«Engine Hot, AC Off», But Temp Gauge Reads Cold. What's Wrong?

The car's performance depends on the temperature gauge working properly. The gauge also helps to keep the engine temperature in check. If the temperature gauge gives false readings, your engine can overheat or not operate within the optimum temperature without your knowledge. This eventually causes engine damage.

Engine hot, AC off message highlights

• Common reasons:no coolant, engine temperature sensor dead
• How to fix:check coolant level, check temp sensor
• Possible consequences:overheated engine will die quickly
• Priority level:High
• Can you drive?Not recommended
• DIY repair:Possible
• Repair price range:$100-$450

How Does a Temperature Gauge Work?

The Temperature Gauge sits inside the car dashboard and indicates the coolant temperature inside the engine's cooling system in your Chevrolet. You can use it to determine whether the engine is overheating, operating at its normal temperature, cool, or fluctuating. The engine's cooling system absorbs heat through a coolant that runs throughout it. 

The electric temperature gauge is simply a voltmeter (indicates voltage). The car's computer serves as the interface between the sending unit and the temperature gauge. The gauge's face scale indicates temperature, but the gadget relays the voltage to the ECU. A bimetallic "hairpin" assembly-two distinct metals linked together- makes up the gauge. It has a needle that's connected to this assembly. The gauge needs a transmitting unit and an electrical circuit to read the temperature. 

A Thermistor (temperature-dependent resistor) serves as the sending unit. It is exposed to engine coolant on one side. Changes in coolant temperature cause changes in its variable resistance. The sending unit's resistance turns highest when the engine is cool, such as when the engine is off or has just been started, obstructing the flow of electrical current.

This is demonstrated by the gauge's indicator, which stays at the low end of the reading range. The resistance of the transmitting unit lowers as the engine coolant warms and the gauge's indicator increases. If the engine remains at working temperature, it will stay in the middle of the reading scale. A warning sign doesn't appear until the engine overheats.

Possible Reasons For a Cold Temperature Reading Issue in the Temperature Gauge

If the temperature reading keeps reading low when the engine is hot and AC is off can point to various reasons. Some common reasons for the issue are:  

1) Faulty Engine Control Unit

Although rare, the Engine Control Module (ECM) may act up and cause false temperature gauge readings. If the ECM can't detect a change in the voltage signal from the sensors due to damage, it can't effectively control the dashboard temperature gauge or the radiator fan. This can make it keep reading cold even when the engine is heating up. The ECU may get damaged due to exposure to water, short circuits from the wires, or frequent car vibrations. 

2) A Defective Coolant Temperature Sensor

You can find the main temperature sensor near the thermostat housing or the radiator's base. The coolant temperature sensor gauges the temperature of the engine coolant using a resistor circuit. The sensor receives a constant 5V reference voltage from the ECM, which it then adjusts based on the coolant temperature to produce a proper voltage signal.

An inaccurate "cold" signal may be sent to the ECU by a broken coolant temperature sensor, fooling it into thinking the engine isn't hot enough. The engine will then continue to heat up and eventually overheat due to the ECU adjusting the fuel injection, ignition timing, and variable valve timing.

3) A Malfunctioned Temperature Gauge

A bad gauge cluster on your instrument panel can also be a probable cause for the cold temperature reading issue. It can display false feedback by reading low when in actual sense, the engine temperature is high. In modern vehicles, most temperature gauges are integrated into the instrument cluster. The faulty readings may be due to poor electrical parts soldering of the temperature sensor or the gauge itself. 

4) Corroded or Bad Electrical Wiring

The accuracy of the temperature gauge might also be affected by electrical or wiring problems. The transmission between the temperature sensor and the gauge can be interfered with by a blown fuse, loose connections, or corroded wire insulators. This can result in inaccurate temperature readings such as the temperature gauge reading cold.

How to Diagnose Issues Leading False Temperature Gauge Reading

When the temperature gauge inappropriately displays a temperature that's too low, too high, or one that's fluctuating, it's hard to determine whether it's a gauge issue or something else. This requires a proper diagnosis to zero in on the issue. Below are some of the ways to help you make a diagnosis.

1) Use an OBD Scanner 

It's always wise to use an OBD scanner to examine the fault codes before diagnosing the coolant temperature sensor or other parts that may cause the gauge to always read "cold". This also applies to other modern car engine issues where you should always use an OBD2 scanner to view the error codes.

You can learn a lot of useful information from these codes and save time and money for diagnosis. It'll require you to connect your diagnostic tool to the OBD 2 port and scan for error codes to appear. The DTC codes will give you an idea of issues such as circuits or sensors. 

2) Testing If It's an Issue with the Sensor

Vehicles have one or two temperature sensors for the ECM or temperature gauge. Here's how to test the temperature sensor to determine if it's faulty. Find the sensor near the thermostat and unplug it. Connect a multimeter to the sensor's external parts. Set the multimeter to DC current and use the sensor to test for the voltage of cold water with ice cubes in a container. The multimeter should read a voltage of around 5V.

You'll then use it to test for the voltage of hot boiled water, which should be about 25V. Compare the readings you get to appropriate ohm values from a chart that corresponds to your car model.

3) Testing for Broken Wiring 

If you find your sensor is in good condition, you can move on to test the electrical connections. Either the wiring between the gauge and the ECU, or the wiring between the sensor and the ECU, could be the issue.

Look for any loose connections or frayed wires connecting these parts. The best technique to identify damaged wiring is to use a multimeter to check the voltage and electrical continuity at different locations throughout the circuit. Determining any irregularities in the readings may help locate the wiring issue. However, this needs some electrical skills.

4) Testing Whether the Temperature Gauge is Dysfunctional

You can examine the wiring inside the gauge cluster or the whole instrument cluster (if the gauge is integrated into it) for any signs of corroded wires. If you have some electrical knowledge, you may also utilize an Ohm tester to test the cluster temperature gauge.

5) Testing If it's an issue with the Engine Control Unit

To determine whether the engine control unit is receiving the right temperature information, check the DTC codes with an OBD2 scanner in the engine. Ensure the ECU and gauge use the same sensor if you can discover the temperature data in the engine control unit but not at the gauge cluster. If so, you'll require to measure the engine control unit's temperature output.

How to Fix the Temperature Gauge Reading Cold Issue

Whether the issue is due to a faulty temperature sensor, a bad temperature gauge, a defective ECM (Engine Control Module), or issues with the electrical wiring, the solution will depend on the precise cause. Below are potential fixes to each scenario:

• Defective ECM

You'll have to replace the ECM if you find it faulty. To guarantee correct installation and compatibility, it is advised to consult a certified mechanic before replacing an ECM because this often requires programming and configuration, which can be difficult to handle for the average DIYer.

• Faulty Sensor

The best course of action is to replace the temperature sensor if it's defective. Ensure you buy the right sensor for the make and model of your car. To install the sensor, unplug the old sensor's electrical connector and take it out of the way. Restore the electrical connector after positioning the replacement sensor in the same location. When installing the new sensor, take care not to overtighten it.

• Malfunctioned Temperature Gauge

Calibration may be the answer if the sensor is working properly, but the temperature gauge is not giving reliable readings. You can use the owner's manual for your car or consult an expert mechanic to find out the precise calibration steps for your gauge.

You may need to change the gauge if calibration cannot resolve the issue. Typically, the instrument cluster must be removed from the dashboard, and a new gauge must be installed in place of the damaged one. If you're unfamiliar with this kind of repair, consider hiring a professional.

• Bad Electrical Connections

Based on the nature of the problem, you may need to patch up damaged wires, replace faulty connectors, or even replace a portion of the wiring harness. If electrical repairs are not your wheelhouse, you may need professional help.

Bottom Line

There you go-why your temperature gauge could be reading cold when the engine's hot and the AC is off, how to make a diagnosis and fix the issue. Remember to prioritize routine maintenance to preserve the accuracy of temperature measurements and keep your car in top shape. It would be best to seek expert advice if you run into problems or have questions about the procedure.