Are all exhaust gas temperature sensors the same?

As a result of this original equipment (OE) pedigree, independent workshops can also trust DENSO to supply replacement exhaust gas temperature sensors (EGTS) that are the perfect match for the vehicles they service and repair on a daily basis. However, it is also useful for technicians to know that there are two types of EGTSs, and to understand what distinguishes them and why.

Overview

Both types feedback important information to the vehicle’s engine management system (EMS) to help it to manage engine performance by providing real time temperature data to the engine control unit. They both also use the physical principle that the electrical resistance of some materials varies when the temperature changes, but one type uses negative thermal coefficient (NTC), and the other positive thermal coefficient (PTC). However, the variation in resistance to determine a change in temperature, is common to each design.

It is worth noting that in addition to managing engine performance, emission aftertreatment systems such as the exhaust gas recirculation and the injection of diesel exhaust fluid, also rely on accurate temperature control to ensure their effective operation.

PTC EGTS

PTC sensors have traditionally been the preferred technology for vehicle manufacturers (VM) that prioritise relatively low production costs and a wide field of applications. With this type, as the temperature increases, so does the resistance and in a linear manner. Largely due to their popularity, there are several industrial standards in place according to which these sensors can be measured.

Failure modes

Common failure modes include thermal runaway, where the device overheats and fails, and electrical overstress, which can damage the thermistor.

Lifetime expectancy

The lifetime expectancy of PTC EGTSs can vary, but they are generally designed to last for several years under normal operating conditions. Usually, PTC sensors are built in an open structure type, allowing them to respond quickly to temperature changes. However, this also means they are directly exposed and vulnerable to overheating or contamination.

NTC EGTS

In way of contrast, with NTC sensors, as the temperature increases, the resistance decreases and generally in a logarithmic, as opposed to linear, manner. However, the exact behaviour depends on its actual production process. They also offer greater sensor control, so are usually preferred for applications that need very accurate and fast temperature sensing and control of temperature changes and flow dynamics, such as quickly changing gas pressures, flow rates and temperatures. As a result, they are more expensive to manufacture, but are preferred by VM engineers, when accuracy and long operating lifetime is a priority compared to their cost. Furthermore, their specific production processes and operating behaviour makes these sensors unique. Consequently, a highly sensitive EMS can easily trigger errors, or show irregular readings, when an original DENSO NTC sensor is replaced by an imitation.

Failure modes

These sensors can fail due to thermal shock and moisture ingress, as well as mechanical stress, and over time, they can also experience drift in their resistance values.

Lifetime expectancy

NTC type sensors typically have a longer lifespan, often lasting 10 to 15 years or more, especially if they are hermetically sealed to protect against environmental factors.

Typical failure

Irrespective of the type, certain vehicle models are more prone to issues with EGTSs due to factors such as the design, their placement, and the complexity of the exhaust system. While it's difficult to pinpoint specific models without greater context, common issues include:

• Vehicles with diesel particulate filters (DPF): Faulty EGTSs can trigger unnecessary DPF regeneration, which can be inconvenient for the driver.
• High-performance or turbocharged engines: These engines often have more complex exhaust systems, making the sensors more susceptible to damage or failure.
• Older vehicles: Over time, wiring and connections can degrade and become unreliable, and the overall efficiency of the engine and exhaust system, of which these sensors are a part, can also decline. Overheating, leakage, carbon deposits, clogged soot filters or excessive oil contamination are among the most common causes for EGTS failures.

To further aid the correct diagnosis of the problem, it's always a good idea to check the specific service history and common issues for the model when analysing a vehicle that has symptoms that seem to relate to these kinds of temperature sensors.

Aftermarket solutions

DENSO Europe has included EGTSs in its EMS aftermarket range since 2013, although to date, these have been exclusively NTC type sensors. Recently however, the offering has been extended to include a range of PTC type sensors, which naturally provides its customers with a wider opportunity to supply and fit an OE quality replacement solution.

Ensuring its aftermarket programme gives its many customers a competitive advantage, is an important objective for the DENSO team, and a great example is the work put into consolidating the range of EGTSs it offers. This is a challenge that requires the kind of in depth OE knowledge that DENSO has accumulated as a trusted VM OE supply partner, over many years.

As an illustration, the product team has been able to consolidate 8 OE part numbers into a single DENSO sensor part number (DET-2018), whereas its nearest competitors require several to fulfil the needs of this single OE reference.

In short, the DENSO consolidation advantage allows its customers to hold less stock, which reduces complexity, delivers more sales and therefore, greater revenue.

In addition, DENSO has now introduced 35 new EGTSs that cater for 1,420 more applications, which are linked to 187 OE part numbers and cover in excess of 36 million vehicles.

Highlights include a host of VAG group vehicles from Audi to VW, as well as multiple applications for models from Chrysler, Jeep, Mercedes, Porsche and Smart. Together with the existing DENSO range, these EGTSs provide independent workshops with the ideal replacement solution.

The full details can be found here, in TecDoc and on the DENSO E-Catalogue.

Hands on technical support

As well as providing independent workshops with the OE products they need, DENSO also supports professional technicians with technical advice and direction through online learning, which is completely free and available by signing up to the DENSO e-learning platform.

Of the many courses on the site is one specifically on EGTSs, which is explicitly designed to improve the diagnostic skills and repair efficiencies of those with the responsibility of replacing these important components within the EMS system.