P26AA: Engine Coolant Bypass Valve A Position Sensor Maximum Stop Performance

P26AA is the upper-limit partner of P26A9: the controller cannot confirm or learn the maximum end stop of valve A, or the reported value at full travel lies outside the expected window. No manufacturer publishes criteria for this code. GM's Volt logic for P26A9 shows how such checks work: the valve is driven to an end, given a fixed time to arrive and relearned several times before a fault is stored. Subaru's bulletin 09-80-21 names the P26A family in general for sensor corrosion on its thermo control valve. CARB expects the OBD system to notice when such an output ignores its commands, wherever a test of its response is feasible. Likely causes are mechanical binding near full open, a damaged stop, or a sensor that saturates early.

Symptoms

• Check engine light
• Reduced cooling under high load
• Heater or engine temperature not as expected

Most likely causes

• Valve binding before reaching its maximum position
• Worn or broken mechanical end stop or gear
• Position sensor out of calibration near its upper end

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. Compare the reading with other engine data while varying the engine speed.
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 P26AA means

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

Frequently asked questions

Can I keep driving with code P26AA?

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 P26AA?

The most common causes are: valve binding before reaching its maximum position; worn or broken mechanical end stop or gear; position sensor out of calibration near its upper end.

Related codes

• P26A9Engine Coolant Bypass Valve A Position Sensor Stop/Minimum Stop Performance
• P26A5Engine Coolant Bypass Valve A Position Sensor Circuit Range/Performance
• P26A3Engine Coolant Bypass Valve A Range/Performance
• P26ABEngine Coolant Bypass Valve A Stuck/Open

Sources

• law.cornell.edu: 13 CCR 1968.2
• gsi.ext.gm.com: 16OBDG01 Hybrid Diagnostics
• static.nhtsa.gov: MC 10208664 0001

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.