What could cause poor fuel economy if the IAT is reading -10° with ECT at 212 degrees F?

The correct choice points to the impact of a faulty Intake Air Temperature (IAT) sensor on fuel economy. When the IAT sensor reads an unusually low temperature, such as -10°F, while the Engine Coolant Temperature (ECT) is at a normal operating temperature of 212°F, it indicates a discrepancy in the perceived air temperature entering the engine.

Modern engine control modules (ECMs) use data from the IAT sensor to optimize fuel delivery and timing adjustments based on the density and temperature of the incoming air. An inaccurate low reading from the IAT sensor can lead the ECM to assume that the air is denser and cooler than it actually is, causing it to add more fuel to the air-fuel mixture to maintain an optimal combustion environment. This overly rich fuel mixture can result in poor fuel economy as the engine runs inefficiently, consuming more fuel than necessary.

In this context, a faulty ECT sensor would not directly contribute to poor fuel economy since both temperatures being reported are significant in their own right. A cracked MAP sensor vacuum line affects manifold pressure readings but does not directly imply an erroneous IAT signal. Similarly, a stuck closed EGR valve leads to increased nitrogen oxides and could affect emissions but may not directly correlate