by Michael Decipha Ponthieux
Posted: 9-15-2012
Last Updated: 8-1-2016


A HEGO, Heated Exhaust Gas Oxygen (o2 sensor) is a NARROWband (emphasis on narrow) LAMBDA sensor that reacts to oxygen content and in return generates a voltage.
Yes its a voltage generator, but can it power your house? I suppose if you had enough of them then possibly, anythings possible; but the most accurate answer here
would be NO. A lean mix (less fuel than stoich, lambda greater than 1.000) results in a voltage output of less than approx 0.4v, a rich mix (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 0.400 volts. Typically a healthy o2 will switch with a
lambda difference of 0.10 AFR from stoich, so a lean lambda value of 1.01 should result in a hego voltage less than 0.400 volts and a rich lambda value of 0.99 should result
in a hego voltage greater than 0.400 volts.

Some folks aren't too good with numbers and thats understandable, im not saying i understand cuz i don't, numbers don't scare me none, but if numbers aren't your
'cup of coffee' then we can simplify this in a very simplistic thought.

When thinking of HEGO voltage all you have to remember is Less = Lean, approx 0.400 volts is the breakpoint which determines that. If you have a logic based mentality,
when a HEGO is sitting on a desk in front of you if you carefully fondle and molest it with a volt meter, it will produce 0.000 volts = lean (no combustion) when exposed to
fresh air. if it doesn't, it would be in your best interest to vacate the premises promptly, this will enhance your chances to possibly probe another HEGO in the foreseeable future.

A HEGO is considered 'switched' when the voltage crosses the breakpoint, it matters not if it was lean and switched rich, or if it was rich and switched lean. "Switched" simply means that it crossed the threshold.

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 excess
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 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 turbo folks 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 don't take to kindly to pressure. Some of good 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. HEGO's were redesigned to include a 4th wire which is a dedicated ground. Most 92+ fords utilize a 4 wire
HEGO (excluding the foxbody).

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, at WOT all HEGOs should be reporting rich 0.8v+, if they
do not and the HEGOs are reporting accurate then the engine is lean = not good under high loads.

Hego Wiring

ECU Hego Ground Pin

Henry thought it was wise to ground the hego through the ecu, some eec 4s (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.

BE ADVISED, at this time you probably contain the information you sought after to resolve your HEGO issues (if any). If your a slacker, now would be the time to invert "switch" your
laziness before proceeding. In fact I forbade continuation without a proper and mechanically sound HEGO configuration/setup. The following abstract will briefly depict typical HEGO

HEGO Operation

A HEGO (properly functioning) should switch rapidly resulting in a full scale assault on your hego voltage indicator rapidly jumping from 0.000 volts up to 0.800 volts in a rapid fashion. A typical good functioning and warm hego should switch at least 3-5 times per second. A lazy or weak hego will typically switch very slow and take 2-3 seconds per switch in extreme cases. Cold hegos function similarly.


If you've set your ECU to correctly bias the hego, you will witness the HEGO camp out on one end of the voltage scale for a while (the side biased), then switch briefly and quickly retreat back like a coward to camp out again.

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.

I will not get into closed loop operation as that is indirectly related to HEGO's.

Continue reading on to the FUEL Write Up.

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