Guide · MegaSquirt

EGT on MegaSquirt

MS3 and MS3Pro read an exhaust gas temperature amplifier on a 0 – 5 V input, set up on TunerStudio's EGT/Thermocouple Settings page with a temperature at 0 V and at 5 V. This guide covers the inputs, the wiring, that setup, EGT actions and MS2/Extra's fixed scaling.

Checked against the MS3 TunerStudio reference and INI, the msextra.com hardware manuals, the MS2/Extra INI and AMP's MS3Pro manuals for firmware 1.5.2 and 1.6.1, September 2026.

00What you need

• An MS3 (with or without MS3X), an MS3Pro, or an MS2/Extra or MicroSquirt, connected to TunerStudio.
• An EG5 for the ECU's 5 V reference, or an EG12 for switched 12 V.
• A Type K probe and a bung or O2-port adapter for it, bought to the specifications under Recommended to use with.
• A multimeter.

Electrical

01Pick an input

MS2 and MS3 read 0 – 5 V inputs at 10-bit resolution. Which inputs are ready to use depends on the board.

• MS3 with MS3X: three inputs work as they are: pin 21 SpareADC, pin 22 EGO2 and pin 23 EXT_MAP, listed in TunerStudio as Spare ADC (ADC13), EGO2 (ADC12) and EXT_MAP (ADC11). If the MS3X card has a pull-up resistor fitted for that input (R67 for SpareADC, R66 for EGO2, R65 for EXT_MAP), remove it for the EG.
• MS3 without MS3X, and MS2 on a V3.0 or V3.57 board: JS4 and JS5 connect straight to the processor and need the protection circuit from the hardware manual (1 kΩ and 0.22 µF) first.
• MS3Pro: Analog In 1 – 3 on the original, 1 – 7 on EVO, 1 – 8 on EVO+ and Ultimate, and 1 – 4 on Mini (firmware 1.6.0 or later). Plug-and-play pinouts vary by vehicle.
• MS3 Gold Box: the plugs are different, so use the pinout in its hardware manual.
• MicroSquirt (cased): pin 29 SPAREADC (ADC6) and pin 5 SPAREADC2 (ADC7).
• Already in use? EGO2 and EXT_MAP may be. Each input takes one sensor, so pick a free one.

02Wire it

• The 5 V reference is shared with the TPS and any external MAP sensor, and MS3's is rated 0.1 A. The EG5 draws under 2 mA.
• EG12: MegaSquirt has no 12 V output, so use a fused switched 12 V feed.
• Use the sensor ground, not chassis. Every 4 – 5 mV of ground offset reads as about 1 °C.
• An EG5 goes on a 5 V sensor supply only, never 12 V. It has no reverse-polarity protection either, so check red and black before you power up.
• Neither output can go much above 5 V. The EG5 can't rise above its own 5 V supply, and the EG12 is capped at about 5.3 V.

Probe side

• The thermocouple cable is cut square. Take the jacket back, strip the yellow and red, and splice yellow to the probe's + and red to −. The cable is Type K extension, so the splice can sit anywhere along its run.
• Check the polarity. A reversed probe reads right at room temperature, so once it is spliced, warm the probe's tip: the reading must rise. If it falls, swap the probe's two leads at the splices. On a probe whose leads are not marked, the − lead is the one a magnet sticks to.
• Need more reach? Extend from the probe end with Type K extension wire, never copper.
• Ungrounded probes only. An ohmmeter from either lead to the sheath should read open.
• Mount the module somewhere cool, on the cabin side of the firewall or an inner fender: under 50 °C for an EG5, under 100 °C for an EG12.

Software

03Set it up in TunerStudio

On MS3 and MS3Pro:

1. Open Advanced Engine → EGT/Thermocouple Settings.
2. Set No. EGT inputs. If there's an Input configuration setting, set it to Manual.
3. Enter Temp At 0V and Temp At 5V. Units follow the project's temperature display setting.
   EG5: 0 and 1000 °C, or 32 and 1832 °F.
   EG12: 0 and 1250 °C, or 32 and 2282 °F.
4. Under Data capture, set EGT 1 Channel to the input you wired.
5. Burn, then power-cycle the ECU. Changing the number of EGT inputs or a channel needs a power cycle.

Temp At 5V is past the EG5's range on purpose. The line through 0 V and 5 V is the same line that passes through the calibration points above. And watch the manual's example of 5 V = 2282 °F/1500 °C: 2282 °F is 1250 °C.

One calibration covers every EGT channel, so an EG5 and an EG12 can't share it. Put the second one on Advanced Engine → Generic Sensor Inputs instead, with Transformation set to Linear and its own 0 V and 5 V values.

MS2/Extra and MicroSquirt have no EGT page. The EGT gauges (in Sensor inputs 2) read ADC6 and ADC7 with scaling fixed in the INI: 0 – 1000 °C across 0 – 5.1 V by default (978 °C at 5 V), or 0 – 1250 °C across 0 – 5 V with EGTFULL set at the top of the MS2/Extra INI. EGTFULL matches an EG12; an EG5 on the default reads about 2 % low.

A dash means past that model's range.

04See it and log it

• Gauges: category EGT inputs, gauge EGT 01. The EGT1 log field appears once the channel is enabled.
• The EGT warn and EGT shutdown indicators show what the EGT actions are doing. EGT fault lights when the sensor check flags the input.
• A generic sensor shows under Generic sensor inputs, logged under its Field Name.

05Optional: EGT actions and the check engine light

On MS3 and MS3Pro, the same settings page can add fuel or shut the engine down above a temperature. The warning LED is the one set up for AFR safety.

1. Turn on EGT Actions, set Warn Temperature, and set Added Fuel and Add To for enrichment above it.
2. For a shutdown, turn on Shutdown Engine and set Time > Warn Temp and Max Temperature. The shutdown options themselves are under AFR safety.
3. In CAN-bus/ Testmodes → Check Engine Light, turn on Sensor Checking. EGT Sensor Check with Minimum EGT and Maximum EGT then flags a bad sensor (flash code 9). EGT Triggers Limp Mode is under CAN-bus/ Testmodes → Limp Mode.

Troubleshooting

Every reading is a few percent off

Check that the black wire is on the sensor ground and that the 5 V reference reads close to 5.0 V with the engine running.

No EGT reading

Check that No. EGT inputs is at least 1, EGT 1 Channel is set to the input you wired, and the ECU has been power-cycled since.

It reads near full scale on a cold engine

The probe is disconnected or open. Both modules drive their output high when the probe circuit is open, so an open probe reads hot, not cold.

The reading falls as the exhaust heats up

The probe's leads are reversed at the splices. A reversed probe reads right at room temperature, then falls toward 0 °C as its tip heats. Swap the probe's two leads.

The reading jumps with revs, fans or the alternator

You have a grounded probe, or the black wire is on chassis ground. Fit an ungrounded probe and use the sensor ground.

It reads high once the engine bay heats up

The module is mounted somewhere too hot, or there's copper wire between the probe and the module.