P0116

A trouble code records what a control module measured. It does not identify which part failed. Test before replacing anything.

Safe to drive

Usually driveable, but check the coolant level first. If the temperature gauge is erratic or high, stop driving.

Urgency

Diagnose soon

Will it pass emissions

No

If ignored

Fuel economy suffers and emissions rise, and the vehicle will likely fail an emissions test because affected monitors will not complete. Cooling fans may run constantly or not at all. If the cause is low coolant or a failing thermostat, the vehicle is on a path to overheating, which can warp a cylinder head or fail a head gasket — repairs an order of magnitude more expensive than the sensor.

Check engine light

Solid

Clearing the light

A completed drive cycle is normally required before the monitor re-runs and the light can clear.

• Cooling fan runs constantly or not at all
• Cabin heater weak or excessively hot
• Possible long warm-up time or hard cold start

Many vehicles show no symptom at all beyond the warning light. The absence of a symptom does not mean the fault is not real.

These are the causes this code can have. For the order to work through them in, see what to check first.

Ordering reflects how often each cause is responsible in general, not a probability for your vehicle. Confirm by testing.

In order. Each of these is cheaper or faster than what follows it, and each one can make the rest unnecessary.

1. Coolant level in the radiator and the overflow tank, on a cold engine only

   A sensor not fully immersed in coolant reads air instead of liquid and produces exactly this erratic, implausible behavior. Never open a hot cooling system — the contents are pressurized and will scald.

2. Coolant temperature against intake air temperature after an overnight soak

   Both should read within a few degrees of each other and of actual ambient. A meaningful gap on a fully cold engine identifies the drifted sensor immediately, at no cost.

3. How long the engine takes to reach normal operating temperature

   Most engines reach operating temperature within roughly ten to fifteen minutes of driving. Much longer than that, with weak cabin heat, points at a stuck-open thermostat rather than the sensor.

4. Whether the cooling fans run constantly with a cold engine and the A/C off

   Constantly running fans over-cool the engine and can prevent it from ever reaching the temperature the PCM expects, which sets this code without any sensor fault at all.

5. Any recent cooling system service

   Air pockets left after a coolant change are a classic cause. So is a disturbed or damaged sensor connector, or a thermostat installed with the wrong temperature rating.

1. Verify the code and read freeze frame

   Tools Scan tool with freeze frame

   Note engine run time, coolant temperature and intake air temperature at the moment the code set. A code that set at key-on with a large coolant-to-intake gap indicates sensor drift. One that set after extended run time with coolant still low indicates a warm-up problem.

   Expected

   Freeze frame data that clearly favors one of the two failure modes

2. Compare coolant and intake air temperature on a fully cold engine

   Tools Scan tool, thermometer

   After an overnight soak and before starting, read both values on the scan tool and compare against an accurate thermometer. This is the single most informative test for this code.

   Expected

   Coolant and intake air temperature within a few degrees of each other and of actual ambient

3. Graph coolant temperature through a full warm-up

   Tools Scan tool with graphing

   Start cold and log coolant temperature continuously while the engine idles and then drives. Watch for a smooth, continuous climb to a stable plateau. Note dropouts, flat spots, sudden jumps, or a plateau well below normal operating temperature.

   Expected

   A smooth rise to a stable operating plateau, with no steps or noise in the trace

4. Cross-check the sensor reading with an infrared thermometer

   Tools Infrared thermometer, scan tool

   With the engine warm, point an infrared thermometer at the thermostat housing or the metal near the sensor and compare against the scan tool value. Aim at bare metal, not at a painted or plastic surface, which reads differently.

   Expected

   Scan tool and infrared readings reasonably close. A large persistent offset confirms a drifted sensor

5. Evaluate the thermostat

   Tools Infrared thermometer

   From a cold start, feel or measure the upper radiator hose temperature. With a healthy thermostat the hose stays relatively cool while the engine warms, then heats suddenly when the thermostat opens. A hose that warms gradually from the very start indicates a thermostat stuck open, letting coolant circulate before the engine is ready.

   Expected

   A distinct, sudden change in hose temperature at the moment the thermostat opens

6. Measure sensor resistance against temperature

   Tools Digital multimeter, service information

   Unplug the sensor and measure resistance cold, then again at operating temperature. These are negative temperature coefficient thermistors, so resistance drops sharply as temperature rises. The correct resistance-to-temperature values are vehicle-specific — look them up rather than guessing.

   Expected

   Resistance matching the manufacturer's chart at both temperatures

7. Check circuit resistance and the sensor ground

   Tools Digital multimeter, wiring diagram

   Measure voltage drop on the sensor ground circuit with the sensor connected and the engine running. Added resistance in the ground path offsets the reading in a consistent direction and mimics a drifted sensor perfectly.

   Expected

   Very low voltage drop on the ground circuit, typically well under a tenth of a volt

8. Confirm the repair with a cold-start drive cycle

   Tools Scan tool

   After the repair, let the vehicle sit until fully cold, then clear codes and drive through a complete warm-up. This code's checks only run on a cold start, so verifying on a warm engine proves nothing.

   Expected

   Coolant temperature reaching and holding normal operating temperature, with the code not returning

The sensor is inexpensive and often easy to reach, but not always — on some engines it sits under an intake manifold and turns a small part into several hours of labor. Thermostat cost varies enormously depending on whether it is a standalone part or integrated into a housing assembly with the sensor built in. If the root cause is a coolant leak, that repair dominates the bill.

• Replacing the sensor without checking coolant level. A sensor sitting in an air pocket behaves erratically no matter how new it is.
• Opening a hot cooling system to check level. The system is pressurized above the boiling point and will spray scalding coolant.
• Confusing this with P0117 or P0118. Those are circuit faults, where the voltage is out of range entirely. P0116 means the voltage is fine and the value is wrong, which is a different diagnostic path.
• Replacing the wrong sensor. Many engines have a separate sender for the gauge and one for the PCM, and their locations vary by engine.
• Replacing the thermostat with one of the wrong temperature rating, which fixes nothing and can set the code again.
• Verifying the repair on a warm engine. The rationality check needs a full cold soak before it will run again.

• Thermostat
• Coolant Temperature Sensor

• P0115Engine Coolant Temperature Sensor Circuit MalfunctionSeverity level 3 of 5: Diagnose soon.moderate
• P0117Engine Coolant Temperature Sensor Low InputSeverity level 3 of 5: Diagnose soon.moderate

Definition

Generic code set (SAE J2012 / ISO 15031-6) via the OBDexopen database, dedicated to the public domain under CC0-1.0.

Diagnostic guidance

Written and reviewed in-house. See our editorial policy.

Review status

Reviewed

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