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Engine Temperature Fluctuates

A temperature gauge that rises and falls while driving usually has a sticking thermostat, low coolant, air in the system, a faulty temperature sensor or a cooling fan problem.

Normal risk

Watch it closely and get it checked soon. If it reaches the red zone, stop safely.

Short answer · Why does my engine temperature go up and down?

Most gauges are buffered to stay steady in the normal range, so visible swings mean something is wrong. A thermostat that sticks then opens suddenly gives up-and-down readings. Low coolant or an air pocket can make the sensor read gas instead of liquid. A cooling fan that does not start lets the temperature climb in traffic. A faulty sensor gives erratic readings.

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Safe checks you can do

  1. Check the coolant levelWith the engine cold.Never open the cap when hot.
  2. Note when it changesIn traffic, at speed or on hills.
  3. Check that the fan runsIt should run when the engine is hot at idle.

Questions that narrow it down

  1. Does it rise in traffic?Cooling fan.
  2. Does it drop at speed?Thermostat stuck open.
  3. Is coolant low?Leak or air.

Possible causes

Listed without ranking. Use the questions above to see which ones fit your car.

  1. Sticking thermostatSwings up and down.
  2. Low coolantSensor reads air.
  3. Air in the systemAfter repairs.
  4. Faulty temperature sensorErratic reading.
  5. Cooling fan faultRises in traffic.

Questions about Engine Temperature Fluctuates

Can I keep driving?

If it stays out of the red, carefully; stop if it overheats.

Is a thermostat expensive?

Usually not, though access varies by car.