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Low ECT voltage = ECM thinks coolant is extremely cold. The ECM adds extra fuel for a "cold engine" that is actually warm, causing a rich condition. You may notice black smoke, poor MPG, and a rich exhaust smell along with P0117. See top-rated scanners on Amazon ↗
🗺️ Where Is the Problem?
Blueprint view - P0117 ECT sensor low voltage (short to ground or internal failure)
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 P0117 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 Shorted to Ground
A low-resistance short to ground on the ECT signal wire drags the voltage to near zero. The PCM interprets this as a reading of -40°F or lower, triggering P0117. Check for chafed wiring in contact with the engine block.
🔩 Part
$5–$30
👨🔧 Labor
$50–$150
⚡ DIY Difficulty
Medium
35%
#2 - Check First
Failed ECT Sensor (Low Resistance)
The thermistor inside the ECT sensor has failed with abnormally low resistance, sending too high a current and too low a voltage to the PCM. Measure sensor resistance at operating temperature - should be 200–300 ohms at 212°F.
🔩 Part
$10–$50
👨🔧 Labor
$20–$60
⚡ DIY Difficulty
Easy
12%
#3 - Less Common
Wiring Short to Ground
The signal harness between the ECT sensor and PCM may be contacting the engine block, frame, or another ground point. Inspect carefully along the entire routing of the harness.
🔩 Part
$5–$30
👨🔧 Labor
$50–$150
⚡ DIY Difficulty
Medium
8%
#4 -
Actual Engine Overheating Mimicking Fault
In rare cases, extreme overheating can cause sensor resistance to drop to an unusual range, triggering a P0117 alongside a real overheating condition. Always verify coolant temperature with a scan tool before assuming it is just a sensor fault.
🔩 Part
$0–$200
👨🔧 Labor
$0–$500
⚡ DIY Difficulty
Varies
🚗 Most Affected Vehicles
🔧 Step-by-Step Diagnosis
- Verify with Live Scanner Data - Connect a scan tool and check the ECT temperature reading with a cold engine. If the reading is below -20°F on a warm day, the circuit has a low-voltage fault, not an actual temperature condition.
- Check for Wiring Short to Ground - Unplug the ECT sensor. With the connector unplugged, check resistance between the signal terminal and vehicle ground. Any resistance below 10k ohms indicates a wiring short.
📍 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