P259F: High boost-control limit means actual boost exceeds the target

P259F is turbocharger/supercharger A boost control position at high limit. Ford’s diesel example detects actual boost substantially above desired boost. Its published comparison is desired pressure minus actual pressure, so the malfunction appears as a negative error.

Read the sign and the engine variant together

The 6.7L criterion is an error below −120 kPa, with a typical ten-second duration. The 3.0L example uses below −80 kPa and eleven seconds. Coolant-temperature, manifold-pressure, and air-mass inputs must be satisfactory.

The listed speed and torque requirements also vary by engine. A diagnostic comparison needs the matching target and operating window; an actual-pressure value by itself cannot establish the specified negative error.

This is separate from a rapid overpressure threshold

The same page describes P0234 using exhaust-pressure limits and a different duration. P259F’s boost target/actual control-limit check should not be merged with that separate threshold monitor simply because both concern overboost.

Useful verification preserves the pressure relationship over time and confirms the engine-specific monitor. The high-limit title does not, by itself, prove that an actuator has physically reached a stop or that the turbo mechanism is stuck. Ford’s summary supplies a sustained control-error criterion, leaving mechanical, electrical, and sensor isolation to the correct vehicle procedure before a repair decision.

Source and applicability

This explanation follows Ford’s 2023–2024 diesel OBD summary. Match the vehicle’s engine and calibration before applying these monitor values; a generic code alone does not establish fitment. Ford PDF, page 45.