The 2021 Lincoln Navigator's P0126 code indicates the Engine Coolant Temperature is below the thermostat's regulating temperature range after the vehicle has operated long enough to warm up. This code is closely related to P0125 but specifically indicates temperature below expected regulation range.
What P0126 Means
P0126 indicates the coolant temperature sensor shows temperatures below what the thermostat should maintain. If your thermostat is rated at 195°F and temperatures stay at 170°F or lower during normal operation, P0126 sets. The engine isn't reaching its efficient operating temperature.
Navigator Cooling System
The 2021 Navigator's 3.5L twin-turbo EcoBoost V6 produces significant heat and requires precise temperature management. The turbos and intercooler system add complexity, but the basic cooling circuit still relies on a thermostat to maintain operating temperature.
Common Causes of P0126
A stuck-open thermostat allows coolant to flow continuously through the radiator. The engine never builds up to operating temperature because heat is constantly being removed. This is especially noticeable in cold weather.
An incorrect thermostat (wrong temperature rating) may open too early. Using a thermostat rated lower than OEM specification prevents the engine from reaching designed operating temperature.
A coolant temperature sensor reading low (but not out-of-range enough to trigger sensor codes) can cause P0126. The ECU thinks the temperature is lower than it actually is.
Diagnostic Approach
Use an infrared thermometer to measure actual engine temperature and compare to scan tool readings. If actual temperature is correct but sensor reads low, replace the sensor. If actual temperature is indeed low, test or replace the thermostat.