Temperature Gauge in 2000 Honda Civic LX

Question edited for clarity and written English 'sense' to 'since', 'don't' to 'doesn't' etc.

Your coolant temperature sensor has likely failed if 'changed it' was the coolant temperature sensor and not the gauge. The scan tool gets the data from a few places as vehicles have more than one temperature sensor. The one on the radiator is the one that normally tells the ECM that the fan needs to run, but there is often another near the thermostat.

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https://www.google.com/search?q=2005+honda+civic+temperature+gauge+not+working

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Yes as long as heater is hot & cooling fan comes on & off without temperature gauge going into the red all is well

Ground the temperature gauge wire to the engine using a jumper wire, then watch the temperature gauge needle. If the gauge now reads Hot, replace the temperature sending unit. Check the fuse for the temperature gauge circuit. If the fuse is good, check for voltage at the ignition terminal on the gauge with turn the ignition key to the "on" position. Using a voltmeter. If there is no voltage, there is a problem in the circuit between the ignition key and the gauge or the fuse panel and the gauge temperature. Fix the problem and test again. If there is voltage, replace the gauge.

Turn the ignition key to the on position and ground the temperature gauge wire to the engine using a jumper wire, then watch the temperature gauge needle. If the gauge now reads Hot, replace the temperature sending unit. Otherwise, check the fuse for the temperature gauge circuit. If the fuse is good, turn the ignition key to the "on" position. Using a voltmeter, check for voltage at the ignition terminal on the gauge. If there is no voltage, there is a problem in the circuit between the ignition key and the gauge or the fuse panel and the gauge temperature. Fix the problem and test again. If there is voltage, replace the gauge.

Watch the temp gauge when the electric radiator fans come on. The temp sensor that controls when the fan comes on may be faulty.

faulty sensor or a damaged wire to the sensor

iam suspecting the temperature gauge here ,something is or could be wrong with its electrical conductivity ,or the cooling fan is coming on too late which could be caused by a similar problem with the fan temperature switch

this is normal, the cooling fan of the engine doesn't come on until u are abouut 2/3 of the way to hot zone of gauge, about 225 degrees F. let me kniow if that is the case.

replace temperature gauge and insprct/replace speedometer/rpm cable

u check anifreeze