What could lead to an engine running below its normal temperature?

An engine running below its normal temperature can often be attributed to a thermostat that is stuck open. The thermostat plays a crucial role in regulating the engine’s operating temperature by controlling the flow of coolant through the engine and radiator. When the thermostat is stuck open, coolant continually flows through the engine, preventing it from reaching the optimal operating temperature. This can lead to various performance issues, such as reduced fuel efficiency and increased emissions, as the engine is unable to operate effectively in its designated temperature range.

In the case of other potential options, a low oil pressure issue does not directly impact the engine's coolant temperature, nor does an obstructed air filter, which primarily affects air intake and engine performance rather than cooling. A thermostat stuck closed would instead cause the engine to overheat, as coolant would not circulate properly. Thus, the failure of the thermostat to close properly is the clear cause of the engine running at a temperature that is below normal levels.