P26A8: Engine Coolant Bypass Valve A Position Sensor Circuit Intermittent/Erratic

P26A8 targets a feedback signal that jumps or drops out briefly instead of staying wrong. No manufacturer publishes a definition for this exact code. The closest documented logic is in GM's 2011 Volt table, where under P26A9 the controller flags a reading that changes by more than 6 % between samples ten steps apart (8 of 10 or 80 of 100 samples) although no movement was commanded. Subaru's bulletin 09-80-21 lists P26A-series codes as consequences of moisture reaching the sensor of the thermo control valve on 2019–2021 models. CARB asks for rationality checks on inputs that affect emissions. In practice a loose terminal, a cracked solder joint or a worn sensor track causes it, often only with vibration or heat.

Symptoms

• Check engine light that may come and go
• Erratic heater output or engine temperature
• Code returns after clearing, often on rough roads

Most likely causes

• Loose or spread terminal in the valve connector
• Worn or contaminated sensor track giving dropouts
• Harness rubbing on engine parts and shorting intermittently

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. Gently move the wiring with the engine running and watch whether the reading changes.
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 P26A8 means

• P: Powertrain: engine and transmission
• 2: Generic code: same on every make
• 6: Control modules and outputs
• A8: Specific fault in that system

Frequently asked questions

Can I keep driving with code P26A8?

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 P26A8?

The most common causes are: loose or spread terminal in the valve connector; worn or contaminated sensor track giving dropouts; harness rubbing on engine parts and shorting intermittently.

Related codes

• P26A4Engine Coolant Bypass Valve A Position Sensor Circuit
• P26A5Engine Coolant Bypass Valve A Position Sensor Circuit Range/Performance
• P26A6Engine Coolant Bypass Valve A Position Sensor Circuit Low
• P26A7Engine Coolant Bypass Valve A Position Sensor Circuit High

Sources

• law.cornell.edu: 13 CCR 1968.2
• gsi.ext.gm.com: 11OBDG01 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.