P0118
Engine Coolant Temperature Circuit High Input
Powertrain Engine Cooling Coolant Temperature Sensor Circuit 🟡 Moderate — Fix within a week
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What This Actually Means
In plain language — no jargon

The ECU detects the coolant temperature sensor is reading hotter than physically possible, like a thermometer stuck at 212°F when the engine just started. This usually means the sensor or its wiring is faulty, not that your engine is actually overheating.

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Symptoms You May Notice
3 known symptoms for this code
Check Engine Light illuminated
Incorrect or erratic coolant temperature gauge readings
Poor cold-start fuel mixture or rough idle
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Embedded Systems Insight
What the ECU/ECM is actually computing

The ECU monitors voltage from the Coolant Temperature Sensor (CTS), which decreases as coolant heats up. When the sensor reads abnormally high voltage—typically above 4.8V—the ECU interprets this as an impossible coolant temperature and triggers P0118. The circuit is tested during both key-on and running conditions.

Voltage & Parameter Thresholds

Parameter Normal Range Fault Condition
CTS Voltage 0.2V to 4.8V (−40°C to 130°C) >4.8V (temperature reading exceeds max safe limit)
Coolant Temp Reading −40°C to 130°C >130°C continuously at startup/idle
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Diagnostic & DIY Fix Guide
Check these in order — from cheapest to most complex
1
Coolant Temperature Sensor
Unplug the CTS connector, inspect for corrosion or moisture, clean with electrical contact cleaner, and reconnect; if still faulting, replace the sensor.
2
CTS Wiring Harness
Inspect the sensor wiring loom for cracks, pinches, or exposed conductors between the engine block and ECU connector; repair or replace damaged sections.
3
ECU Connector
Check the engine control module connector pins for corrosion or looseness on the CTS signal line and reseat the connector firmly.