P26AF: Engine Coolant Bypass Valve B Stuck/Open

P26AF is the functional fault for valve B remaining open against its command. No manufacturer documents this code publicly; Ford, for instance, monitors its valve B (transmission cooling) only electrically through P26AC/P26AE. How an open-stuck check can work is shown by Ford's hybrid logic for P26AB on valve A, which compares two coolant temperatures for 60 s after the valve should have changed state. CARB requires output components to be checked for proper functional response where feasible. A valve stuck open on an auxiliary coolant path wastes heat during warm-up, so the engine or transmission takes longer to reach temperature; causes include deposits, a broken return spring or gear, or a seized actuator.

Symptoms

• Check engine light
• Longer warm-up, higher fuel use on short trips
• Lower cabin heat in cold weather

Most likely causes

• Valve held open by deposits or debris
• Seized actuator or broken drive gear
• Coolant temperature sensor bias misleading the check

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 continuity of the wire between the component and the control module.
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 P26AF means

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

Frequently asked questions

Can I keep driving with code P26AF?

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 P26AF?

The most common causes are: valve held open by deposits or debris; seized actuator or broken drive gear; coolant temperature sensor bias misleading the check.

Related codes

• P26B2Engine Coolant Bypass Valve B Stuck Closed
• P26ABEngine Coolant Bypass Valve A Stuck/Open
• P26B0Engine Coolant Bypass Valve B Range/Performance
• P26ACEngine Coolant Bypass Valve B Control Circuit/Open

Sources

• law.cornell.edu: 13 CCR 1968.2
• fordservicecontent.com: OBDSM1901 HEV
• fordservicecontent.com: OBDSM2400OUTPUT 16Jul2024 WSM final V2 MCS7001

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.