P0118 Code: Engine Coolant Temperature Circuit High

What the P0118 code means

The coolant temperature sensor is a negative temperature coefficient thermistor: its resistance drops as the engine warms up. The computer applies a five volt reference and measures how much voltage is left. A cold engine means high resistance and high voltage, and a hot engine means low resistance and low voltage.

The P0118 code is set when that voltage stays above the upper limit for a sustained period, normally above 4.6 volts. That does not match any real temperature. It means the circuit is open and the computer is reading its own reference.

This reading is no minor input in engine management. It shapes mixture richness during warm-up, spark advance, idle speed, when the cooling fans come on and when the system enters closed loop.

That is why a sensor reporting an eternally cold engine leaves the vehicle enriching the mixture permanently, with fuel use through the roof, spark plugs fouling and the fans spinning nonstop. The engine runs, but badly and expensively.

It helps to understand why this code makes daily driving so much more expensive even when the engine seems fine. During warm-up, the computer deliberately enriches the mixture because a cold engine needs more fuel to run smoothly. That phase is designed to last a few minutes and end as soon as the coolant reaches temperature.

With a sensor that reports an eternally cold engine, that phase never ends. The vehicle runs in permanent warm-up mode, with excess fuel at all times, a raised idle and the fans active. The extra spending is easy to notice at the pump, and over time the unburned excess fuel damages the catalytic converter and dilutes the oil.

P0118 code symptoms

• Check engine light on.
• A clear increase in fuel use.
• Cooling fans running permanently, even with the engine cold.
• Hard starting when hot, from excess fuel.
• High idle that does not come down even after the engine warms up.
• Black smoke and a gas smell.
• Dash temperature needle at zero or erratic.
• Spark plugs blackened with soot.
• The warm-up phase never ends: high idle and a permanently rich mixture.
• Smell of unburned gas at the tailpipe even with the engine hot.

Likely causes of the P0118 code

1. Connector unplugged or poorly seated, very common after work on the engine.
2. Wire cut or with a loose connection in the signal or ground circuit.
3. Sensor failed internally with an open circuit.
4. Connector pins corroded, green or backed out.
5. Coolant leak onto the connector, which causes corrosion.
6. Low coolant level, which leaves the sensor with no liquid around it.
7. Shared sensor ground with high resistance.
8. Wrong replacement sensor with a different resistance curve.
9. Wiring damaged by engine heat.
10. Sensor broken during installation, by forcing it into a plastic housing or overtightening it.
11. Deteriorated sensor O-ring, which lets coolant leak onto the connector.

How to diagnose the P0118 code step by step

1. Read the temperature in live data with the engine cold. It should look like the ambient temperature. A reading of minus 40 degrees confirms the open circuit.
2. Jump the two connector pins with the key on. If the reading jumps right away to a very high temperature, the wiring is good and the sensor is the bad part.
3. Measure the sensor resistance cold and compare it with the reference table.
4. Check the five volt reference on the signal pin with the sensor unplugged.
5. Verify the ground continuity all the way to the computer.
6. Inspect the connector for green corrosion, moisture or traces of coolant.
7. Wiggle the harness with the engine running while watching the reading, to catch wires broken inside.
8. Check the coolant level, because a sensor with no liquid around it does not measure properly.
9. Check whether the idle comes down as the engine warms up. If it stays high after several minutes, the computer is still in warm-up mode.
10. Check the sensor O-ring. When it deteriorates, coolant wets the connector and corrodes it from the inside.

How to fix the P0118 code

• Plug the sensor back in if it was loose and clean the pins with contact cleaner.
• Replace the sensor once the jumper test confirms the wiring is good.
• Repair the damaged wire with solder and heat-shrink tubing, not with quick-connect crimps.
• Clean or replace the corroded connector and protect it with dielectric grease.
• Fix the coolant leak that caused the corrosion.
• Top off and bleed the cooling system if the level was low.
• Use a sensor with the exact part number for the vehicle, because resistance curves are not interchangeable.
• Replace the O-ring when you fit the new sensor and tighten moderately in plastic housings.
• Change the oil if the engine ran for a long time on a permanently rich mixture, because the fuel dilutes it.

After the repair, check with the engine cold that the displayed temperature matches the ambient, and with the engine hot that it reaches its working value.

Technical reference data

Approximate resistance values for a typical coolant sensor:

• Voltage that sets the code: above 4.6 volts sustained on a 5 volt reference.
• P0117 is the opposite code: low voltage, which is the same as an absurdly hot engine.
• Idle during warm-up: between 1,000 and 1,500 rpm, dropping to 600 to 850 rpm once at temperature.
• An idle that does not come down with the engine hot confirms the system is still in warm-up mode.

Brand-specific notes

Many engines have two temperature sensors: one for the computer and one for the dash gauge. A gauge that reads correctly does not rule out the P0118 code, because it may be reading the other sensor.

Before you order the part, unplug the sensor and plug it back in firmly. A poorly seated connector after an oil change or a repair explains a notable share of cases, and it costs nothing to check.

Frequently asked questions about P0118