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High ECT voltage = ECM thinks engine is extremely cold. When the ECM sees a high-voltage ECT signal it may never command the cooling fan on, risking an actual overheating event. Monitor the temperature gauge closely while diagnosing. See top-rated scanners on Amazon ↗
🗺️ Where Is the Problem?
Blueprint view - P0118 ECT sensor high voltage (open circuit or failed sensor)
These are statistical causes across ALL vehicles - your exact car may rank differently
For example, on a Honda 4-cyl the downstream O2 sensor causes P0118 64% of the time, but on a GM 5.3L V8 the catalytic converter is the cause 71% of the time. Get a probability ranking built specifically for your year, make, model, and mileage.
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🎯 Top Causes & Probability
45%
#1 - Most Likely
ECT Sensor Open Circuit (Broken Wire)
With an open circuit, no current flows and the PCM sees the full 5V reference voltage, which it interprets as an extremely cold temperature. This is the most common cause of P0118 and is usually a broken signal wire.
🔩 Part
$5–$30
👨🔧 Labor
$50–$150
⚡ DIY Difficulty
Medium
35%
#2 - Check First
Failed ECT Sensor (High Resistance)
A sensor with abnormally high internal resistance restricts current and raises the signal voltage. Measure resistance across the sensor terminals at operating temperature - above 1,000 ohms indicates failure.
🔩 Part
$10–$50
👨🔧 Labor
$20–$60
⚡ DIY Difficulty
Easy
12%
#3 - Less Common
Wiring Fault Between Sensor and PCM
Corrosion, a pushed-back terminal, or a broken connection at the PCM end of the harness creates the same open-circuit condition as a broken wire. Check both ends of the circuit.
🔩 Part
$5–$30
👨🔧 Labor
$50–$150
⚡ DIY Difficulty
Medium
8%
#4 -
Engine Not Reaching Operating Temperature
A stuck-open thermostat can cause the engine to never fully warm up, and in some conditions the ECT reading stays near the high end of the normal cold range, borderline triggering P0118. Check thermostat operation with scan tool live data.
🔩 Part
$15–$60
👨🔧 Labor
$50–$150
⚡ DIY Difficulty
Easy
🚗 Most Affected Vehicles
🔧 Step-by-Step Diagnosis
- Read Live ECT Sensor Data - Connect a scan tool. With a warm engine, the ECT reading should be 180–210°F. If it reads -40°F or below, there is an open circuit condition.
- Check Sensor Connector and Pin Retention - Unplug the ECT sensor connector. Inspect for pulled-back pin terminals and corrosion. Wiggle each wire while watching for the code to clear on a live scanner.
📍 Find a Trusted Shop Near You
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Tips for Choosing a Shop
- Ask if they charge a diagnostic fee and whether it applies toward the repair
- Request a written estimate before approving any work
- Ask specifically about the part brand - OEM vs. aftermarket matters for this code
- Check Google reviews for recent mentions of the specific repair you need