and the gear may not align properly with other gears, making abnormal noise, and at worst, the bolt could break and interlocking between the gears would be lost, in which case the engine would stop and would not restart. (2) The fan shaft gear, which drives the cooling fan for the engine, does not have sufficient strength, and it may resonate at low revolutions and wear. If this problem is left unattended, the gear may be damaged, and in the worst case, the teeth of the gear could break, in which case the engine would stop and would not restart.