P2B5C: Engine Coolant Flow Control Valve Supply Circuit Low

With P2B5C the supply voltage reaching the driver of the coolant flow control valve motor is too low. GM monitors this on the 3.0-litre Duramax (25OBDG04B): if the motor driver sees less than 6 V while the powertrain relay delivers more than 11 V, the difference points at the feed path, and a Type B fault follows after 80 hits in 100 samples. Typical causes are a blown or loose fuse, a corroded relay contact or a damaged supply wire; 21-NA-149 includes P2B5C among the LM2 codes linked to a chafed harness. Without enough voltage the motor cannot position the valve, so engine temperature control falls back to a default. A weak battery alone rarely sets it, because the relay check would fail as well.

Medium severity. Can I keep driving?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.

Symptoms

MIL on after two drives
Valve does not move, temperature control in fallback mode

Most likely causes

Blown or loose fuse, corroded relay contact in the valve feed
Supply wire damaged or chafed in the engine harness
High resistance at a ground or splice of the feed circuit

What to check, in order

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

With the engine cold, check the coolant level; never do this when the system is hot
With the connector unplugged, check whether the signal wire is making contact with ground.
Check the actual temperature in live data
Make sure the fan comes on when it should
Check the thermostat and connector
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 P2B5C means

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

Frequently asked questions

Can I keep driving with code P2B5C?

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 P2B5C?

The most common causes are: blown or loose fuse, corroded relay contact in the valve feed; supply wire damaged or chafed in the engine harness; high resistance at a ground or splice of the feed circuit.

Related codes

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.