P0118

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

Safe to drive

Driveable short term, but expect poor fuel economy and possible hot-restart trouble. Repair promptly — where the engine is running rich, the catalytic converter is what pays for the delay.

Urgency

Diagnose soon

Will it pass emissions

No

If ignored

Where the PCM is applying enrichment the engine does not need, fuel consumption climbs, spark plugs foul and fuel washes oil off the cylinder walls; over weeks to months, unburned fuel reaching the catalytic converter can overheat and destroy it, turning a modest sensor repair into a converter replacement. The engine may also become hard to restart when hot. Where a substitute coolant value is in use, the immediate driveability cost is smaller, but the circuit fault is still there, the light stays on, and affected monitors may not complete — so an emissions test will not pass. Live fuel trims tell you which of the two situations you are in.

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. Whether the sensor connector is fully seated and locked

   A connector knocked loose during other work, or one that never latched, is among the most common causes and takes seconds to find. It costs nothing to check.

2. The coolant temperature value on the scan tool with a warm engine

   A warm engine reporting minus forty degrees confirms an open circuit immediately, before any testing begins.

3. The connector terminals for corrosion, spread pins or a backed-out terminal

   A terminal that has pushed out of the connector body looks perfectly connected from the outside while making no contact at all.

4. Long term fuel trim at idle and at cruise

   This tells you whether the PCM is actually commanding unneeded enrichment or has fallen back on a substitute coolant value, which decides how urgent the repair is for the catalytic converter.

5. Whether other sensors sharing the same ground circuit are also setting codes

   A lost shared sensor ground produces a cluster of unrelated-looking codes. Recognizing that pattern saves you from replacing several good sensors.

6. Recent service history around the engine

   Broken sensor bodies and pinched wiring are common after coolant service, intake work or timing jobs — these sensors are brittle when hot and easy to crack on removal.

1. Confirm the fault with a live reading

   Tools Scan tool

   With the key on, read the coolant temperature value. A reading at the extreme bottom of the scale, typically minus forty, on an engine that is not that cold confirms an open circuit.

   Expected

   A reading pinned at the minimum of the table, not tracking actual engine temperature

2. Inspect the connector before testing anything

   Tools Flashlight, magnifier, pick

   Unplug the connector and examine both halves. Look for backed-out terminals, green corrosion, spread female contacts, and a cracked sensor body. Reseat firmly and recheck the reading before moving on.

   Expected

   Clean, tight, fully seated terminals. Many faults resolve here

3. Jumper the signal wire to sensor ground

   Tools Fused jumper wire, scan tool, wiring diagram

   With the sensor unplugged and the key on, bridge the signal terminal to the sensor ground terminal on the harness side with a fused jumper. This simulates zero resistance, which the PCM should read as maximum temperature. Use a wiring diagram to identify the terminals — pin assignments vary by vehicle.

   Expected

   The reading swings to maximum temperature and typically sets P0117. That proves the harness and PCM are good and condemns the sensor

4. If the jumper produces no change, test the harness

   Tools Digital multimeter, wiring diagram

   Measure for reference voltage on the signal terminal at the harness with the key on. No reference voltage means an open signal wire or a module problem. Then check continuity on the sensor ground wire back to the PCM.

   Expected

   Reference voltage present on the signal wire and near-zero resistance on the ground wire back to the module

5. Measure the sensor's resistance directly

   Tools Digital multimeter, thermometer, service information

   With the sensor out or unplugged, measure across its terminals at a known temperature. An internally open sensor reads infinite. Compare against the manufacturer's resistance chart, which is vehicle-specific.

   Expected

   A finite resistance matching the chart for the measured temperature

6. Check live fuel trims to gauge the actual fueling consequence

   Tools Scan tool

   With the engine running, read short and long term fuel trim. Heavily negative long term trim means the PCM is genuinely commanding unneeded enrichment and the engine is running rich. Trims near normal suggest a substitute or modeled coolant value has taken over. Either way the circuit fault still needs repair, but this decides how much converter risk is accumulating.

   Expected

   Fuel trims within roughly plus or minus 10% if a substitute value is in use; a large negative long term trim indicates real rich running

7. Wiggle-test the harness for an intermittent open

   Tools Scan tool, assistant

   With the reading displayed live, flex the harness along its route and tap the connector. An intermittent open drops the reading to minimum momentarily, which pins the fault to a specific section.

   Expected

   A steady reading throughout. Any dropout localizes the break

8. Check for a shared sensor ground fault

   Tools Digital multimeter, wiring diagram

   If other sensors are also reporting implausible values, check the common sensor ground at the module and at any splice point. Repairing one ground can clear several codes at once.

   Expected

   Low voltage drop on the shared ground under load

9. Verify with a full cold-to-warm cycle

   Tools Scan tool

   After the repair, clear codes and watch the temperature reading climb smoothly from ambient to operating temperature. Confirm the fans cycle normally and that fuel trims settle rather than sitting heavily negative from residual rich correction.

   Expected

   A smooth temperature climb, normal fan cycling, and fuel trims within roughly plus or minus 10%

Almost always a low-cost repair when caught early — the sensor is cheap and the connector fix is cheaper. The expense comes from delay: where the engine really is running rich, months of it can add a catalytic converter replacement, commonly well over a thousand dollars, to a repair that started at under two hundred.

• Replacing the sensor before jumpering the harness. The jumper test proves in one minute whether the sensor or the wiring is at fault, and the wiring is at fault often enough to matter.
• Overlooking a backed-out terminal. The connector clicks home and looks perfect while the pin makes no contact inside.
• Replacing the sensor and cracking the new one during installation. These sensors thread into hot metal and are easy to overtighten or damage; the torque figure is vehicle-specific, so look it up rather than guessing.
• Assuming the engine must be running flat-out rich. Many vehicles substitute a modeled coolant value once the code matures — read fuel trims to find out what is actually happening before quoting converter damage.
• Dismissing it as a harmless light at the other extreme. Where enrichment really is being applied, sustained rich running is a genuine threat to the catalytic converter.
• Confusing this with P0117. P0118 is high voltage meaning an open circuit; P0117 is low voltage meaning a short to ground. They are opposite faults with opposite tests.
• Replacing the gauge sender rather than the PCM sensor on engines fitted with both, since which unit feeds which system varies by engine.

• Coolant Temperature Sensor
• Coolant Temperature Sensor Wiring

• P0117Engine Coolant Temperature Sensor Low InputSeverity level 3 of 5: Diagnose soon.moderate
• P0119Engine Coolant Temperature Sensor Circuit IntermittentSeverity 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

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Review status

Reviewed

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