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V3 MicroSquirt® - QuickStart Guide

Exhaust Gas Oxygen Sensor

An exhaust gas oxygen sensor (EGO) is very useful for setting up your MicroSquirt® EFI controller volumetric efficiency table, and while it is highly recommended, it is not essential.

Your MicroSquirt®™ EFI controller can read from one or two oxygen sensors. People who have engines with separate cylinder banks (V6, V8, etc.) will have to make a choice:

It is very important that there be no exhaust gas leaks upstream of the oxygen sensor. Certain conditions can draw ambient air into the exhaust, causing the MicroSquirt® EFI controller to compensate for an apparently lean condition. This will falsely create a rich mixture in your system that can be difficult to diagnose. Those who have converted late-model “emission” engines, should be careful to understand and modify the operation of any air injection systems they have to be sure air is not being pumped into the exhaust ahead of the oxygen sensor(s) during “closed-loop” operation.

Closed loop refers to those times when an EFI computer is using the feedback on the mixture provided by the oxygen sensor to effectively control the injected amounts. For the MicroSquirt® EFI controller, this is when the engine:

See the Settings/Enrichments section of TunerStudioMS.

Open Loop” refers to those times when the MicroSquirt® EFI controller ignores the feedback from the oxygen sensor. Note that the MicroSquirt® EFI controller also allows you to set limits on how much (EGO + Limit (%)) and how fast (EGO Step (%) and Ignition Events per Step) the oxygen sensor feedback can influence the injected amount.

One, three and four wire narrow band O2 sensors [NB], and two wide band sensors [WB] are currently available on the market. MSD offers a heated sensor under part number 2330.

Narrow band O2 sensors are designed to measure stoichiometric [chemically correct] air/fuel mixtures [A/F] of 14.7:1 to allow catalytic converters to work efficiently. Narrow band sensors always have one wire for the sensing function. Additional wires are for the heater and its ground (3 wire sensor), and possibly an additional wire to ground the sensor itself (4 wire). The sensor needs to be quite hot to operate. The heater keeps the sensor at operating temperature under more conditions.

Examples are:

The difference between the heated (3 or 4 wire) O2 sensor and a non-heated (one wire) sensor is the A/F ratio sensing of warm up and low load conditions. The heated sensor uses an internal coil to heat the ceramic element to the desired 400° Celsius in 30 or 40 seconds. This temperature is also maintained when the car is at idle for extended periods of time or is under low load conditions where the exhaust gas temperatures fall below 400°C.

Under other operating conditions the exhaust gas temperature will be much greater than 400° C. and heating is not necessary. The non-heated sensor relies on the exhaust gas heat to keep it at its operating temperature. This works most of the time but there is still times that it might drop below its desired operating temperature and show a leaner than actual mixture as its output drops to zero.

A 1-wire sensor is as good as a 3-wire provided that it is always at operating temperature. If you cruise around for a bit with the engine at low load, the O2 sensor COULD cool down. If you do not have exhaust gas temperature [EGT] monitoring then you cannot be sure. Once warm, a 3-wire O2 sensor will stay warm. For most of us the one wire will prove to be adequate. A 4-wire has a shielded cable. You only need to ground the shield at one end. In many installations there is not enough voltage drop from the manifold to ground to make shielding worth the bother, but every little helps. So the more wires the O2 sensor has, the more situations in which the sensor will be active and accurate, but you are still stuck with knowing whether you are rich or lean, but not by how much.

MicroSquirt® EFI controller software has full support for Wide Band (WB) EGO sensors (12x12 AFR table, etc.).

With a narrow band sensor, we can really only tell for certain whether we are rich or lean, but not by how much. If you look at the graph, you can see that for a narrow band sensor, the 12.5:1 AFR required for maximum power can give O2 voltage from 0.8 to 0.95 (depending on exhaust gas temperature), yet this same range of O2 voltages can indicate mixtures from 10:1 to 14.5:1. So we cannot use it reliably to set mixtures for full power. With a wide-band sensor, 12.5:1 corresponds to 2.08 volts, and 2.08 volts means 12.5:1. Thus there is no ambiguity over AFR and voltages. We can measure any mixture in the range we are likely to use, from full power through to maximum economy.

To configure the very popular Innovate LC-1 for use with the MicroSquirt® EFI Controller and MegaTune, see the instructions here: DIYAutoTune LC-1 Configuration Instructions.

Some people have asked, "Why even have the VE table if you have a wideband?". The answer has some technical aspect in that the sensor and controller response times might not be adequate, but there are a number of practical considerations for going to 'EGO only' control of fuelling as well. An EGO sensor (either narrow band or wide band) has a number of conditions under which it could mislead the MicroSquirt® EFI Controller. Among these are:

That's your MicroSquirt® controller code has limits on the EGO correction, and doesn't use EGO feedback under a number of conditions. Under the conditions they do work, though, a wide band sensor is a very nice tool to have.

If your car did not come with an oxygen sensor, you can add one. The thread for all oxygen sensors [including wide-band] is: 18mm x1.5mm - i.e., a metric thread 18mm in diameter with a pitch of 1.5mm, the same as 18mm spark plugs.

To fabricate a bung to mount your sensor, you can go to your local automotive parts store and look in the section with all the HELP products. Pick up a package of "18mm Spark Plug Anti-foulers". Cut off the externally threaded part, and weld the rest to your manifold or down pipe. This works wonderfully and you can do 2 cars for 4 bucks. You can also get them from a speed shop under Holley part number Holley's 534-49 or MSD part number 2335 for about $10. Or you can go to muffler shop and ask for an O2 bung. And they can weld them in for you too.

The hex portion of the oxygen sensor is is 22mm, and a 7/8" wrench will work for installing/removing. Unless otherwise specified, the oxygen sensor should be torqued to 30 lb·ft (40 N-m). Apply anti-seize (ex. GM #5613695) to the threads before installing.

If you have installed a heated sensor, you will need to wire the heater in the sensor. Connect one heater wire into ignition-switched 12 volts, the other heater wire goes to ground. The heater wires are the often thicker than the signal and ground wires, and are sometimes white. O2 sensor heaters typically are about 18 Watts (1.5 Amps), so use an appropriate wire gauge and fuse. The heating element is Positive Temperature Coefficient PTC  (non-linear) resistor. When it is cold it has low resistance and draws about 2.4 Amps at 12 volts. As it heats up its resistance increases and current reduces down to much lower values (below 0.5 Amps). Thus it is self-regulated and when warm the current draw can be neglected. Most new cars have it connected in parallel with the fuel pump (which draws 8 Amps and more).

Colin Gebhart and Scott Campbell have an excellent page on the wiring of various oxygen sensors at:


If you are using a wide-band sensor, you will have to calibrate your MicroSquirt® controller to read it correctly. If you are lucky, you may have one of the wide band sensors already programmed into TunerStudioMS.

If your sensor is not in TunerStudioMS's list, then you will need two voltage/AFR pairs (i.e. the voltage of the sensor's controller at two different air/fuel ratios). Ideally these AFRs will span the operating range. Your Wide Band Controller should come with this information.

Then you need to open TunerStudioMS, and under 'Tools→Calibrate AFR Table' either select your sensor or enter the Volts/AFR pairs. Then click the 'Write to Controller' button to burn the table. The table will stay in your MicroSquirt® controller's memory until you reburn it, or load new code.

Also note that this sensor's calibration is NOT retained in the MSQ when you load new code and restore your settings by loading a previous MSQ file. You must recalibrate this and other sensors.

If you have any questions or problems that can't be answered from the links above, or a search the MicroSquirt® manual:

you can ask questions at the MicroSquirt® support forum which is at: Click the links for more information.

MicroSquirt® and MicroSquirt® controllers are experimental devices intended for educational purposes.
MicroSquirt® and MicroSquirt® controllers are not for sale or use on pollution controlled vehicles. Check the laws that apply in your locality to determine if using a MicroSquirt® or MicroSquirt® controller is legal for your application.
©2011 Bruce Bowling and Al Grippo. All rights reserved. MicroSquirt® and MicroSquirt® are registered trademarks. This document is solely for the support of V3 MicroSquirt® boards from Bowling and Grippo.