P0118 Code: Engine Coolant Temperature Sensor 1 Circuit High
The coolant sensor signal sits near 5 volts, so the PCM thinks the engine is frozen solid at -40°F.
At a glance
Severity: Low
- Meaning
- Engine Coolant Temperature Sensor 1 Circuit High
- System
- Fuel & Air Metering · Powertrain
- Safe to drive?
- With caution
- Typical repair cost
- $0–$350
- DIY difficulty
- Easy
Inside the PCM, the coolant temperature signal wire is pulled up to 5 volts through a fixed resistor, and the sensor's thermistor pulls it back down as the engine heats up. When nothing is connected, or the path to ground is broken, the voltage floats up to almost the full 5 volts. P0118 sets once the signal stays above roughly 4.8 to 4.9 volts, which corresponds to about -40°F, the coldest value most scan tools can display. The usual culprit is an open circuit: a connector left unplugged or not fully latched after cooling-system work, a broken wire, a pushed-out terminal or a sensor whose element has cracked open. Since the PCM now fuels for an arctic cold start, the engine tends to run rich until the PCM swaps in a substitute temperature.
Can I keep driving? Short trips are fine, but don't drive for weeks with it: the extra fuel washes oil off the cylinder walls, fouls spark plugs and can overheat the catalytic converter. Keep an eye on real engine temperature, because the gauge may no longer tell you.
How to read P0118
- P Powertrain
- 0 Generic (SAE)
- 1 Fuel & air metering
- 18 Specific fault
Symptoms
- Check engine light on, often immediately after starting
- Live data shows coolant temperature stuck at -40°F regardless of how long the engine has run
- Strong fuel smell, sooty tailpipe or black smoke on a warm engine
- High fast idle that never settles, or hard starting when the engine is already hot
- Temperature gauge stays on cold, or the needle drops to the bottom
- Radiator fans run nonstop as a safety fallback on many vehicles
Possible causes
Ranked by how often each cause is behind this code (editorial estimate).
- Sensor connector unplugged, half-latched or knocked loose, very often after a thermostat, water pump or intake job
- Sensor element open internally or cracked housing (reads infinite ohms)
- Corroded or spread terminal in the connector making intermittent contact
- Broken signal or ground wire in the harness, typically at a flex point near the engine
- Open in the shared sensor ground, which usually sets intake air temperature codes at the same time
- Signal wire shorted to a voltage source, or a PCM input fault
Diagnosis step by step
Read the temperature and check the obvious
Key on, look at coolant temperature in live data. A flat -40°F on a warm engine confirms an open circuit rather than a drifting sensor. Then look at the sensor itself: many P0118 cases turn out to be a connector that was never clicked home after other work.
Tools: OBD-II scanner, Flashlight
Bridge the connector with a jumper
Unplug the sensor and connect a fused jumper wire between the two terminals of the harness side. The reading should jump to its maximum hot value, often 280°F or more. If it does, wiring and PCM are good and the sensor is open. Remove the jumper right away so the fans don't keep running.
Tools: OBD-II scanner, Pliers
Measure the sensor across its pins
An open sensor shows OL (infinite) on the ohm scale. A working one reads roughly 2,000 to 3,000 ohms at 68°F and drops to around 200 to 300 ohms near 195°F. Flex the pins lightly while measuring; a reading that cuts in and out means a cracked element.
Tools: Multimeter
Check reference and ground at the harness
With the sensor unplugged and the key on, the signal terminal should show about 5 volts to battery negative, and the ground terminal should show near 0 volts with good continuity to the engine. Missing ground, especially if an intake air temperature code is also stored, points to the shared sensor ground wire or its splice.
Tools: Multimeter
Inspect the terminals up close
Look into the connector with a light. Green corrosion, a terminal that has backed out of its cavity, or a female contact that has spread open will all give an open circuit that comes and goes with engine vibration. Drag a spare male pin through each socket; it should feel slightly tight.
Tools: Flashlight, Screwdriver
Repair, clear and verify
Replace the sensor or connector as needed, top off the coolant you lost and bleed the system. Clear the code and watch a cold start: coolant should read the same as intake air temperature, then climb smoothly. The fans should stop running and the rich smell should be gone within a drive.
Tools: OBD-II scanner, Gloves
Where the parts are
- 1Ignition coil
- 2Spark plug
- 3Fuel injector
- 4Camshaft sensor
- 5Crankshaft sensor
- 6Engine computer (ECM)
- 7Intake manifold
Repair options & costs
| Repair | DIY? | Estimated cost |
|---|---|---|
| Reconnect or properly latch the sensor connector | Yes | $0–$50 |
| Replace coolant temperature sensor | Yes | $60–$200 |
| Replace connector pigtail with new terminals | Yes | $40–$160 |
| Repair broken signal or sensor ground wire | No | $100–$300 |
| Replace fouled spark plugs after prolonged rich running | Yes | $100–$350 |
Rough US estimates for parts and labor; shop rates vary by region. Not a quote.
Notes by brand
Volkswagen
Many older VW four-cylinder engines (Golf IV and Jetta generation) use a four-pin dual coolant sensor that feeds both the ECU and the gauge. It is a known wear item; a flickering gauge alongside P0118 or erratic readings usually means the sensor or its connector is on its way out.
Emissions / smog test impact
Will fail
A lit check engine light fails the OBD-II portion of a state emissions test. After the fix, drive several complete warm-up cycles so the readiness monitors can run, since most of them wait for a valid coolant temperature.
Frequently asked questions
Why does an unplugged coolant sensor read -40°F instead of zero?
The PCM feeds the sensor through a pull-up resistor, so with no sensor connected the signal rises to about 5 volts. The PCM's lookup table converts that voltage to the coldest possible temperature, which is -40°F on most vehicles.
Can P0118 damage my catalytic converter?
It can if you ignore it. The PCM adds cold-start fuel, and unburned fuel ends up burning inside the converter. A few days won't hurt, but weeks of rich running can overheat the catalyst and foul the plugs.
Why is my car hard to start only when it's already warm?
A hot engine needs very little extra fuel to start. Because the PCM thinks it's -40°F outside, it sprays a full cold-start charge into a hot engine and floods it. Holding the gas pedal to the floor while cranking often helps, because many PCMs cut fuel in that clear-flood mode.
Could this be the gauge sender instead of the PCM sensor?
Some engines have two separate sensors, one for the PCM and one for the dashboard gauge. P0118 always refers to the sensor the PCM reads, so check the service information for which one it is before buying a part.