What an EGT probe actually is
An EGT probe is a thermocouple: two different metals joined at the tip. Heat that junction and the pair generates a very small voltage that rises in a known way with temperature. Measure the voltage, apply the calibration for that metal pair, and you have a temperature.
Thermocouples are grouped by letter (J, K, T, N, E, B, R, S and others). Each letter uses a different metal pair, which sets its temperature range, accuracy and lifespan. In the car world, K-type is the standard. It pairs a nickel-chromium leg with a nickel-alumel leg and covers roughly -328 to 2300 °F (-200 to 1260 °C). It's cheap, tough and accurate, with a maximum error of about 4 °F (2.2 °C), which is under 1%.
"EGT sensor" really describes the job, not the part. The same K-type thermocouple can measure oil, coolant, air or surface temperatures if you can get it where it needs to be.
Why you need an amplifier
Across its whole range, a K-type thermocouple only produces around 60 millivolts. That's far too small for an ECU input to read reliably. A thermocouple amplifier boosts that signal, converts it to a temperature and sends it to the ECU over CAN. Haltech offers the TCA2 dual channel and TCA4 quad channel amplifiers, plus eight-channel versions for per-cylinder monitoring on bigger engines.
Choosing a probe
| Probe style | Best for | Why |
|---|---|---|
| 1/4" open tip | Exhaust gas | The usual EGT choice. The exposed junction reacts in about 200 ms |
| 1/8" open tip | Exhaust or air where space is tight | Smaller body, same fast response |
| 1/8" closed tip | Fluids like oil or coolant | The sealed tip survives being submerged, but responds in about 800 ms |
| Washer style | Surface temperatures | Clamps under a bolt head |
Response time matters more than people think. If you want to catch a single misfire or a brief lean spike, a closed tip is simply too slow. For exhaust work we almost always use the 1/4" open tip thermocouple, which Haltech offers in a range of lead lengths so you can reach the amplifier without joins.
Installing probes so the numbers compare
EGT readings are most useful when you compare cylinders against each other. That only works if every probe sees the same conditions:
- Same distance from the cylinder head on every runner.
- Same depth into the pipe on every probe.
- Naturally aspirated engines: put the tip in the middle of the pipe.
- Turbo or supercharged engines: keep it shallow, no more than about 1/4" (6 mm) into the pipe.
If one cylinder reads high, check its probe position before you start adding fuel to it.
Getting the polarity right
Haltech's K-type probes come terminated in a two-pin mini K-type plug, so they just plug into the amplifier. If you're terminating your own, polarity matters, and a reversed thermocouple reads nonsense. On K-type:
- Yellow is positive, the chromium leg.
- Red is negative, the alumel leg.
The mini K plug is marked + and - inside, which makes it easy to check.
Setting EGTs up in the Haltech software
With the probes plugged into the amplifier and the amplifier connected to the ECU's CAN bus, head to the EGT entries under Sensors:
- Wiring: assign each probe to an amplifier channel and pick the sensor type. Choosing the Haltech K-type option loads the calibration for you.
- Display: set the minimum and maximum display temperatures and the warning limits.
- Diagnostic: watch each channel for sudden spikes or drops, which can flag a failing probe and set an error code.
EGTs can also protect the engine. Under Engine Protection, the cylinder shutdown settings can use individual EGTs to shut down a cylinder that's in trouble.
Troubleshooting
| Symptom | Usually |
|---|---|
| Temperature falls as the engine heats up | Probe wired backwards |
| One cylinder always reads differently | Different probe depth or distance from the head |
| Reading jumps or drops out | Damaged probe or connector, check the diagnostic page |
| No EGT data at all | Amplifier not on the CAN bus, or channel not assigned in Wiring |
EGT FAQ
Should I tune by EGT or by lambda?
Both. A wideband tells you mixture, while EGT shows how each cylinder is actually burning. Together they spot problems neither catches alone.
Can I use a thermocouple on an ordinary ECU input?
Not directly. The signal is too small, which is why an amplifier is needed. For thermistor-style sensors on a pulsed input, see using an SPI for temperature sensors.
How many EGT probes do I need?
One in the collector is useful, but one per cylinder is what really helps tuning. Our guide to which sensors you need covers the rest, or book a free consultation.
