O2 Sensor Simulator

[Bob Blick] built this simulator from a 555 and few other common parts. Just when I thought I’d seen all the uses for the 555. The oxygen sensor on a cars exhaust is used to determine how efficiently the fuel mixture is to an engine. These can be a real pain to replace, so it can be helpful to fake the signal to the ECU.

18 thoughts on “O2 Sensor Simulator

  1. Question #2 on the site:

    The car eventually will get too rich or too lean because it isn’t using the oxygen sensor anymore. This is a diagnostic aid, not a long-term replacement for an oxygen sensor.

    Just thought I’d use this to point out that using this to fake an o2 sensor permanently (Or for any actual use of the car) is a bad idea, unlike what the post suggests.

  2. This is not a replacement for a “difficult to replace” oxygen sensor. This could be used in drag racing, but in most cases it won’t matter (the O2 sensor isn’t used for WOT conditions on any car I know of), maybe only to keep the ECU from throwing a check engine light.

    In most cases, the ECU will know that this isn’t a properly operating oxygen sensor, so it will mostly ignore it. You’re in real trouble if the ECU /does/ think it’s a real oxygen sensor, and decides to lean out the mixture, possibly burning up valves and causing serious problems.

    Someone please tell me I’m wrong and that there is a reason for such a /hack/.

  3. David, this is unless you want the motor to run rich or lean. In my case the engine is supercharged and has to run rich and leans out as boost increases. If I still had my stock o2 sensors I would have my check engine light on all the time. Luckily there was a company that made a module that plugged into the o2 sensor connector and simulated a normal signal. Unfortunately the company went out of business but at luck would have it, I now have another solution. You can probably use this as a long term solution without damage to you engine, but you probably won’t pass emissions inspections.

  4. “In most cases, the ECU will know that this isn’t a properly operating oxygen sensor, so it will mostly ignore it….Someone please tell me I’m wrong…”

    Nope, absolutely right… Autospeed mentioned something about temporarily interrupting the O2 signal line in an article awhile back. When the ECU detects the O2 sensor(s) went dead – it tends to run on the rich side. Just fyi if you’re trying to add a little bit more…

    The reason why to build might have something to do with “because I can.”


    Is a sensor I used on my old car. The ECU was a little over sensitive on the downstream (post cat) sensor. So the check engine light would come on prematurely – that sensor did not control fuel trim whatsoever, so no worries there. Popped this on and the ECU was happy.

  5. I wouldn’t try this on a car with a catalytic converter. If you get the mixture too wrong it will most likely destroy the cat = expensive repairs and the car probably not passing the emissions test when it’s MOT time!
    I would only use it purely for diagnostics, and even then only if you truly know what you are doing.

  6. I’ve seen this before and I think that its just plain wrong.
    Someone has measured the signal from the sensor, determined that it switches high/low and then assumed that that is a function of the sensor. Its not, in closed loop mode the ECU drives the fuel mixture rich/lean to determine the average fuellng error. It then adds an adjustment to the fuelling. The switching is a function of the control loop. With this circuit the ECU is driving the fuelling rich/lean and receiving a completely unsynchronised signal back. The frequency of switching is a function of the temperature of the sensor, its much slower to respond at lower temperatures.
    The other big problem is that a standard O2 sensor isn’t very stable with temperature so the ECU runs open loop over 75/80 per cent throttle. For drag racing you’re back where you started.

  7. This can’t be used on your pre cat o2, or it will make your engine run like crap most likely, because it controls the air/fuel mixture. It is for the post cat o2, which tells your computer if the cat is working or not, so it is completely harmless to use this on the post cat o2. Dont use it on the pre cat o2 though. Or you can just waste a bunch of money to buy a $80-220 o2 sensor when yours goes out.

  8. with the newer cars they have a simple catalytic converter check, just an O2 sensor that says that the emissions is lower on the aft side of the cat, and the car does not do any tuning from it, I would like to see how to make a small discreet plug that goes in place of the O2 sensor to stop the MIL light from coming on when a high flow cat is in place

  9. So let’s just say that “somenone” has a 2003 chevy s10 4.3l with a post cat o2 sensor. And that person “accicdently” took out their cat’s and replaced them with a straight pipe wich then turned on the check engine light. Will that effect my fuel mixture. Some have said yes some have said no. Does anyone know 100% either way?

  10. ok this would be used for the O2 sensors located after the catalytic converters. The sensors off the manifold before the cats are a linear sensor that is determining air/fuel mixture, however it is not entirely accurate to a value. This is constantly re-calibrating itself based on the emissions. The post-cat sensors measure emissions, and based on what those sensors see for emissions, carbon or nitrogen based depending on rich/lean, the sensors before the cats recalibrate. However above like 2500 rpm the car runs open loop and therefore the engine ignores the readings anyways. The post-cat sensors are fast acting switches, but this circuit hooks up to the signal wires and only relates to the signal going back, not the heater. This circuit can be used long term on those sensors because you are basically showing a timed voltage drop, which simulates as if the O2 sensor is working properly, however it is not a replacement, simply to be used in addition to the sensor when you remove the catalytic converters to stop the service engine light from coming on. Your car will revert to running at how the sensors before the catalytic converter were calibrated in the factory, slightly rich or slightly lean during closed loop. don’t quote me on it but it is all credible information.

  11. Nick, it will affect your fuel mixture, but only slightly. I took my cats off my ’99 Trans Am with an LS1, the gas mileage was very close to the same as before, and it runs slightly richer, not big deal, you just dump a little more fuel

  12. these mods are only a bandaid on a bigger problem. if an O2 sensor is going out, either side of the cat, it should be replaced to make the ECM run correctly. altho the lower end, postcat, may send a signal, it is probably to the extent of a thermocouple effect, and as the tempature, mixture, and FULETYPE (yes, some cars dont like high octane, especialy a E85, less btu rating for the ethanol, hence higher octane) or racing fuel, so the mixtures change drasticaly, but the O2 sensors will both read consistanty “off” and that then leads to why this wont work. as far as replacing a missing postcat sensor like with straitpipes, you will eventualy throw the mixture anyway with the lost backpressure and mostlikely a high performance air filter, in conclusion, this will most likely cause more problem than help.

  13. I have a 98 honda civic lx. My check engine light is on so i got a diagnostic reading and it says p0135 and p0141 are no good so I bought the lower sensor not the primary sensor cause the lower sensor was missing when the mechanic lifted my car up and where the sensor insert hole was there was only a bolt the same type used for the oil tank that was inserted there to cap the hole up and I thought that could of have been the source of the problem so i bought the lower sensor but still check engine light stays on when he put the new sensor .

    So i got another diagnostic reading and instead of getting the codes the mechanic checked to see what is on or off while the car was on and it says that my o2 sensors are both off. I bought this car not too long ago and the person who had this car before me did some modification to it he took off the bulb for the check engine light so that he can sell the car without any hassels. He cut one of the wire harness sensors where the lower sensor was missing and put a male plug-in end of the wire harness (when the end of the wire harness is suppose to be a female plug-in port)for the lower o2 sensor not the primary sensor without connecting a o2 sensor to it. And I really dont/cant understand why he did this.

    So if you can help me before I bring my car to an electrian and spend big bucks please let me know and I will be so greatful.

    Oh yeah I followed where the wires goes for the o2 sensors with my hands and eyes and they lead me to the back of the engine and i saw a cut wire (all black with a yellow stripe) could this be the problem cause the mechanic says he doesnt see any other cut wire that might connect to it so please please please help me… Thanks you so much for taking the time to read this…
    My friend said i have a flexi pipe

  14. i farm and would like to take the cat converter off of my 2020 for f 350 v 10 will the o2 sensor cause a problem or do i need a o 2 sensor simulator or am i just stuck with the cat converter

  15. This circuit is for experimentation only. The article on this circuit is printed on another web site and explains what the circuit will and wont do.
    There are circuits that will add or subtract voltage on top of the front o2 sensors voltage, making it possible to richen or lean out your mixture while driving( this is not one of those).

    There are also circuits using the 555 chip that will replace the signal from your rear o2 sensor to keep your check engine light from coming on. These just put out a fixed signal( voltage and frequency) that will fool the computer into thinking there is still a real o2 sensor in place. You would use these if you removed your cat and didnt want to have codes pop up / check engine light on.

    In states like California you cant just pull the check engine light bulb or put in an ECM that has that function removed, because they plug a code reader in to check it, and you will fail smog.They also look for the o2 sensor extensions and if the see those, you will fail also.

    Look here for a good rear o2 sensor simulator that will pass the toughest smog: MKIV.com .

    The rear o2 sensor has NO affect on the fuel mixture.

  16. I have an 85 caprice 4.3tbi that is giving me an O2 sensor code. It only has 1 sensor. It currently is misfiring on what seems like one cylinder. It was retrofitting with a dual exhaust (not by me) and doesn’t have an O2 sensor installed. Can a dummy sensor be used in this case to resolve the issue, or will it continue. If I can use the dummy, will I still need to install it in a bung on the exhaust?

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