P0118 indicates the Engine Coolant Temperature (ECT) sensor input to the engine control module (ECM/PCM) is higher than expected. The usual fixes involve testing and replacing the sensor or repairing the wiring, with the possibility of a computer issue if the sensor and harness test fine.
Understanding P0118 and its potential impact
P0118 is a diagnostic trouble code that signifies a high input reading on the ECT circuit. When the PCM sees a voltage or signal outside the expected range, it lights the check engine light and stores the code. In practice, the car may run poorly, the temperature gauge might read abnormally, and fuel economy can suffer. Although an actual overheat condition can cause engine damage, P0118 most often points to a faulty sensor or wiring rather than a true temperature rise.
What commonly causes P0118
The following list outlines the most frequent sources of a P0118 code. Use these as a guide when you start diagnostics.
- Faulty Engine Coolant Temperature (ECT) sensor
- Short to power (5V reference) or short to ground in the sensor circuit
- Open circuit or high resistance in the ECT harness
- Weak PCM/ECM input or internal fault (less common but possible)
Note: Real-world symptoms can vary by vehicle make and model. If the cooling system is genuinely overheating due to a stuck thermostat or low coolant, you may see related cooling symptoms in addition to the code.
Diagnostic steps to identify the cause
Before you begin repairs, gather the right tools and confirm the fault with live data and thorough inspection. The steps below are a practical approach used by many technicians.
- Check with a scan tool: read the live ECT value at different temperatures (cold start and after engine warms). Compare the reading to the expected coolant temperature and to the vehicle’s gauge behavior.
- Inspect the ECT sensor and harness: look for damaged wires, loose connectors, corrosion, oil or coolant intrusion, and bent pins at the sensor plug.
- Test the ECT sensor resistance: with the engine cool, disconnect the sensor and measure resistance across its terminals using a multimeter. Compare to the manufacturer’s spec (often around a 10kΩ resistance at ~20°C, but verify your service manual).
- Check the 5-volt reference and ground: verify that the PCM’s reference voltage (and the sensor’s ground) are present and solid. Look for shorts to 5V, to ground, or to other circuits.
- Inspect for open/high resistance in wiring: use a wiring diagram to trace the circuit and check continuity along the harness and through connectors.
- Rule out genuine overheating: ensure coolant level is correct, the radiator cap is sealing, the thermostat isn’t stuck, and the cooling fans operate properly during temperature rise.
After performing these tests, you should be able to determine whether the issue is the sensor, the wiring, or the PCM. If the readings are within spec and wiring checks pass, the PCM could be at fault, but that is usually a less common outcome and often involves professional testing.
Recommended fixes based on findings
Depending on what you find in the diagnostics, apply the appropriate fixes. The goal is to restore an accurate ECT signal to the PCM and eliminate any misreadings caused by hardware faults.
- Replace the ECT sensor if its resistance or voltage readings are out of spec or if the sensor is visibly damaged or contaminated.
- Repair or replace damaged wiring or connectors at the ECT sensor. Clean or reseal connector pins as needed to ensure a solid electrical connection.
- Repair any shorts to power or ground in the sensor circuit. This may involve shielding, re-routing wires, or replacing a compromised harness portion.
- Replace the wiring harness or add connectors if corrosion or damage is extensive and cannot be repaired.
- If the sensor and wiring test good, consider PCM-related troubleshooting with a professional, as a PCM fault is relatively rare but possible.
- Do not ignore a true overheat condition: if cooling system components are failing (stuck thermostat, failed water pump, blocked radiator), address those issues promptly to prevent engine damage.
After completing repairs, clear the code with a scan tool and recheck for any reoccurrence. If the code returns, re-run diagnostics or seek professional help to evaluate the PCM more thoroughly.
Practical tips and safety considerations
Handle coolant and electrical components with care. Allow the engine to cool before touching any parts of the cooling system or electrical connectors. If you are unsure about any step, consult your vehicle’s service manual or a qualified mechanic.
Summary
P0118 points to a high input on the Engine Coolant Temperature circuit, most often caused by a faulty ECT sensor or wiring issues. Start with a sensor and wiring inspection, test sensor resistance against specifications, verify the 5V reference and ground, and rule out genuine cooling system problems. Replace the ECT sensor or repair harnesses as needed, clear the code, and test drive to confirm the fix. If the code returns, professional diagnostic help may be required to assess the PCM and more advanced electrical concerns.
How do I reset my temperature sensor?
First, turn off the engine and disconnect the battery for about 15 minutes. Reconnect it to reset the gauge. Some cars may need you to check or replace the coolant temperature sensor if this does not work.
How much does it cost to fix a P0118 code?
Cost Outlook
PCM re‑programming (factory tool): $120‑$180 plus software license fees if not already owned by the shop. PCM replacement (including VIN‑matched programming): $600‑$900 for the unit, $200‑$300 labor.
Can I still drive with a PO118 code?
You really shouldn't drive with the P0118 code. The engine can overheat and cause extensive and expensive damage. The engine control module could also switch to default settings, which leads to poor performance and increased fuel consumption. It's best to address the problem as soon as possible.
What sensors can trigger a P0118 code?
The P0118 code refers to issues with the Engine Coolant Temperature (ECT) circuit and its accompanying sensor. The ECT is a thermistor, or an electrical resistor, whose resistance is significantly reduced by heating. The ECT is located in the coolant passage in your engine's cylinder head.