Cirrus SR22: Exhaust gas temperature probe failures and fixes

The Exhaust Gas Temperature (EGT) probes in a Cirrus SR22’s Continental IO-550-N engine are critical sensors providing real-time data to the sophisticated engine monitoring system, typically integrated into the Garmin Perspective (or older Avidyne) avionics. EGT readings are invaluable for engine management, allowing pilots to precisely lean the fuel mixture for optimal performance, fuel efficiency, and to avoid harmful engine operations. For Cirrus SR22 owners in California, Arizona, and Nevada, understanding EGT probe failures and their fixes is essential for effective engine management and safe flight.

At The Aero Center, we specialize in the advanced systems of the Cirrus SR22. Our 24/7 maintenance center ensures prompt and accurate diagnosis of engine instrument issues, including EGT probe failures, minimizing your aircraft’s downtime. We operate with the principle of Authority, ensuring all diagnostics and repairs adhere strictly to Continental’s and Cirrus’s maintenance guidelines and FAA regulations.

IMPORTANCE OF EGT READINGS

EGT is a primary indicator of the air-fuel mixture entering a cylinder.

  • Peak EGT: Represents the stoichiometric mixture where all fuel and oxygen are consumed, resulting in the highest exhaust gas temperature.
  • Rich of Peak (ROP): Excess fuel flow provides cooling, leading to lower EGTs. Used for high power operations and engine cooling.
  • Lean of Peak (LOP): Excess airflow provides cooling, also leading to lower EGTs. Used for fuel efficiency and specific engine operations, particularly on advanced engines like the IO-550-N in the SR22.

Accurate EGT readings are vital for:

  • Mixture Leaning: Allowing pilots to precisely lean the mixture for desired operations (e.g., best power ROP, or specific LOP settings).
  • Engine Health Monitoring: Consistent EGT readings across cylinders, along with Cylinder Head Temperature (CHT), help detect issues like fouled spark plugs, clogged fuel injectors, sticky valves, or induction leaks. Anomalies in EGT can signal an issue long before it becomes critical.
  • Preventing Detonation/Pre-ignition: While CHT is the primary indicator, EGT trends can support proper mixture management to avoid harmful combustion events.

COMMON EGT PROBE FAILURES AND SYMPTOMS

EGT probes are thermocouples, typically Type K, designed to withstand extreme exhaust temperatures (up to 1,300°C / 2,372°F)¹. They are robust but subject to wear due to heat, vibration, and corrosive exhaust gases.

  1. Erratic or Fluctuating Readings:
    • Symptom: The EGT reading for a single cylinder jumps around wildly, changes rapidly by hundreds of degrees, or fluctuates erratically, even if the engine is running smoothly. This is a very common symptom of an intermittent connection or a failing probe itself².
    • Cause: Often due to internal wire breakage, loose or corroded connections in the wiring harness, or a failing thermocouple junction within the probe itself.
  2. Abnormally High or Low Readings (Constant):
    • Symptom: One cylinder’s EGT consistently reads significantly higher or lower than the others (e.g., 200-300°F different), without any other accompanying engine roughness or CHT anomaly.
    • Cause: A degraded probe that is still somewhat functional but providing an inaccurate voltage output. Less likely to be an engine issue if CHTs are normal and the engine runs smoothly².
  3. “Stuck” or Unresponsive Reading:
    • Symptom: The EGT reading for a cylinder doesn’t change with mixture adjustments or engine power changes, or it might display a fixed value (e.g., ambient temperature or a high/low extreme) or a “red X” on the Garmin display.
    • Cause: Complete internal breakage of the probe wiring, a totally failed thermocouple junction, or a complete open circuit in the wiring.
  4. Slow to Respond:
    • Symptom: The EGT reading is sluggish to react to mixture changes compared to other cylinders.
    • Cause: Often due to contamination on the probe tip or internal degradation reducing its sensitivity.

TROUBLESHOOTING EGT PROBE FAILURES (FOR A&P MECHANICS)

Troubleshooting an EGT probe issue typically involves a systematic process to differentiate between a faulty probe, wiring problems, or an actual engine anomaly.

  1. Pilot Interview & Data Analysis:
    • Gather detailed information from the pilot: exact symptoms, flight conditions when observed, recent maintenance.
    • Crucially, if the Cirrus is equipped with a data logging system (like Cirrus’s CAPS or aftermarket engine monitors), download and analyze the engine data. Trends in CHT, fuel flow, manifold pressure, and other EGTs can help confirm if it’s a sensor issue or a genuine engine problem².
  2. Visual Inspection:
    • With the cowl removed, visually inspect the suspected EGT probe. Look for:
      • Physical Damage: Kinks, cracks, or severe carbon buildup on the probe tip.
      • Connection Integrity: Check the security and cleanliness of the probe’s wiring connections (often spade connectors) at the probe and further down the harness. Look for corrosion, fraying, or loose crimps.
      • Mounting: Ensure the probe is securely clamped or screwed into the exhaust riser. A loose probe can give erratic readings or break off.
  3. Swap Test (Preferred Initial Mechanic Step):
    • If a specific cylinder’s EGT is suspect, a common diagnostic step is to swap the suspect EGT probe with a known good probe from another cylinder².
    • Run the engine (or simulate conditions that caused the symptom). If the anomaly moves with the probe to the new cylinder, the probe itself is faulty. If the anomaly stays with the original cylinder location, the issue is likely in the wiring harness for that cylinder or an actual engine problem with that cylinder.
  4. Resistance/Continuity Test (Ohm Meter):
    • EGT probes are thermocouples, which generate a tiny voltage when heated. However, a basic continuity or resistance check with an Ohm meter can help identify a completely failed probe.
    • Disconnect the probe wires from the harness. A good Type K thermocouple probe should show very low resistance (typically less than 1 Ohm) between its two leads. An “open circuit” (infinite resistance) indicates a broken internal wire or thermocouple junction.
    • Also, check for continuity between either lead and the probe body (grounded). Most aviation EGT probes are “ungrounded” or “isolated” thermocouples, meaning there should be no continuity to the probe body. Continuity to the body can indicate an internal short.
  5. Wiring Harness Check:
    • If the probe tests good or the problem moved to the wiring (after a swap), check the continuity of the individual wires in the harness from the probe connection point back to the engine monitoring unit. Look for intermittent breaks or high resistance in the wiring.
    • Twisted pair wiring and shielding are critical for EGT probes due to their low voltage output; damage to shielding can introduce electrical noise.

FIXES FOR EGT PROBE FAILURES

  1. Probe Replacement:
    • If the probe itself is confirmed faulty (e.g., by a swap test or continuity check), it is replaced with a new, FAA-approved EGT probe specific to the Cirrus SR22’s engine and monitoring system.
    • Cost: EGT probes for the Cirrus SR22 typically range from $120 to $250 per probe (part cost)¹. Labor for replacement is usually minimal unless access is difficult.
    • Installation: Ensure proper installation, secure clamping/screwing into the exhaust riser, and correct connection of the thermocouple leads (polarity is crucial; typically Yellow is positive, Red is negative for Type K thermocouple wire¹).
  2. Wiring Repair/Replacement:
    • If the issue is in the wiring harness, the damaged section is repaired or replaced. This often involves replacing crimped spade connectors with new, clean ones, or splicing in a new section of approved thermocouple extension wire if the damage is further along the harness.
    • Caution: When splicing thermocouple wires, it’s critical to use the correct thermocouple-grade extension wire (Type K) and specialized thermocouple connectors or methods that maintain the integrity of the thermocouple circuit. Standard electrical connectors can introduce errors.
  3. Connection Cleaning:
    • Sometimes, simply cleaning corroded or dirty electrical connections at the probe and within the harness can resolve erratic readings.
  4. Engine Diagnostics (if not probe/wiring):
    • If the EGT probe and wiring test perfectly, and the symptom persists, the problem lies within the engine itself. This could involve:
      • Clogged Fuel Injector: Leaner mixture, higher EGT.
      • Fouled Spark Plug: Incomplete combustion, higher EGT, often rough running.
      • Sticky Valve: Can cause erratic or high EGTs due to improper sealing.
      • Induction Leak: Leads to leaner mixture in a specific cylinder, causing higher EGT.
      • Ignition Issue (e.g., failing magneto lead): Can affect combustion efficiency and EGT.

For Cirrus SR22 owners in California, Arizona, and Nevada, recognizing the subtle signs of an EGT probe failure and seeking professional diagnosis from The Aero Center is a smart investment. Accurate EGT data is fundamental to maximizing engine life and ensuring safe, efficient flight operations.

The Aero Center is located at William J. Fox Airfield KWJF | Lancaster, CA. Contact us at 209.885.6950 for questions or appointments.

Footnotes:

  1. Michigan Avionics. “EGT Thermocouple Probe Type?”. https://support.michiganavionics.com/portal/en/kb/articles/egt-thermocouple-probe-type
  2. SavvyAviation. “Data-Driven Diagnosis”. https://www.savvyaviation.com/data-driven-diagnosis/
  3. McFarlane Aviation Products. “EGT and TIT Thermocouple Probes”. https://www.mcfarlaneaviation.com/products/category/egt-and-tit-thermocouple-probes/
  4. Zenith Aircraft Builders and Flyers. “EGT Probe Failure Symptoms”. https://zenith.aero/forum/topics/egt-probe-failure-symptoms
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