O2 Sensor

Last updated: April 12, 2000

O2 sensor

Intro to O2 Sensors

See the article on how the O2 sensor is used by the engine computer.


Date: Tue, 11 Apr 2000 21:07:17 -0400
From: "\(Mr\) Sandy Linthicum" (sandy-linthicum@nc.rr.com)

Closed loop is only intended for light load cruising. you go out of closed loop the moment you start loading the engine.

Latency of the o2 sensor is about 1/2 second (ie. readings are 1/2 second old when read). No prob for cruising but instant death at full power WOT.


Date: Wed, 14 Apr 1999 16:32:46 -0500
From: "Kevin T. Wyum" (aspi@winternet.com)

Here's the scoop as I understand it on this issue. First let me state that I have or have had both on my car. Both being the PFS hooked into a standard O2 and heated O2 as well as the fancy $2000 wide range lambda which happens to be built into my Motec currently on the car. The 4 wire wide range sensor can be had for a little less than $400 but yes that's what's there.

I'm not up on my reading with respect to the function of the wide range, but have done a reasonable amount on the normal and heated O2's in the past.


A normal O2 is actually very accurate, but only in a certain range which happens to be close to stoich or 14.7 : 1 or .45 volts as an output from a normal sensor. What happens is that the further from stoich you go with a normal O2 sensor the numbers become more inaccurate with respect to an actual oxygen ratio present. A wide range sensor is just like its name suggests, more accurate than a normal sensor at mixtures further from stoich.

Relative Numbers

I'm currently using a wide range for the motec, because that's the only thing it supports as well as my car being so far from any known 3rd gen configuration I can't use relative numbers. In this case I'm not speaking of the PFS data logging of numbers, only what is displayed on the older boxes with keypad. I would differ with what others have stated in that you should ignore the numbers displayed from a normal O2 sensor. While you can't extrapolate an actual AF ratio from the voltage on a normal O2 sensor once it deviates any significant amount from stoich you can use the voltage as a benchmark that others have used with success. If you recieve the same voltage as another person has on a normal O2 sensor you can assume that your AF is similar to theirs at that given time.You may not know exactly what AF that happens to be but you really don't need to know. Since we do have relative numbers that have worked for others in the past I would suggest that you can safely tune your car within reason using numbers from a standard O2 sensor up to the point of about 450+HP. So what voltage numbers are reasonable? From my experience of a few years doing it that way I found a good benchmark of .82 volts to be the leanest you want to allow during WOT and higher boost, above 10-12psi. Voltage higher than that will be richer and a bit safer until you reach a point of being excessively rich, which can actually cause damage of its own, such as clogging cats, diluting lubrication in the housings etc. So .82 -.86 is a good balanced range to be in. Just watch the keypad numbers at wide open throttle in 3rd and 4th gear at 5000-8000 RPM. The number should remain fairly constant. If the numbers drop below .82 let off the throttle and increase the fuel in that range and test again. If higher than .86 remove fuel from that range and test again. A little side observational info. When running leaded race gas the heated O2 does read a slight bit differently given all other conditions being the same. Off the top of my head it was about .76 = .82 or something like that. If someones using C16 or something ask me directly about numbers to be used.


From: Rob Robinette

How the O2 Sensor Works

The oxygen sensor detects oxygen in the exhaust in a way similar to a battery cell. Platinum layers on the inside and outside of the probe act as electrodes. Zirconium dioxide between the layers of platinum acts as an electrolyte. The inside layer of platinum is exposed to the air in the engine bay, the outer layer is exposed to the exhaust. When the O2 sensor is hot it generates positive voltage between the two layers of platinum. Negatively charged oxygen ions in the exhaust are attracted to the platinum and reduce the voltage that the Engine Control Unit and air/fuel gauge reads.

That's why a rich mixture = higher voltage from the sensor (more gas = fewer negative O2 ions to drop the voltage) . Just remember "L=L", Lean = Low voltage. You can attach a Volt Ohm meter (black to ground, red to the O2 sensor) to monitor your engine's air/fuel ratio. Full throttle with 0.82 volts is best performance but slightly higher voltage (richer) is safe. At part throttle the ECU will bounce the ratio from rich to lean. You only have to worry about full throttle. I suggest you have a copilot read the voltage while you accelerate at full throttle.

The 3 wire sensors are used to give power and ground to pre-heat the sensor so it starts working quicker, it has no effect after the sensor gets warmed up.

Replacement O2 Sensors

Date: Thu, 12 Jun 97 12:49:55 -0500
From: "Linthicum, Sandy"

I bought the NAPA part below and have it installed on my 93-R1. If I remember it was less than $60. It works fine and lets me mix in leaded race gas (100-115 octane) at the track for the same price per gallon as the no lead 100 octane race gas. Means I have to buy a lot less race gas to raise the overall octane to a meanful level. Leaded gas will eventually kill the heated O2 sensor but it takes a while (probably several seasons). It will probably kill an unheated O2 sensor on the first tank. By the way, one of my concerns was if I was going to have to use a funnel or get a gas can to use leaded fuel since the filler on cars is designed to not allow leaded gas in unleaded cars. At Road Atlanta at least, all the race gas is pumped from the same nossels (leaded & unleaded) so it is not problems (except for the Porsche PCA members who had been filling up with leaded Race Gas for 2 days in their new 911's with full emmissions intact - leaded gas also kills the CATs).

You will also probably want to get the 3 wire connector harness to permanately replace the wiring harness side connector for the O2 sensor. I got this at the Ford parts counter. It is a generic aftermarket item and cost me $10. Have the O2 sensor with you because there are two plug types and if you have my luck the first one you get will not fit.

Netblazer post:

You want one off a Ford Escort of all things. I grabbed the wiring harness connector off one at a wrecking yard (make sure you don't tell them what you are taking or they will FLIP since its part of the engine harness :)

NAPA part No. OS204

Should have 3 wires (2 white, and 1 black) hook the white wires to pos, 12, and gnd, and hook the black wire to the wire where the old one hooked up.


Date: Thu, 8 Jan 1998 10:32:38 -0500
From: Rob Robinette (robinett@postoffice.dca.net)

Napa part # OS204 for 86 Ford Escort, $39, 3 wires, black to O2 sensor, two white to power and ground (optional).

The oxygen sensor is located on the exhaust pipe (pre-cat) between the right front fender and the fire wall in the engine compartment. It is sticking out from the exhaust pipe and has one wire attached to it. There is a plastic connector on this wire above the sensor so you can disconnect it.

You will have to solder the new O2 sensor's black wire to this connector for reinstallation. The two white wires on the Napa sensor are for 12 volt power and ground and heats the sensor. A heated sensor supposedly works better with high octane racing fuel. You can leave these white wires disconnected.

It's a good idea to spray the sensor with a penetrating lubricant the night before you do the change to make the removal a little easier. I would also recommend putting some anti-sieze paste (available at Napa) on the threads of the new sensor before you install it.

Overall the O2 sensor replacement is a straightforward job. Good luck.


Date: Thu, 05 Mar 98 13:15:38 -0500
From: "Linthicum, Sandy" (linthias@sandy-ntws.usps.gov)

For immediate replacement, you do NOT need to connect ground and 12V for the heating element. It works OK without heating it, just takes a few minutes to warm up.

Buy it and buy the mating connector to put in your wiring harness at the same time (i.e.- test it). About $10. If NAPA doesn't have it try a Ford dealer.


Date: Tue, 03 Mar 1998 08:53:56 -0800
From: Spencer Hutchings (hutch@qnis.net)

About a year ago I "retired" my stock O2 sensor.

I tried a plain $25 Bosch sensor, reacted very slow. Junk, trashed it. Then I heard about this great Escort O2 sensor everybody was using on this list. I got the part number from Sandy Lithium and bought one. It was reading a little leaner that my stock 65k sensor was reading. But it was doing so very consistently and quickly. I trashed it and got another one just like it thinking that I either got a bad one or that I somehow screwed it up by getting anti-sieze on the sensor or something.

Guess what? The second one acted the exact same way. Now I'm thinking my car is running lean. I send my ECU to XS Engineering to get reprogramed. Same readings, little richer, not much. I call XS and b%tch. They tell me it's got to be my O2 sensor or my stock FPR. They run the fuel maps at like 11.5:1 or 12:1. I replace my stock FPR with an adjustable one they sell made by SX. O2 readings don't change. I don't want to jump to any harsh conclusions, but I think the Escort O2 sensor is junk. I think the O2 sensors on our cars actually cost around $200 bucks because they are, sort of, high quality sensors. I had a long talk with Eric at XS about O2 sensors. The only one he really trusts is his Motec (whatever that is) He uses the sensors out of the new 96-98 Porsche Twin Turbos when he can get somebody to spend the $1000 bucks for one.

O2 sensor

New connector for the 3-wire sensor:

O2 sensor


Date: Wed, 4 Mar 1998 09:33:26 +0000
From: "David Lane" (dlane@peabody.jhu.edu)

Spencer Hutchings is upset because his Escort 02 sensor gives leaner readings on his A/F meter than he was used to seeing from the stock sensor.

When I was talking with the guys at Tri-Point, they said that they liked to use Mazda O2 sensors with aftermarket engine management systems because these systems were in closed loop mode (using readings from the sensor) much more of the time. Tri-Point's experience was that the original equipment sensor was more stable over time than less costly alternatives, and more accurate.

Earlier Net discussion on the subject highlighted the fact that O2 sensors of all kinds are not very accurate (in an absolute sense) when they are reading mixtures that extend near either end of their ranges. Most of us with A/F meters figure where the car runs best--usually near the rich end of the A/F meter display--and just look for repeated readings. Many of the meters themselves are purposefully vague about the readings--with actual values only being stated for the ends of the range, and for the 14.7 to 1 reading.

That having been said, with the stock engine management system NOT depending on the O2 sensor in full throttle situations, it comes down to the same air/fuel mix being interpreted as a different voltage by various O2 sensors. I can understand Spencer's frustration that his ECU is seeing leaner readings from the Escort O2 sensor than from the original. It just isn't "right." On the other hand, getting used to a slightly different light pattern on his A/F meter should not be a major hassle, and the worse that would happen from a driving standpoint is that the ECU would make the mix a little richer during closed loop operation--namely at relatively steady state cruising.

There is a significant difference in cost between the O2 sensors in question, so I guess each of us has to decide what the best value is, what we can live with, and what will drive us crazy.

As for Spencer, he needs to go back to the stock unit. I thank him for bringing up the differences to the list. It is valuable information. Maybe someone wants to buy his Escort O2 sensor.


And one more response to Spencer's issues above. --Steve

Date: Tue, 03 Mar 98 12:36:14 -0500
From: "Linthicum, Sandy" (linthias@sandy-ntws.usps.gov)

If I had changed nothing but the sensor, then I would simply ajust my interpertation of my monitor's information accordingly, not go crazy swaping components trying to get exactly the same readings.

I cannot compare the Escort sensor to the stock sensor technically, anymore than you did, but no one I am aware of has had anymore trouble using it than they did the stock sensor.

If values are different on your Cyberdyne, then either its impedence is too low and it is dropping the voltage level of the sensor more than it did with the stock sensor or maybe there is simply a small variance in the designs. None of the stock sensors are suitable for precision measurement, which is why closed loop operation is only used for idle and light to moderate throttle. The O2 data is not used to meter fuel at all when you have to worry about running lean (WOT or on boost), it uses its preset air/fuel maps.


Date: Wed, 24 Feb 1999 09:08:45 -0600
From: "Justin Arnold" (aarnold@pdq.net)

I got a 4 wire sensor from pep boys for $36.99. 2 white wires (positive and ground for the heating element) black (signal) and grey (sensor ground). Works REALLY well. Just thought you all might want to know

OOPS.... fergot to say that it is off of a '98 camaro. pep boys, and it is a Borg Warner part.


From: Bob TMS Murray
Date: March 24, 1999

My next O2 sensor will be from Motec. Theirs is actually quite reasonably priced at ~$95 - higher than a Ford or OEM, but we're not talking about a lot of $ in the first place. I would investigate this - as if it's good enough for WSC, it's probably sufficient for anything I'll ever build.


Date: Wed, 24 Mar 1999 20:46:36 -0500
From: Sandy Linthicum (sandy_linthicum@mindspring.com)

As another note, Motec's sensor and injector components are simply off the shelf standard GM, Ford, Bosch components - significantly marked up.

Nothing wrong with that, and they say they QC all the parts for function and accuracy.

O2 Sensors for Leaded Gas

Date: Fri, 09 Jan 1998 18:15:15 -0500
From: Jay Wallace (jwallace@nist.gov)

Originally From: peter paul fenske (pfenske@direct.ca)
Sender: owner-diy_efi@efi332.eng.ohio-state.edu

For those interested in the Bosch lead tolerant O2 sensor, the part number you want is: 0 258 104 002 also known as LSM11.

The correct "Quoted Life" in a leaded environment is:
at max .6 g/liter Pb, 20-30,000 km
at max .4 g/liter Pb, 30-50,000 km
at max .15 g/liter Pb, 80,000 km

100LL AVGAS contains .56 g/liter Pb.

I've been running mine since Oct 20th or so, without any problems yet on nothing but AVGAS. Cost is roughly $270.

Wide Band O2 Sensors

Date: Mon, 10 Apr 2000 23:01:26 +0000
From: ryan.schlagheck@att.net
Subject: (rx7) [3] Wide Band Lambda O2 Sensor discussion?

Just some info on wide band O2 sensors... Feedback is appreciated.

Demetrios and I have been swapping emails regarding O2 sensors for tuning the Haltech (and any other computer that has adjustable fuel maps). We decided to take this public in an effort to get a relevant thread going.

My orignal intention was to buy a wide-band O2 sensor (Bosch Motorsports, approx. $) and datalog to the Haltech. But hold everything...you can't do that. Wide band O2's use a different type of input that cannot be resolved as simply as the narrow band (the one-wire and three wire O2's in most modern vehicles). Most wide- band sensors, if not all, are a part of a "system", wherein they are attached to a standalone computer to sample (~25-30 samples per second), convert, dampen (to ~5 readings per second), and display the A/F ratio.

The net result of this is that discrete tuning of adjustable ECU's will likely require at least one trip to the dyno (or hook up w/ a buddy who has a wide-band O2). Once that's done, ou can feel free to hook up a 3- wire O2 to datalog for "skips" and giggles.

So, has anyone ever run a wide-band O2 to datalog w/o using a computer to convert the O2 voltage?


Date: Tue, 11 Apr 2000 00:22:11 GMT
From: skan@ticnet.com

Haltech can be configured for choosing what O2 sensor you want to use for your car. My lambda unit has an output port where I can tied to the haltech and configured the haltech to read 0-5V range from the unit and still be accurate. You can simply choose Bosch 0-5V O2 sensor and tied that to your car without the lambda meter. The only problem is that you'll have to convert a/f ratio into voltage. I believed at 12:1 ratio, the voltage is around 78V for 0-1V range so basically you multiply that number by 5 for 0-5volt range and it should be the same.


Date: Tue, 11 Apr 2000 12:41:06 +0000
From: ryan.schlagheck@att.net

As I understand it, after being schooled by Demetrios, the benefits of running the Bosch Motorsports or NGK wide band O2 sensor connected to the Haltech is that you can datalog the results to disk during a run, and make changes afterwards to all the applicable fuel maps. Running in closed loop mode means that you intend to use the O2 sensor as an input to the fuel map adjustments. This effectively uses your base maps but makes +/- fuel adjustments based on the O2 readings.

We talked about splitting the costs of the wide lambda O2 + standalone computer but before I do that, I've decided to buy just the Bosch Motorsports four-wire O2 sensor (this is important that you get a 4-wire for tuning) and datalog/tune with it. If I need more resolution, the box we talked about uses the Bosch sensor, so I/we would only have to buy the box at that point. Also, the sensor lasts about 500+ hours on pump gas and about 50 for straight leaded/C16. The plan would simply be to run the O2 for tuning and datalogging and then hang it up. Fact of the matter is you could run the O2 for MADS the whole weekend through and still have plenty of life left in it. I plan to datalog my MADS sessions and by that time, have an excel spreadsheet with functions to graph 2-way and 3-way plots (yeah, yeah, stay tuned). :-)

O2 Sensor Readings

Date: Thu, 13 Apr 2000 17:50:58 -0700
From: "Ulen, Robert S" (Robert.Ulen@PSS.Boeing.com)
Subject: (rx7) [3] O2 Sensor - Voltage Measurement Test

I hooked up a high quality (Tektronix TX3) voltmeter to my O2 sensor wire and did some voltage measurements while driving around last weekend. I thought I would share the results for those of you are interested. My car is a completely stock 95 with 42K miles. Strong runner with normal 11-9-11 boost pattern (4.6 second 40 to 70 MPH, 3rd gear WOT test).

Here are the results:

O2 Sensor Voltage Measurements


Engine coolant at full operating temperature.

At idle:
V = 45 to 50 mV.

At hiway cruise, 5th gear, 65 MPH, 2400 RPMs:
V oscillates between 0.10 and 0.80 volts.

V = 0.0 when cruising, and throttle is closed (injectors are off). If you had leaky injectors you may see something above 0 volts.

WOT after idling a long time (exh gas temp lowest, and O2 sensor temp lowest) V = 0.98 to 0.99.

WOT after cruising moderate speed on highway (exh gas temp higher than at idle)
V = 0.94 to 0.95.

WOT after extended high loads (higher exh temps, probably around 750 C = 1400 F)
V = 0.91 to 0.92.

RPMs at 3500 in neutral
V = 0.90.

System goes open-loop at approx. 3400 RPM, with or without a load, even parked in neutral.

V = 0.90 to 0.91 even with very slight throttle loads in open-loop mode (above 3400 RPMs or larger throttle openings at < 3400 RPMs the system will go from closed-loop to open-loop control. This is why you never want to cruise on the hiway in 4th gear and expect good gas mileage).


Note that the O2 sensor's output decreases slightly when it gets hotter, even though the A/F mixture is probably still at approx. 12:1. This is normal according to reference info at:


If you are interested on where that strange backwards "S" shaped curve comes from and don't mind some nitty gritty math, down-load my Excel file at the bottom of Rob's referenced page. This derived information confirms the basic curves provided by Autometer.

I know that using the stock O2 sensor is not the most accurate thing to use, but its better than nothing if you can't afford a $2500 wide-band Lambda sensor kit. If the car was ultra lean you should still be able to see it with the stock O2 sensor (less than 0.87 volts at WOT with normal exhaust temps of approx. 750 C (1400 F) which is around 13:1 A/F ratio).

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