Your 8th Gen (FA5/FG2), 9th Gen (FB6/FG4), 10th Gen (FC1/FK7), or 11th Gen (FL1/FL5) Civic may display diagnostic trouble code P0118 — which indicates a high voltage condition in the engine Coolant temperature sensor circuit.
P0118 is defined as “Engine Coolant Temperature (ECT) Sensor 1 Circuit High Voltage”. This code indicates that the ECT sensor circuit voltage is pegged at 4.90V or higher — which corresponds to approximately -40°F (-40°C) on the scanner.
Unlike P0116 (a logic fault), P0118 is a hard electrical failure — an open circuit. The ECU supplies a 5V reference to the ECT sensor, and the thermistor modulates voltage to ground based on coolant temperature. If the circuit is broken — open wire, corroded pin, failed sensor — the voltage remains at 5V and the ECU stores P0118.
Before replacing components, perform basic electrical diagnostics using a digital multimeter (DMM) and a paperclip. The following is a technical breakdown of the typical causes of P0118 on these Honda Civic models and the recommended diagnostic and repair procedures.
P0118 Diagnostic Overview: Open Circuit in ECT Sensor
P0118 (“ECT Sensor Circuit High Voltage”) is the counterpart to P0117. P0117 indicates a short to ground (low voltage), while P0118 indicates an open circuit (high voltage).
The ECT sensor is a thermistor with a negative temperature coefficient (NTC). Resistance decreases as temperature increases. Cold coolant = high resistance = high voltage. Hot coolant = low resistance = low voltage (approximately 0.5V–0.8V on a fully warmed engine).
When the circuit is broken, the 5V reference has no path to ground. The ECU reads 4.90V or higher, interprets the engine temperature as -40°F, and stores P0118.
Common Symptoms:
- Hard starting — especially when the engine is hot
- Rich exhaust / black smoke — the ECU enriches the mixture due to perceived low temperature
- Rough idle — fuel trims are affected
- Cooling fans running constantly — the ECU defaults to fan operation as a safety measure
- Temperature gauge inoperative or pegged
Primary Failure Modes for P0118
1. Disconnected Connector
If work was recently performed near the thermostat housing (clutch replacement, Head Gasket repair, or other engine work), the ECT connector may have been left unplugged. No connection = instant P0118.
2. Corroded or Damaged Harness
On the 8th Gen, the ECT sensor is located above the transmission bellhousing. Transmission work can strain or damage the harness. Engine movement may flex wires until they break internally. Additionally, rodent damage to wiring is a known issue on Honda vehicles.
3. Internal Thermistor Failure
Thermal cycling (heat/cold cycles) can cause internal solder joints or the thermistor itself to crack. Once cracked, the circuit opens — replacement is required.
Platform-Specific Considerations
8th Gen (FA5/FG2, K20Z3 Si) & 8th Gen R18 & 9th Gen (FB6/FG4, K24Z7):
- ECT Sensor 1 is located in the water outlet housing on the cylinder head near the thermostat.
- On the K20Z3 (Si), it is on the cylinder head near the upper radiator hose, above the transmission bellhousing.
- On the R18, it is on the cylinder head near the thermostat housing on the intake side.
- Note: The sensor in the bottom of the radiator is ECT Sensor 2, which controls the cooling fans. Replacing it will not resolve P0118.
- OEM part number: 37870-RTA-005 for most 8th/9th Gen Civics.
10th & 11th Gen Turbo (L15B7, K20C1, FC1/FK7/FL1/FL5):
- ECT Sensor 1 is located on the engine block, transmission side.
- On vehicles with aftermarket tuning (e.g., from DRob or Phearable), P0118 may trigger limp mode and reduce boost to protect the engine.
- OEM part number: Verify by VIN — the 1.5T and 2.0L use different sensors.
Diagnostic Procedure Using a Paperclip and DMM
This diagnostic sequence is derived from Honda’s factory service manual.
Step 1: Verify the Reading with a Scanner.
Connect a scanner with KOEO (Key On, Engine Off) and monitor ECT 1 live data.
Evaluation: A reading of -40°F (-40°C) or 4.90V or higher confirms an open circuit.
Step 2: The Paperclip Jump Test (Harness and ECU Test).
Turn the ignition OFF. Disconnect the 2-pin ECT sensor connector. Use a metal paperclip to jump the two terminals on the harness side (short the signal wire to ground).
Turn the ignition back ON (KOEO) and monitor the scanner.
Evaluation:
- If the reading changes from -40°F to 300°F+ (or drops to 0V), the harness and ECU are functional. The sensor is faulty — replace it.
- If the reading remains at -40°F (4.90V+), there is an open circuit in the wiring harness. Trace the wiring back to the ECU.
Step 3: Resistance Test (Bench Check).
Remove the sensor and test with a DMM set to Ohms (Ω).
Evaluation: AT room temperature (~68°F / 20°C), a functional sensor should read approximately 2,000Ω (2 kΩ). A reading of OL (Over Limit) indicates a failed thermistor — replace the sensor.
OEM Part Numbers and Cost Reference
Verify fitment by VIN before ordering.
| Part | Fits | OEM Part Number | Online (HondaPartsNow, RockAuto) | Honda Dealer |
|---|---|---|---|---|
| ECT Sensor 1 | 8th Gen R18, 9th Gen K24Z7 | 37870-RTA-005 | $15–$45 | $50–$80 |
| ECT Sensor 1 | 8th Gen Si K20Z3 | 37870-PNA-003 (verify by VIN) | $25–$45 | $60–$90 |
| ECT Sensor 1 | 10th Gen L15B7 (1.5T) | Verify by VIN | $30–$55 | $70–$100 |
| Electrical Connector Pigtail | All Generations | Aftermarket / Honda repair pigtail | $10–$15 | $20–$30 |
Recommendation: Use Denso or genuine OEM Honda sensors. Aftermarket sensors may cause erratic cooling fan operation or idle issues. 37870-RTA-005 is the updated part number for many applications.
Repair Cost Estimates
| Repair | DIY Cost | Independent Shop | Honda Dealer |
|---|---|---|---|
| ECT Sensor Replacement | $15–$55 | $100–$180 | $150–$250 |
| Wiring Repair (splice & solder) | $5–$15 (materials) | $100–$200 | $150–$300 |
| Coolant Top-off (Honda Type 2 Blue) | $20–$40 | Included with labor | Included with labor |
| Dealer Diagnostic Fee | N/A | N/A | $120–$200 |
Pricing based on current market rates from HondaPartsNow, RockAuto, and Hybrid Racing.
Note: When replacing the ECT sensor, some coolant loss is expected. Have Honda Type 2 (Blue) coolant available for topping off. Bleed the cooling system with a spill-proof funnel after reinstalling the sensor.
Pre-Purchase Inspection Note
When evaluating a used Civic with a P0118 code, the seller may state: “It just needs a cheap sensor.”
Perform the paperclip jump test yourself:
- Unplug the ECT connector, jump the terminals with a paperclip, and monitor the scanner.
- If the reading jumps to 300°F, the harness is good — the sensor is dead.
- If the reading remains at -40°F, there is a wiring problem requiring more extensive repair.
Recommendation: If the seller has not performed this basic diagnostic step, it may indicate neglect of other maintenance items.
Diagnostic Checklist
☐ P0118 = ECT Sensor Circuit High Voltage — open circuit
☐ Check ECT reading at KOEO — -40°F (-40°C) or 4.90V+ confirms open circuit
☐ Paperclip jump test — jump ECT connector pins on harness side. If reading flips to 300°F+, the sensor is faulty — replace it
☐ If reading stays at -40°F after jump test — wiring has an open circuit between sensor and ECU. Trace the harness
☐ Check sensor resistance — at room temp, should be ~2,000Ω. OL = failed sensor
☐ Inspect for rodent damage — Honda wiring is susceptible to rodent damage
☐ Verify connector is fully seated — may have been left unplugged after recent work
☐ 8th Gen K20Z3 Si: ECT 1 is on the head near the thermostat, above the bellhousing
☐ 8th Gen R18 & 9th Gen K24Z7: ECT 1 is in the water outlet housing near the thermostat — OEM part 37870-RTA-005
☐ 10th/11th Gen Turbo: ECT 1 is on the block, transmission side — verify by VIN
☐ Use OEM or Denso sensors — aftermarket sensors may cause driveability issues
☐ Top off with Honda Type 2 (Blue) coolant — and bleed the system properly
☐ Pre-purchase inspection: Perform the paperclip test before purchasing