P2B5B: Engine Coolant Flow Control Valve Control Circuit High

P2B5B covers a short to battery voltage on the motor leads of the coolant flow control valve. GM's diesel table 25OBDG04B evaluates it through the driver stage as well: a current above 9 A with a lead tied to supply counts as a failure, and the usual 80-of-100 rule with 12.5 ms sampling stores a Type B code. GM bulletin 21-NA-149 names P2B5B in its LM2 harness-chafing condition together with blown fuses and rough running, which points at wiring rubbing on powered circuits rather than at the valve. Because the driver shuts down to protect itself, the valve no longer reacts and warm-up control is lost. Ground short and open are P2B5A and P2B59.

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, possibly further codes from the same harness
Blown fuses or rough running if the harness is chafed
Weak heater output during warm-up

Most likely causes

Motor lead chafed against a powered circuit (GM LM2, 21-NA-149)
Corroded or wet connector bridging to supply
Driver stage in the engine control module faulty

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
Check the sensor's ground and make sure the signal wire is not receiving voltage from another circuit.
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 P2B5B means

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

Frequently asked questions

Can I keep driving with code P2B5B?

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 P2B5B?

The most common causes are: motor lead chafed against a powered circuit (GM LM2, 21-NA-149); corroded or wet connector bridging to supply; driver stage in the engine control module faulty.

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.