What Does P1255 Mean?
The engine coolant temperature (ECT) sensor circuit is reading an abnormally low voltage, indicating a short to ground. This causes the ECM to interpret the coolant as being extremely hot (since ECT sensors are negative temperature coefficient thermistors where low resistance equals high temperature). The ECM may activate cooling fans continuously, enrich the fuel mixture excessively, or trigger unnecessary overtemperature protection strategies.
Common Causes
35%
Damaged wiring harness with signal wire shorted to ground due to chafing or pinching
30%
Failed ECT sensor with internal short circuit
20%
Water intrusion or corrosion at the ECT sensor connector causing low-resistance path to ground
15%
Faulty ECM coolant temperature input circuit
Diagnostic Steps
1
Read freeze frame data and check for companion code P1256 (open/short to positive), which would indicate an intermittent wiring issue.
2
With the engine cold, use a scan tool to read the ECT value — if it reads an unrealistically high temperature (e.g., 250+ degrees F), the short to ground is confirmed active.
3
Disconnect the ECT sensor connector and check if the scan tool reading changes to an extremely cold value (short to B+ behavior) — this confirms the sensor or wiring is at fault, not the ECM.
4
Measure resistance across the ECT sensor terminals with the sensor disconnected. Compare to the manufacturer's resistance-vs-temperature chart for the current ambient temperature.
5
If the sensor reads correctly, inspect the harness from the sensor to the ECM for chafing against the engine block, exhaust manifold, or other grounding points.
6
Replace the ECT sensor if its resistance is out of specification.
Estimated Repair Cost
$50 - $200
Parts + labor, varies by vehicle and location
The engine coolant temperature (ECT) sensor circuit is reading an abnormally low voltage, indicating a short to ground. This causes the ECM to interpret the coolant as being extremely hot (since ECT sensors are negative temperature coefficient thermistors where low resistance equals high temperature...
The most common cause of P1255 (Engine Coolant Temperature Sensor Short To Ground) is: Damaged wiring harness with signal wire shorted to ground due to chafing or pinching
Typical repair costs for P1255 range from $50 to $200, depending on the vehicle, location, and whether you do it yourself or go to a shop.
A shorted ECT sensor may cause continuous cooling fan operation, poor fuel economy from rich running, and masking of actual overheating conditions. If the ECM cannot accurately read coolant temperature, real overheating events may go undetected, risking engine damage.
Start by connecting an OBD2 scanner to read the code and any freeze frame data. Then follow the diagnostic steps specific to P1255 to identify the root cause.
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Quick Info
Category
Powertrain
System
Engine Cooling System
Difficulty
Type
Manufacturer
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