P259C reports that closed-loop boost control of turbocharger A was not reached within the expected time after its enabling conditions were met. Instead of trimming the actuator from boost or position feedback, the controller stays in open-loop or default mode. CARB 13 CCR 1968.2 requires diesel OBD systems with feedback boost control to report it when control fails to start inside a time window set by the manufacturer, and P259C is the code for that case. No manufacturer publishes thresholds for P259C. Ford's 2023 diesel summary shows why it matters: its underboost check for turbocharger A runs only while boost feedback control is active. Typical root causes are inputs the loop needs, such as a faulty boost or position sensor, or a stored actuator fault. P259D is the turbocharger B equivalent.
Medium severity. Can I keep driving?Drive without demanding power and go to a repair shop soon. If you see a lot of blue smoke or hear a metallic noise, shut off the engine.
Symptoms
- Engine warning lamp on
- Sluggish or uneven boost build-up
Most likely causes
- Boost pressure or actuator position sensor fault that blocks closed-loop control
- Actuator A fault, often with its own stored code
- Charge air leak preventing boost from reaching the range where control starts
- Engine control software issue
What to check, in order
Start with the simplest. You can tick off each step.
Good to know
- Repair difficulty: medium
- It can trigger limp mode.
- It affects emissions.
What each character of P259C means
- P: Powertrain: engine and transmission
- 2: Generic code: same on every make
- 5: Speed and idle
- 9C: Specific fault in that system
Frequently asked questions
Can I keep driving with code P259C?
Drive without demanding power and go to a repair shop soon. If you see a lot of blue smoke or hear a metallic noise, shut off the engine.
What causes code P259C?
The most common causes are: boost pressure or actuator position sensor fault that blocks closed-loop control; actuator A fault, often with its own stored code; charge air leak preventing boost from reaching the range where control starts.
Related codes
- P259DExcessive Time To Enter Closed Loop Turbocharger B Boost Control
- P2263Turbocharger/Supercharger A Boost System Performance
- P0046Turbocharger/Supercharger Boost Control A Circuit Range/Performance
- P0236Turbocharger/Supercharger Boost Sensor A Circuit Range/Performance
Sources
Values and procedures vary by manufacturer. If it affects brakes, steering, fuel or high voltage, see a professional. Updated: 2026-11-04. How we build these pages.