P2B5F: Engine Coolant Flow Control Valve Position Sensor Circuit High

P2B5F is set when the valve position signal sits above the top of its window. The 2025 Duramax calibration (3.0 L, 25OBDG04B) treats a SENT raw value over 98 % of 5 V as implausibly high and stores a two-trip Type B code after 80 of 100 readings, sampled every 6.25 ms while the SENT link itself is healthy. A signal shorted to the 5 V reference or to supply, a moisture path in the connector or an internal sensor fault are the usual explanations; GM's 21-NA-149 lists P2B5F with the LM2 harness-chafing codes as well. Coolant flow control switches to a fallback, which can show as slow warm-up or weak cabin heat. Low counterpart: P2B5E.

Symptoms

• MIL on after two drives
• Slow warm-up or weak cabin heat

Most likely causes

• Signal wire shorted to the 5 V reference or to battery voltage
• Moisture or corrosion bridging contacts in the valve connector
• Position sensor in the valve faulty

What to check, in order

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

1. With the engine cold, check the coolant level; never do this when the system is hot
2. Check the sensor's ground and make sure the signal wire is not receiving voltage from another circuit.
3. Check the actual temperature in live data
4. Make sure the fan comes on when it should
5. Check the thermostat and connector
6. Measure the sensor and replace it if its readings do not make sense

Good to know

• Repair difficulty: medium
• It affects emissions.

What each character of P2B5F means

• P: Powertrain: engine and transmission
• 2: Generic code: same on every make
• B: Subsystem depends on the code range
• 5F: Specific fault in that system

Frequently asked questions

Can I keep driving with code P2B5F?

Watch the temperature gauge. If it climbs or a warning appears, stop the car in a safe place and turn off the engine: overheating can cause serious damage.

What causes code P2B5F?

The most common causes are: signal wire shorted to the 5 V reference or to battery voltage; moisture or corrosion bridging contacts in the valve connector; position sensor in the valve faulty.

Related codes

• P2B5DEngine Coolant Flow Control Valve Position Sensor Circuit
• P2B5EEngine Coolant Flow Control Valve Position Sensor Circuit Low
• P2B60Engine Coolant Flow Control Valve Position Sensor Circuit Range/Performance

Sources

• gsi.ext.gm.com: 25OBDG04B
• static.nhtsa.gov: MC 10224856 0001
• law.cornell.edu: 13 CCR 1968.2

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.