P26AC: Engine Coolant Bypass Valve B Control Circuit/Open

Which valve carries the letter B depends on the maker. Ford names it in its gasoline OBD summaries for 2020–2023 and 2024–2025: valve B is the active transmission cooling valve, which sends engine coolant to the automatic transmission oil cooler, while valve A heats the transmission. P26AC is set when the smart driver in the PCM reports a fault on that output for 5 s; in 2024 Ford also files it under the extended code P26AC_13 for an open circuit. CARB lets makers combine open, low and high faults of an output into one code, which is why P26AC is often the only electrical code used. With the valve unpowered, transmission oil may run hotter or colder than intended.

Symptoms

• Check engine light
• Transmission fluid temperature higher than normal when towing
• Usually no change in normal driving

Most likely causes

• Broken wire or unplugged connector at valve B
• Open solenoid or motor winding in the valve
• Faulty output driver in the PCM

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 P26AC means

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

Frequently asked questions

Can I keep driving with code P26AC?

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 P26AC?

The most common causes are: broken wire or unplugged connector at valve B; open solenoid or motor winding in the valve; faulty output driver in the PCM.

Related codes

• P26ADEngine Coolant Bypass Valve B Control Circuit Low
• P26AEEngine Coolant Bypass Valve B Control Circuit High
• P26AFEngine Coolant Bypass Valve B Stuck/Open
• P26B0Engine Coolant Bypass Valve B Range/Performance
• P2681Engine Coolant Bypass Valve A Control Circuit/Open
• P26B7Engine Coolant Bypass Valve C Control Circuit/Open

Sources

• fordservicecontent.com: OBD Operations Summary Gasoline 2020 2023
• fordservicecontent.com: OBDSM2400OUTPUT 16Jul2024 WSM final V2 MCS7001
• 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.