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How to Know if Temp Gauge Is Bad (9 Symptoms)

More than we would like to admit, a well-functioning temp gauge is crucial to our vehicle performance. Whether you have a turbo-charged, fuel-injected wheeler, or a more dated model, keeping your engine temperature in check is a requirement. Conversely, failing to do so leads to adverse repercussions, including engine overheating, stalling, and permanent engine damage.

There are several ways to spot a temp gauge gone awry. By feel and thorough inspection, you should be able to determine if symptoms such as incorrect temp readings, unusual vibrations, poor idling, and non-working cooling fans point to a defective gauge or some other engine component.

Understandably, some of these indicators may overlap with more complex engine problems. To put some distinction between the two, here are the nine most common ways how to know if a temp gauge is bad:

  1. Erroneous temp gauge readings
  2. Extreme polarities
  3. Illuminated Check Engine Light
  4. Increased fuel consumption
  5. Starting issues
  6. Non-working cooling fans
  7. Engine overheating
  8. Black smoke
  9. Other performance quirks

Determining what causes your temperature gauge’s strange behaviors may not be easy. But if you can recognize its symptoms, you are already halfway through solving the problem.

Signs of a Faulty Temperature Gauge

Car Temperature Gauge

1. Erroneous Temp Gauge Readings

The clearest sign that you have a bad transmission temp gauge is an erroneous temperature reading. For instance, you may notice your temperature gauge rising, but your car is not overheating. Similarly, it may just stay on “cold” in the middle or go straight to high. These anomalies can be attributed to several factors, including a bad water pump or a broken temperature gauge sensor.

Based on reported incidents by vehicle owners, the four most common reasons behind high-temperature readings with no overheating are the following:

  • A bad water pump that fails to circulate the coolant throughout the engine
  • Faulty temperature gauge sensor caused by dirt or debris contamination
  • Damaged temperature gauge that misrepresents actual coolant temperature
  • Clogged radiator due to age or poor quality

It is important to note that this incident should not be automatically blamed on failing components. One of two things could be happening in this scenario – either nothing is wrong with the engine, but the temperature reading is off the charts, or the engine has gone past its normal operating temperature but has not yet reached the point where the PCM/ECM would consider it as overheating.

Either way, you will need to check everything surrounding the engine that directly or indirectly affects your vehicle’s air-fuel mixture and cooling system. Doing so will allow you to close in on one of these triggers.

2. Extreme Polarities

This symptom is probably the second most common indicator of a faulty temperature gauge. And by this description, I mean the temp gauge can be either too hot or too cold but does not match the engine temperature.

The best way to go about it is to get a reading of the actual temperature of your vehicle’s engine and compare it to that of the temp gauge. Provided you have ascertained nothing is wrong with your engine, the mismatched reading would then point to one of the following potential problems:

  • Defective temperature sensor or sending unit
  • Malfunctioning circuit in the temp gauge
  • Faulty wiring
  • Contaminated coolant system
  • Damaged thermostat
  • Corroded sensor, temp gauge, and ECU sensors
  • Broken temperature gauge
  • Failing ECU

3. Illuminated Check Engine Light

Being a universal problem indicator, your Check Engine Light going off signifies a myriad of things – among them a potential fault with your temp gauge. The CEL and temp gauge warning lights are set off simultaneously in most cases. Even if they did not, an activated Check Engine Light is a call to perform an immediate inspection of your vehicle, preferably with a high-spec OBD-II or DBM-III scanner like a Foxwell NT809 Bi-directional Scan Tool (view on Amazon).

4. Increased Fuel Consumption

A faulty car temperature gauge indicates that you have bad coolant temperature or ECT sensors. Experienced car owners know too well that defective ECT sensors send false signals to a vehicle’s PCM or onboard computer. This results in an incorrect air-fuel mixture and, consequently, a misuse of fuel that translates to poor gas mileage.

5. Starting Issues

Starting the Car

Because the function of a temp gauge has largely to do with air-fuel mixture, having a faulty one can translate to difficulties in starting your car. In worse situations, you may even have to deal with a no-start. Compared to the other symptoms in this guide, this predicament has a broader scope – meaning you will have to look beyond your temperature gauge and into other potential triggers of your starting issues.

6. Non-working Cooling Fans

Some cars with engines mounted longitudinally or transversely use an electric cooling fan for their cooling system. And this is one important fact you should know about your vehicle. I say this because electric cooling fans utilize a DC electric motor that is turned on or off with a thermo switch, module, or computer control – levers largely dependent on coolant temperature to operate.

As established earlier, a bad temp gauge = a bad ECT sensor = incorrect air-fuel mixture. Eventually, the flawed air-fuel mixture will cause the electric cooling fan to operate intermittently or not do so at all. Although highly unlikely, it is also possible for your cooling fan to give out earlier than your temp gauge. In this scenario, the former becomes the telltale sign pointing to the latter.

7. Engine Overheating

When your vehicle’s cooling fan breaks down, engine overheating becomes inevitable. At this point, signals sent to the onboard computer would already be distorted (not to mention incorrect) – resulting in a flawed air-fuel mixture exacerbating the situation. The said situation may not necessarily be the case for vehicles that use a separate ECT sensor for the cooling fan. But for those that share the same sensor, owners might be looking at fixing the temp gauge alongside this component.

8. Black Smoke

Black Exhaust From a Car

Unlike the other items in this list, black smoke from your car’s tailpipe is more an indication than a symptom of a bad temp gauge. If a faulty temp gauge is determined but left unaddressed, this is one of the signs your vehicle is likely to manifest. Of course, black smoke can trace back to your fuel mixture running rich too. But then again, that will only happen if your temp gauge is broken and incorrect signals are sent to the PCM/ECM.

9. Other Performance Quirks

A faulty temp gauge is typically accompanied by unusual vehicle behaviors such as shuddering at low speeds or loss of power. The best explanation is that a temp gauge indicates whether or not a car has the correct air-fuel mixture. Any upset to the air-fuel mixture is guaranteed to result in gauge malfunction and performance issues. Mechanics and car enthusiasts cannot stress enough how crucial air-fuel mixture is to vehicle performance.

How to Diagnose a Faulty Temp Gauge

Although some of the symptoms linking to a bad temp gauge are easy to spot, it can prove tricky to narrow down the issue to a specific one. As a vehicle owner, you will need to do a considerable amount of tinkering before arriving at a prognosis. A visit to the local mechanic may be inevitable in the long run. But until such time, here are some steps for you to determine whether your temp gauge is at fault or your engine is overheating:

1. Get Up to Operating Temperature

Run your mill for approximately 20 minutes to get it up to operating temperature (or until the temp gauge indicates it is running hot).

2. Inspect Your Coolant Tank

Next, pop the hood and visually inspect your coolant tank. After running your engine for the specified time, your coolant level should be at the top line or marking that reads “full/hot.” If it is not, you could be running low on coolant or have a potential coolant leak.

3. Inspect the Coolant Color

Note the color of the coolant as well. It should be a translucent green or orange, depending on its original color when you first added it. If it appears cloudy, rusty, or muddy at the slightest, the coolant could be contaminated (which would explain why your engine is running hot).

4. Check the Radiator Cap

Let your engine cool down afterward and check the radiator cap if loose, as this contributes to inconsistent temp signals.

5. Check the Radiator Hose and Engine Head

While at it, check the radiator hose and the engine head (near the thermostat) – they should be hot to the touch. If not, you may be looking at a clogged radiator or a defective water pump.

6. Confirm the Cooling Fan Is Running

Verify that your cooling fan is running. Normally, a cooling fan activates at 230° F (110° C). A non-working cooling fan would mean that your engine is overheating.

7. Check the Radiator Hose and Engine

To test if you have a stuck thermostat, see if your radiator hose and engine are warm.

8. Compare Temperatures

Compare the temperature of your lower and upper radiator hoses. Under normal circumstances, the lower hose should be cooler than the upper one. You may consider issues with coolant circulation or a potential coolant leak if this condition is not met.

I would not wish it, but you should be able to uncover more triggers as you go about these procedures. But if all these steps and components check out while your temp gauge is still acting weird, your temperature gauge is likely the culprit.

Word to the Wise:

A coolant temperature sensor (CTS, ECT, or ECTS) measures the temperature of the coolant/antifreeze mix in your vehicle’s cooling system and ultimately controls your temp gauge. So if the latter is malfunctioning or going haywire, chances are something is wrong with your coolant temperature sensor too.

How Much Does a Temp Gauge Repair Cost?

Ultimately, your expenses will be determined by what is causing your temperature readings to fluctuate and temp gauge to go bad. Potential repairs and associated costs will differ for each item we covered in this guide. Depending on the reason you uncover, the non-exhaustive table below should give you a good idea of the cost:

ComponentEstimated Cost
Coolant$20 – $100
Other faulty sensors$100 – $300
Temperature gauge$150 – $300
Temperature sensor or sending unit$150 – $375
ECT Sensor$150 – $430
Thermostat$200 – $350
Radiator$150 – $1,200
Water pump$720 – $1,820
Cylinder Head Gasket$715 – $2,000
PCM, ECM, or ECU$800 – $2,000

Conclusion – How to Know if Temp Gauge Is Bad

To recap, here are the nine most common signs of a bad temp gauge discussed in today’s article:

  1. Erroneous temp gauge readings
  2. Extreme polarities
  3. Illuminated Check Engine Light
  4. Increased fuel consumption
  5. Starting issues
  6. Non-working cooling fans
  7. Engine overheating
  8. Black smoke
  9. Other performance quirks

I hope you find this guide useful, as it is never advisable to drive around with a broken temp gauge. I strongly suggest urgently addressing the issue the moment you experience any of these symptoms. Doing so will spare you from not being able to use your car’s defroster or heating system (a huge bonus when driving in cold or inclement weather). But more importantly, you would be able to prevent unnecessary repair expenses – not to mention permanent damage to your car engine and its components.