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If your Ford F-150 is displaying fault code P1538 (or P1537), it indicates that the intake manifold runner control (IMRC) in bank 2 (and bank 1 for '37) is stuck open. The IMRC is a system that uses two intake runners to provide each cylinder with sufficient air. These runners are usually closed but open when the driver requests more power from the engine. When one of these codes appears, it means the system is not disengaging, resulting in constant extra power. While this might seem appealing, it can lead to decreased fuel efficiency and potential engine over-revving at idle. Typically, this issue can be resolved by checking the vacuum hoses connected to the IMRC.
The PCM noticed that the IMRC monitor signal at a closed throttle was lower than it should be when the key was turned on or the engine was running.