P26C0: Engine Coolant Bypass Valve D Stuck/Open

GM uses P26C0 for the block rotary valve, which controls coolant flow through the engine block between two mechanical end stops; its tables label it as the engine block coolant valve stuck open. The ECM counts a failure when requested and measured position differ by at least 10° while the raw feedback sits at 50° or more, i.e. on the open side. The 2019 L3B table grades it Type A, one trip; the 2025 table 25OBDG03B lowers the gap to 5° and grades it Type B. The check is suspended during the valve's calibration run and needs at least 11 V. GM bulletin 21-NA-149 lists P26C0 among codes caused by a chafed engine harness on 2019–2021 trucks. An open block valve lets the block stay cooler than intended, slowing warm-up.

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

Check engine light
Longer warm-up, especially in cold weather
Higher fuel consumption on short trips

Most likely causes

Block valve jammed on the open side
Chafed engine harness disturbing drive or feedback (GM 21-NA-149)
Position feedback reporting a wrong angle

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 continuity of the wire between the component and the control module.
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 P26C0 means

P: Powertrain: engine and transmission
2: Generic code: same on every make
6: Control modules and outputs
C0: Specific fault in that system

Frequently asked questions

Can I keep driving with code P26C0?

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 P26C0?

The most common causes are: block valve jammed on the open side; chafed engine harness disturbing drive or feedback (GM 21-NA-149); position feedback reporting a wrong angle.

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.