by: Michael Decipha Ponthieux
Posted: 2012-09-15
Last Updated: 2023-06-21


A HEGO, Heated Exhaust Gas Oxygen sensor (an o2 sensor) is a NARROWband (emphasis on narrow) LAMBDA sensor that reacts to oxygen content and in return generates a voltage.
A lean mix (less fuel than stoich, lambda greater than 1.000) results in a voltage output of less than approx 0.420 volts, due to no combustion to react and create voltage. Likewise a rich mixture (more fuel than stoich, lambda less than 1.000) causes the oxygen sensor to 'react' to the fuel content and generate a voltage greater than approx 0.420 volts. Typically a healthy o2 will switch with a lambda difference of 0.01 lambda from stoich. Thus a lean lambda value of 1.01 should result in a hego voltage less than 0.420 volts and a rich lambda value of 0.99 should result in a hego voltage greater than 0.420 volts.

Hego Switch Voltage

The hego switch point is the voltage at which the hego reads either rich or lean. When thinking of HEGO voltage all you have to remember is Less = Lean, approx 0.420 volts is the switch point which determines that. Logically thinking a HEGO resting on a desk in front of you (or still in the package) will produce 0.000 volts = lean (no combustion) when exposed to fresh atmospheric air.

Typically the o2 sensor switch point doesn't change much but can sometimes be affected by exhaust. Reducing the o2 sensor switch point from 0.420 to 0.100 is not too uncommon on heavily modified vehicles that tend to run rich. On the other side of the spectrum older less efficient big block and larger cube engines typically want a higher o2 sensor switch point possible 0.550 to 0.750 volts. It is generally not necessary to change the hego voltage switch point as it is a physical attribute of the hardware specifically.

A HEGO is considered 'switched' when the voltage crosses the switch point. It matters not if it was lean and switched rich, or if it was rich and switched lean. "Switched" simply means that it has crossed the threshold. This can be observed while a hego is controlling fuel in closed loop by watching the lambse (commanded afr) ramp leaner commanding a higher AFR until the hego switches lean (voltage drops below 0.420 volts) and then it will began ramping the lambse richer reducing afr until the hego switches back rich in a never ending "closed loop". Closed loop operation is dependent upon the HEGOs as the HEGOs have full control of fueling in closed loop.

Hego Placement

A HEGO requires a significant amount of heat to function accurately, a great deal of folks (dummies and guru's alike) do not realize that an engine that idles for an excessive amount of time WILL cause the hego's to cool down and read inaccurately. As a result the hego will generate less voltage and the ECU in closed loop will dump fuel to get it to switch == not good since its not really lean just cold and not reading correctly.

HEGO placement will effect its readings as well. Ideally you want the HEGO as close as reasonably possible to the head as this will not only insure it remains warm but will also reduce the hego delay so the ECU can make corrections faster and more accurately.

Exhaust piping diameter will further exacerbate a faulty reading. A larger diameter pipe, lets say 9" for example (fear not the 9 inches), will just about render a HEGO null at idle and low rpm (low flow condition) since the ratio of exhaust volume to pipe volume is significant. The 9" exhaust was just to make the visualization clear (im one for size), this is a factor to be considered with much smaller diameter piping as well. Although not typically an issue it can arise with turbos running larger diameter downpipes I.E. 5"+

Speaking of which, an o2 should always be mounted AFTER THE TURBO. Mounting one in the hotside can cause inaccurate readings from not only the excessive heat but they also don't take to kindly to pressure. Some people have had ok results, most do not. I recommend to avoid it when possible.

4 Wire Hego's

It is imperative that the exhaust be completely sealed (no exhaust leaks) and that the HEGO's get a good clean and solid ground. The older 3 wire sensors do not have a dedicated ground; they receive their grounding through the exhaust pipe == not ideal. Due to this issue HEGO's were redesigned to include a 4th wire which is a dedicated ground. Most 92+ fords utilize a 4 wire HEGO.
Note: On those older 3 wire o2 sensor setups such as the 80's speed density fox body- the hego ground reference wire is typically an o-ring on the rear of the head that MUST BE CONNECTED TO GROUND.

One issue that can have you 'chasing your ass' is faulty heater power +12v. A HEGO with the heating element burnt out or not receiving heater power at all will typically result in the hego reporting lean at higher loads where the excess exhaust flow has cooled the sensor. This is something to be mindful of and should be verified at WOT all HEGOs should report rich with a voltage greater than 0.800 volts. If they do not and the HEGOs are reporting accurate then the engine is lean == not good under high loads such as half throttle and above.

Hego Wiring

ECU Hego Ground Pin

On some early ecu's Henry thought it was wise to ground the hego through the ecu. Some EEC-IV's (ALL 91- and older ecu's) have a hego ground pin that runs through the engine harness and grounds at the back of the head or the firewall in that vicinity. I don't believe it was ever documented as to the specific drug(s) Henry's engineers were ingesting but one can only infer they were influenced by mind altering substances.

HEGO Operation

A HEGO (properly functioning) should switch rapidly resulting in a small scale assault on your hego voltage indicator rapidly jumping from below 0.420 volts and above 0.420 volts in a rapid guerrilla warfare fashion. A typical good functioning and warm hego should switch a minimum of 3-5 times per second. A lazy or weak hego will typically switch very slow and often take 2-3 seconds per switch in extreme cases. Cold hegos function similarly switching slowly since they cannot generate voltage correctly due to the slow reaction with combustion.

Rear HEGOs

The rear o2 sensor's can serve two purposes. The primary purpose on 2011+ vehicles with front wideband o2 sensors is to CALIBRATE the front wideband sensors. The rear o2 sensors should NEVER BE REMOVED ON 2011+ VEHICLES for this very reason. On older OBD-II vehicles from as early as 1994 up to 2010 the rear o2 sensors were primarily used to verify the catalytic converter capacity to verify the cats were functioning correctly. On these vehicles the rear o2's can also correct the front o2s as well with a control algorithm known as FAOSC.


Fore Aft Oxygen Sensor Correction is a software algorithm that trims the front o2 sensors based on the rear sensor feedback in order to keep the AFR in the range where the catalytic converter is most efficient. This is the primary purpose of closed loop fuel control.


HEGO Bias is yet another software algorithm. In short the bias determines how long it stays on either side of stoich. A 0 bias means it ramps back and forth in closed loop switching normally. If you've set your ECU to bias the hego lean (positive bias) you will witness the HEGO camp out lean for a while (the side biased) and when it switches rich it will quickly go back lean for a while. This is due to the ecu ramping the lambse far lean when the hego switches rich and slowly ramping the lambse down to get the o2 to switch again. This is not a hardware control it is purely software control. The o2 is still physically producing voltage when it reacts with rich combustion.

NOTE: A hego bias relies heavily on the fuel modeling to be dialed in correctly, if fuel has not be dialed in then the hego bias MUST be set to all 0's to disable biasing otherwise fueling issue will arise.

More in depth closed loop operation is detailed in the fuel write up below.

Continue reading on to the FUEL Write Up.

Return Home
Jump to Forum
www.EFIDynoTuning.com - EFIDynoTuning LC - New Orleans, LA