The Clean Air Act is a federal law in the United States that regulates air pollution emissions from motor vehicles and other sources. The act prohibits the manufacture, sale, or installation of any device that is intended to defeat or bypass the emission control systems of a vehicle.

An exhaust gas temperature (EGT) sensor is a device that measures the temperature of exhaust gases as they exit the engine. This information is important for engine performance and efficiency optimization.

An EGT sensor does not violate the Clean Air Act because it is not designed or intended to defeat, bypass, or shut down any emission control devices. Instead, it simply measures the temperature of the exhaust gases and provides data that can be used to optimize engine performance and fuel efficiency.

In fact, many modern vehicles are equipped with EGT sensors as part of their standard equipment to help ensure that emissions are kept within legal limits. These sensors can also help diagnose problems with the engine or emission control systems, leading to faster repairs and more efficient operation of the vehicle.

In summary, an EGT sensor is a legal device that can be installed on a vehicle without violating the Clean Air Act, as long as it is not intended to defeat or bypass any emission control systems.

A quick way to check the EGT probe is to disconnect it from whatever device you are using to read it & put an ohm meter across both output wires (the wire colors are usually red & yellow). If you get an open reading then the probe is probably bad. If you get somewhere between 3 to 12 ohms then it may still be okay.

Most analog output sensors have general specifications such as linearity (or non-linearity), repeatability, and resolution, as well as environmental specifications like operating temperature or shock and vibration, and dynamic specifications like response or bandwidth. All of these specifications represent limits of error or sources of uncertainty related to the sensor's output compared to its input. Many of these terms are fairly easy to understand by their wording alone, but linearity error or non-linearity is not in that category.

Definition of Linearity Error or Non-linearity

Linearity, or more correctly, non-linearity, is a measure of the maximum deviation of the output of any sensor from a specified straight line applied to the plot of the data points of the sensor's analog output versus the input parameter being sensed, which is called the measurand, under constant environmental conditions. The more linear the sensor's output, the easier it is to calibrate and to minimize uncertainty in its output scaling. However, understanding a sensor's non-linearity specification requires understanding the nature of the reference straight line.

Reference Straight Line

There are several possible reference straight lines that could be utilized to express a sensor's linearity error. The optimum choice based on statistics would be a "best fit line". But just what is the criterion for "best fit"? Both experience and statistics favor a line calculated by the "method of least squares", whereby the sum of the squares of the deviations from the desired line is mathematically minimized. Such a best fit straight line (BFSL) is broadly used as a basis for a sensor's linearity error or non-linearity, not merely because it is statistically appropriate but also because it has been validated in real world measurements.

Impact of Other Errors

Because the linearity error applies to the analog output of the sensing system, recognition must be given to other errors that can affect the output besides sensor non-linearity. To fully comprehend what the linearity error specification actually means, there are several pre-conditions that must apply to the measurement process. First, environmental factors like ambient temperature must be reasonably constant or small changes compared to the linearity error.  Next, the repeatability and hysteresis errors in the sensor itself must also be small compared to its linearity error. Third, any non-linearity in the system output caused by ancillary electronics in the measuring system must also be very small compared to a sensor's linearity error. And finally, the resolution of both the sensor and the output reading instrument must be sufficient to react to the small deviations in output caused by linearity error.

Why Worry About Other Errors

Measurement errors cannot simply be added together arithmetically, but are correctly combined by a Root-Sum-Squares (RSS) calculation. So only if these other errors are small will linearity error be the dominant source of measurement uncertainty. Otherwise, the weighting effect of the other errors can lead to serious uncertainties about the measurement results. This is also one of the reasons that trying to measure linearity error is more complicated than it might seem. Not only must there be the ability to minimize the effects of ambient factors like temperature and humidity, but it is important to note that sensor linearity error needs to be measured with equipment having at least ten times the desired precision of the linearity error itself, which usually means highly precise equipment normally found only in metrological calibration or national standards laboratories.


How Linearity Error is Specified

The maximum linearity error using a BFSL reference for a unipolar output sensor is usually expressed as a (±) percentage of Full Scale Output or Full Span Output (FSO). For a bipolar output sensor, its maximum linearity error is expressed as a (±) percentage of Full Range Output (FRO), i.e., from (-) FSO to (+) FSO.


To illustrate the effects of linearity error, consider a sensor with a range of 0 to 2 inches, an output of 0 to 10 V DC, and its linearity error specified as ±0.25% of FSO. The sensor has a scale factor of 5 Volts per inch and an FSO of 10 V DC, so non-linearity could cause an error of ±25 mV in the output, which is equivalent to an error of ±0.005 inches. The user must then decide whether this level of error is tolerable. This is illustrated by Figure 1 below, which shows both the sensor's analog output in blue and its point-by-point error from the reference line in orange. Keep in mind that the units of the error are so much smaller than the unit of output that if shown along the blue line they would be indiscernible in terms of resolution.

Title: Non-Linearity vs Ouput 

Figure 1

To summarize:


1. Linearity error is referenced to a Best Fit Straight Line calculated by the least squares method

2. Low sensor linearity error increases measurement precision and eases sensing system calibration

3. Errors due to a sensor's temperature, repeatability, hysteresis, and resolution can affect output linearity

4. Sensor errors do not simply add up arithmetically but must be combined by a Root-Sum-Squares calculation

5. A sensor's calibration equipment must be a minimum of ten times better than the measurement precision desired


The Sensor Connection warrants that units shipped will be free from defects in material and workmanship for a period of one (1) month from date of shipment. In the event that warranty service is required, The Sensor Connection will, at its discretion, either repair or replace unit(s) or product(s) found to be defective, provided that they are returned prepaid to The Sensor Connection. Our full return policy can be found here.

For additional pricing or technical questions, contact us now to speak to an experienced application engineer!

Most End-User's shouldn't be concerned with cold junction compensation since most modern day electronics that are used to monitor the EGT probe have this function built in so it is transparent to the End-User.
Thermocouples operate from a principal regarding dissimilar metals forming a junction that can be used to measure temperature.  The primary (or "hot) measuring junction is the tip of the sensor, but secondary junctions are formed when the thermocouple is terminated to your electronics (usually copper wires here).  This is typically referred to as the "cold" junction, since you do not want to measure the temperature at this termination point.
 The temperature at this "cold" junction has to be measured to remove the offset to the measurement at the "hot" end at the tip of the sensor.  If no "cold junction compensation" circuitry is used then your measurement would always be the ambient temperature at the termination to the electronics.  The compensation circuit takes the ambient temperature measurement at this termination point out of the equation.
The Cold Junction is measured at the very end of the long wires (red & yellow leads). This point of measurement is typically done where the thermocouple (or, the Muffler Clamp Probe) connects to the measuring electronics.

Most race teams and R & D customers closely guard their EGT tables.
There are many factors that affect EGT reading including:
Fuel type, timing, valve size, cam dwell, the fuel delivery system, turbo, and placement of the probe in the manifold. Even environmental issues including track air temperature, barometric pressure and humidity can affect EGT temperature readings.

The range of our standard muffler clamp is 2.5 inches. This can be made to just about any length for custom EGT orders.  We recently added a larger diameter muffler clamp that can be purchased as a spare part that expands to 5.5 inches (140 mm).

EGT probes are our core business. We take the quality of construction & materials very seriously. With proper application and respect of the probe, our customers can expect many years of service.  The probes are rated to 1200 degrees Celsius, so 720 degrees Celsius is well within the operating temperature range.

Our Instrumentation page has amplifiers that linearize the EGT probe's output, for example to 1 to 5 VDC or 0 to 5 VDC.

For additional technical questions, contact us now to speak to an experienced application engineer!