P259A: Turbocharger Boost Control Position Sensor B Performance - Stuck Low

P259A applies to engines with a second boost control actuator, for instance a twin-turbo layout or the second stage of a two-stage system, whose position is read by sensor B. The code is stored when that actuator's reported position stays at the low end and stops tracking the command even though the signal is electrically sound. No manufacturer documents P259A publicly. Its logic mirrors P2598 on the A side, where Ford compares requested with actual position and reacts to more than 15 % for 25 s, and GM counts samples with vanes more open than requested. CARB 13 CCR 1968.2 requires the boost control system to report faults that stop it from reaching its target. P259B is the high-side counterpart; circuit faults of sensor B fall under P2586-P2590. Reduced power is the usual result.

Symptoms

• Engine warning lamp on
• Weak acceleration, notably in the range where charger B works

Most likely causes

• Sticking vanes, flap or wastegate on charger B
• Weak vacuum or electrical supply to actuator B
• Worn actuator B or misadjusted linkage

What to check, in order

Start with the simplest. You can tick off each step.

1. Check the hoses, clamps, and intercooler
2. With the connector unplugged, check whether the signal wire is making contact with ground.
3. Compare commanded and actual pressure in live data
4. Check the actuator's movement
5. Check the vacuum hoses and the solenoid valve
6. Check the oil and the turbo shaft play before replacing it

Good to know

• Repair difficulty: hard
• It can trigger limp mode.
• It affects emissions.

What each character of P259A means

• P: Powertrain: engine and transmission
• 2: Generic code: same on every make
• 5: Speed and idle
• 9A: Specific fault in that system

Frequently asked questions

Can I keep driving with code P259A?

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 P259A?

The most common causes are: sticking vanes, flap or wastegate on charger B; weak vacuum or electrical supply to actuator B; worn actuator B or misadjusted linkage.

Related codes

• P259BTurbocharger Boost Control Position Sensor B Performance - Stuck High
• P2598Turbocharger Boost Control Position Sensor A Performance - Stuck Low
• P2587Turbocharger Boost Control Position Sensor B Circuit Range/Performance
• P2586Turbocharger Boost Control Position Sensor B Circuit

Sources

• law.cornell.edu: 13 CCR 1968.2
• fordservicecontent.com: obd summary diesel 2023
• gsi.ext.gm.com: 18OBDG04 Engine Diagnostics

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.