What does it mean?
Here the module is not complaining that the reading is too high or too low, but that it is unstable. The signal from coolant temperature sensor 3 jumps, drops out briefly or changes faster than coolant temperature physically can, and the module flags it as intermittent or erratic. Coolant is a large thermal mass, so a genuine reading moves slowly and smoothly; a step of many degrees between two samples can only be electrical. Faults of this kind are almost always mechanical connection problems that come and go with vibration, temperature or moisture, which is why the code often appears long before any driveability complaint does.
Symptoms
- Temperature gauge that flickers, jumps or drops suddenly and then recovers.
- Cooling fans switching on and off unexpectedly with no relation to engine load.
- MIL (check engine lamp) that comes on and later goes out by itself.
- Occasional rough running or a brief stumble that never repeats predictably.
- Heater output that varies between journeys with no obvious pattern.
- Other sensor codes appearing at the same moment if a shared ground is involved.
Likely causes
- Loose or spread terminals in the sensor connector, giving contact that breaks under vibration.
- Corrosion inside the connector, often where coolant has wicked along the wires from a small seepage.
- Chafed harness that touches ground intermittently as the engine rocks on its mounts.
- Cracked thermistor or a failing internal solder joint in the sensor, sensitive to temperature.
- Poor or shared ground point that becomes resistive when hot.
- Wire broken inside its insulation, making and breaking contact as the harness flexes.
- Loose pin in the control module connector or a partially unlatched module plug.
How to fix it
- Graph the sensor value at the fastest sample rate the tool allows and watch for steps that no thermal mass could produce.
- Review the freeze frame for what the vehicle was doing when the code set, such as cold weather, rough road or hard acceleration.
- With the graph running, wiggle the connector and flex the harness section by section to reproduce the dropout.
- Check terminal tension in the sensor and module connectors with a matching pin, and look for green corrosion inside.
- Perform voltage drop tests on the signal and the ground while flexing the harness, rather than a static continuity check.
- Inspect the harness where it passes brackets, the exhaust side and moving components, and reroute or protect it as needed.
- Repair the connection, clean and seal the connector, and only then consider the sensor; verify over several drive cycles in the conditions that produced the fault.