P1126

What it means

On Nissan models P1126 means the engine control unit has judged that the thermostat is not working properly. It usually sets when the engine coolant temperature does not rise as expected after a cold start, or does not reach and hold normal operating temperature, which points to a thermostat stuck open or a related cooling fault. The exact monitoring conditions vary by model and engine.

Symptoms

• Engine warning light on
• Engine slow to warm up or never reaching normal temperature
• Temperature gauge reading low or staying lower than usual
• Weak cabin heater output in cold weather
• Poorer fuel economy than normal
• Rough idle or slightly richer running while cold

Possible causes

• Thermostat stuck open or opening too earlylikely
• Faulty or inaccurate engine coolant temperature sensorlikely
• Wiring or connector fault on the coolant temperature sensor circuitpossible
• Low coolant level or air trapped in the cooling systempossible
• Cooling fan running constantly or a fan control fault overcooling the enginepossible
• Fault within the engine control unit or an outdated calibration, where a manufacturer update is known for the modelrare

How to diagnose

1. Read the stored codes and freeze frame data, noting coolant temperature, engine load and vehicle speed when the code set, and check for other temperature-related codes.
2. Check the coolant level and condition with the engine cold, and look for leaks or signs of air in the system.
3. Inspect the coolant temperature sensor wiring and connector for damage, corrosion or poor contact.
4. With a scan tool, watch live coolant temperature from cold and check it rises smoothly to normal operating temperature.
5. Compare the scan tool reading with the real temperature measured at the thermostat housing or hoses using a suitable thermometer, to test sensor accuracy.
6. Check the cooling fan does not run continuously when the engine is cold or at idle, and that fan control works as the model specifies.
7. Feel the radiator hoses as the engine warms up: the upper hose heating very early suggests the thermostat is stuck open.
8. Check for any manufacturer service information or calibration updates relevant to the model and engine.

How to fix

• Replace the thermostat, along with its seal or housing if required, and refill and bleed the cooling system to the correct specification.
• Replace the engine coolant temperature sensor if its readings disagree with actual temperature.
• Repair or replace damaged wiring or connectors in the sensor circuit.
• Top up or replace the coolant with the correct type, fixing any leak found and bleeding out air.
• Repair the fan control fault causing overcooling, such as a faulty relay, switch or control circuit.
• Apply an official calibration update only if service information lists it as a fix for this fault on the specific model.

Can I keep driving?

This code for other makes

• P1126LexusMagnetic Clutch Circuit Malfunction
• P1126MazdaThrottle Position Narrow Range Sensor Circuit
• P1126OtherThrottle Position (Narrow Range) Sensor Circuit Malfunction
• P1126ToyotaMagnetic Clutch Circuit
• P1126VolkswagenLong Term Fuel Trim Add Air Bank 2 System Too Lean

Nearby codes

• P1125NissanTandem Throttle Position Sensor Circuit Fault
• P1130NissanSwirl Control Valve Control Solenoid Valve