Thermocouple resistance can signal EGT sensor failure and affect engine safety.

Outline / Skeleton

• Hook: Why tiny thermocouples and their resistance matter for the engine’s “temperature heartbeat.”

• 1. Quick primer: what thermocouples do in an EGT system

• The Seebeck effect, two junctions, voltage that tracks temperature difference.

• 2. Where resistance comes from

• Aging metal, corrosion, poor connections, insulation wear.

• 3. How resistance changes readings

• Higher resistance can dampen the signal, introduce drift, or mask true temperatures.

• 4. Real-world consequences

• Misinterpreted engine Health, unsafe operating margins, potential damage if readings lie.

• 5. Signs of trouble

• Erratic readings, slow response, occasional open circuits, corrosion at fittings.

• 6. Troubleshooting and upkeep

• Visual inspection, resistance checks with a multimeter, clean connections, replace bad thermocouples, verify wiring paths.

• 7. Real-world analogy and closing takeaways

• Think of the EGT gauge as a furnace monitor; if the sensor gets stiff, the readout isn’t trustworthy.

• About 1,100–1,350 words.

How resistance in thermocouples can impact an EGT system

If you’ve ever watched an EGT gauge swing and wondered what’s really happening under the hood, you’re not alone. Exhaust Gas Temperature readings are the engine’s temperature heartbeat, giving you a quick pulse on performance, efficiency, and safety. But that heartbeat relies on tiny devices called thermocouples. They’re small, rugged, and incredibly telling—when they’re behaving, that is. Resistance in those thermocouples can throw a wrench in the works, sometimes signaling a problem rather than just providing data.

Let me explain what thermocouples are doing in plain terms. A thermocouple works on a simple principle: when two different metals are joined, they generate a small voltage that changes as the temperature changes. In an EGT system, one junction sits in the exhaust stream and the other is near the engine bay. The wiring from those junctions turns heat into voltage, and the gauge translates that voltage into a temperature reading. It’s a tidy piece of physics wired into an instrument panel—pretty elegant, actually.

But nothing in aviation loves to be elegant at the cost of reliability. Resistance is the quiet saboteur here. Over time, metal can corrode, insulation can wear, and connections can loosen. All of that raises the electrical resistance in the circuit. Think of resistance as friction in a water pipe: the more friction, the harder it is for the signal to get through. In a thermocouple circuit, that friction shows up as a distorted voltage signal. The result? A reading that’s not quite right.

Where resistance comes from is often about age and environment. Thermocouples sit in hot, sometimes dirty exhaust and then have to survive the engine’s vibration, humidity, and thermal cycling. The metal compounds used to make the thermocouple can oxidize or corrode at the junctions where they’re brazed or welded. The insulation that keeps the wires from shorting can degrade. Connections at the sensor head or along the wiring harness can loosen. All of these factors raise resistance, and each one nudges the system toward less accurate readings.

So, how exactly does that resistance distort the readout? In a couple of tangible ways:

• Signal damping and drift: Higher resistance can dampen the tiny thermoelectric signal the system relies on. The result is readings that lag behind real changes in exhaust temperature, or drift gradually away from the true value. In practice, that means your gauge might show a temp that’s a notch off from what’s actually happening in the exhaust.

• Increased noise and offset: Resistance isn’t just a single number; it can introduce noise into the voltage. A noisy signal looks jittery on the gauge, especially during transitions—think throttle changes or during climbout. An offset makes a reading consistently too high or too low, which can mislead you about engine efficiency or fuel-air balance.

• Open circuits and intermittent readings: If resistance climbs so much that a circuit behaves as if it’s open, you’ll get sporadic or completely missing readings. That’s a red flag right there—an open circuit usually means you’re not seeing a reliable temperature, which is a dangerous situation in the cockpit or the engine bay.

• Temperature-dependent changes: Some resistance changes aren’t constant. They can vary with weather, engine temperature, or vibration. The result is readings that seem to wander or bounce in a way that doesn’t match what you know the engine is doing.

What does this mean for engine operation? The short answer: inaccurate EGT data can lead to poor decisions. If you think the exhaust is hotter than it is, you might pull back fuel unnecessarily or, worse, run lean enough to overheat a turbine or piston. If you think the exhaust is cooler than it is, you might push the engine harder, chasing performance that’s not actually there. Either way, you’re straying from what the engine truly needs for safe, efficient operation.

You might be wondering: what would signal that resistance in a thermocouple is the culprit rather than a momentary fluctuation? Here are some signs to watch for:

• Readings that don’t track with RPM, power changes, or time in service. If the gauge seems out of step with what the engine is doing, it’s worth a look.

• Readings that drift slowly over a long flight or multiple cycles, even when the engine environment hasn’t changed.

• Erratic or jumpy readings during throttle transitions. If the temperature jumps unpredictably, resistance could be dampening or injecting noise into the signal.

• Open-circuit indications or completely flatlines in one or more channels. That’s not a gray area—if a sensor isn’t producing a readable signal, it needs attention.

• Visible signs of wear around the sensor head or along the sensor wiring: corrosion, frayed insulation, loose fittings, or burnt smells near the harness.

The maintenance side isn’t mysterious, but it’s precise. Here’s a practical way to approach this in the field or shop environment, without turning the task into a treasure hunt:

• Start with the sensor and its connections. A visual inspection goes a long way. Look for corrosion at the sensor heads, loose terminals, damaged insulation, or any signs of heat damage. Clean or reseat connections as needed, and make sure that all fittings are snug and properly sealed to prevent moisture intrusion.

• Check the wiring path. Ensure the wires aren’t kinked, pinched, or rubbing against sharp edges. Vibration can wear insulation over time, which nudges resistance higher and invites noise.

• Measure resistance. With the engine cooled and powered off, you can measure the thermocouple’s resistance with a multimeter. If you see a resistance value that’s far outside the manufacturer’s specified range, that’s a solid indicator the sensor or its leads are failing.

• Compare channels. If you have multiple EGT sensors, compare their readings to see if one channel is behaving differently. A lone suspect sensor often points to that particular thermocouple or its wiring rather than a system-wide issue.

• Temperature correlation tests. If possible, log EGT against engine parameters during a controlled run. A sensor that consistently lags or drifts when you expect a steady response is a red flag.

• When in doubt, replace. Thermocouples aren’t eternal. If a sensor shows signs of degraded performance, replacing it with a known-good sensor is often the simplest path back to trustworthy readings.

To put it more plainly: resistance issues aren’t a moral failure on the sensor—they’re a reliability check. They tell you something inside the system is not as healthy as it should be. Treat that signal with respect, not dismissal.

A few handy analogies and reminders can make this easier to digest in the heat of the moment. Think of the EGT sensor as the engine’s thermometer, but not a fragile one. It’s more like a sturdy old thermometer that has learned to resist a bit of noise. When the resistance climbs, the thermometer only pretends to be accurate; the numbers you read are the illusion. Your job is to verify, test, and replace as needed so the reading once again reflects reality.

And while we’re on the subject, let me offer a quick tangent that ties into the bigger picture. The EGT system is rarely alone; it’s part of a web of sensors and gauges that paint a complete picture of engine health. Pressure sensors, fuel flow indicators, turbine temperature readings, and manifold pressure all work together. If one sensor starts to drift because of resistance, it can ripple through the entire diagnostic story. That’s why consistent, careful maintenance matters. A single faulty thermocouple isn’t just an isolated nuisance; it can lead to misinterpretations that affect performance, efficiency, and safety.

So, what’s the bottom line? Resistance in a thermocouple can be a practical signal that something in the EGT system is failing. It’s not a mystery to solve with guesswork; it’s a cue to inspect, measure, and, if needed, replace. When the readings are trustworthy, you’re better equipped to tune the engine for performance without compromising safety. And that, in the end, is what good maintenance is all about: keeping the numbers honest so the engine behaves the way it should—steady, predictable, and safe.

If you’ve ever wrestled with an instrument that doesn’t quite show the truth, you know how frustrating it can be. The good news is that resistance is something you can manage with a methodical approach: check connections, verify wiring, test with a meter, and replace when necessary. It’s a small sequence that pays big dividends in reliability and peace of mind.

Takeaway: respect the thermometer, respect the signal. When resistance climbs, treat it as a clear signal to check the health of the thermocouple and its wiring. A little proactive attention now saves bigger headaches later, keeping the engine in its sweet spot—where readings are honest, and performance follows suit.