P26AB: Engine Coolant Bypass Valve A Stuck/Open

P26AB is stored when valve A stays open although it was told to close. Ford's hybrid summaries (2014–2019) define the valve as the switch between a combined loop, with engine coolant through the heater core, and an isolated loop heated by a PTC heater; if the second coolant sensor reads more than 5 °F above the engine sensor within 60 s of a circuit fault, the valve is judged stuck in the combined loop, and P2682 must set first. On 2016–2017 smart fortwo cars a corroding vacuum change-over valve keeps the heater shut-off valve open, so Mercedes-Benz extended its warranty to 10 years. Ram 1500 trucks of 2013–2015 (3.6, 4.7 and 5.7 L) and 3.0 L diesels received PCM flashes that list P26AB.

Symptoms

• Check engine light
• Engine takes long to warm up, higher fuel use when cold
• Heater works with engine off only poorly (hybrids)

Most likely causes

• Valve jammed open by deposits, wear or a broken drive part
• Corroded vacuum change-over valve (smart fortwo 453)
• Engine control software misjudging valve state (Ram flashes)
• Drive circuit fault leaving the valve in its open default

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 P26AB means

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

Frequently asked questions

Can I keep driving with code P26AB?

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 P26AB?

The most common causes are: valve jammed open by deposits, wear or a broken drive part; corroded vacuum change-over valve (smart fortwo 453); engine control software misjudging valve state (Ram flashes).

Related codes

• P26B1Engine Coolant Bypass Valve A Stuck Closed
• P26AFEngine Coolant Bypass Valve B Stuck/Open
• P2682Engine Coolant Bypass Valve A Control Circuit Low
• P2681Engine Coolant Bypass Valve A Control Circuit/Open
• P26A3Engine Coolant Bypass Valve A Range/Performance
• P26A9Engine Coolant Bypass Valve A Position Sensor Stop/Minimum Stop Performance

Sources

• fordservicecontent.com: OBDSM1901 HEV
• static.nhtsa.gov: MC 11031639 0001
• static.nhtsa.gov: MC 10121873 9999
• static.nhtsa.gov: MC 10148688 9999

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.