# RC Low-pass filter for O2 sensor

#### john2k

Joined Nov 14, 2019
87
I'm planning on adding a 1Mohm resistor and a 4.7uF capacitor like the picture below on a o2 sensor that generates a voltage from 0 to 0.9v. The plan is to remove highly frequent voltage changes and allow the lower slow voltage changes to happen. Basically frequently changing voltage reports ineffecient catalytic converter so with the resistor and the capacitor apparently this allows the slower less frequent changes but smoothens out the faster frequent changes. This diagram is a tried and tested one based on youtube and the person who did it has had it over a year. My only question is, is there a need to put maybe a diode across from the + and - just like the capacitor?

Joined Jan 15, 2015
5,604
There is no need for a diode. The design is a simple LP (Low Pass) filter. While it will only pass lower frequencies and filter the spikes what the ECU sees will be less than the O2 sensor sends, that may be a downside waiting for 5 RC Time Constants. I have seen it done using capacitor values from 1.0 uF to the 470 uF shown in your drawing. So in this case:

"The time also called the transient response, required for the capacitor to fully charge is equivalent to about 5 time constants or 5T. This transient response time T, is measured in terms of τ = R x C, in seconds, where R is the value of the resistor in ohms and C is the value of the capacitor in Farads".

In this case 470 seconds for one time constant now we get 470 * 5 = 2350 Seconds or about 39.2 min. That's a bit long. You may want to try some lower value capacitors like 10 uF and see how your results actually look. Some of this also depends on the O2 sensor in question too.

Ron

#### john2k

Joined Nov 14, 2019
87
Thank you Ron, I've got a 4.7uF capacitor and a 1mega ohm resistor. 1x4.7 will give me 4.7seconds right?

#### crutschow

Joined Mar 14, 2008
25,248
1x4.7 will give me 4.7seconds right?
Correct.
That gives a 4.7s time-constant.
Basically you want the minimum time between readings to be about 5 time-constants.
In this case that would be 23.5s minimum.

Joined Jan 15, 2015
5,604
Thank you Ron, I've got a 4.7uF capacitor and a 1mega ohm resistor. 1x4.7 will give me 4.7seconds right?
Yeah and as mentioned by crutschow above you need to multiply that by five for five time constants so about 23.5 seconds. Keep in mind that capacitors have a wide tolerance and thus I use "about" pretty freely.

Ron

#### MisterBill2

Joined Jan 23, 2018
6,696
There is a whole lot MORE TO THIS STORY than what we are being told. The oxygen sensor is indeed a fairly fast device intended to provide feedback to the engine control computer so that it can adjust the fuel/air ratio in real time. And this one must be downstream from the converter. If the outout is fluctuating then you must have a rapidly changing F/A ratio, meaning either a failed injector, an incorrectly firing spark plug, or a leaky valve, although it may possibly be a valve lifter or camshaft problem. A simpler work-around will be to just substitute an adjustable DC voltage for that sensor connection, and set the voltage to wjhere the emissions control computer is happy. But onlyuntil you can repair the engine.

#### john2k

Joined Nov 14, 2019
87
There is a whole lot MORE TO THIS STORY than what we are being told. The oxygen sensor is indeed a fairly fast device intended to provide feedback to the engine control computer so that it can adjust the fuel/air ratio in real time. And this one must be downstream from the converter. If the outout is fluctuating then you must have a rapidly changing F/A ratio, meaning either a failed injector, an incorrectly firing spark plug, or a leaky valve, although it may possibly be a valve lifter or camshaft problem. A simpler work-around will be to just substitute an adjustable DC voltage for that sensor connection, and set the voltage to wjhere the emissions control computer is happy. But onlyuntil you can repair the engine.
The O2 sensor you are referring to is the sensor that is referred to as Air Fuel Ratio sensors (well it is for my car) this is not the sensor i am doing this modification on. The sensor i am doing the modification on is the heated o2 sensor that just reports back after the catalytic converter and provides feedback to let the car know if it's within emissions efficiency.

#### MisterBill2

Joined Jan 23, 2018
6,696
OK, now it is clearer, that there is some sort of noise, or variation, on a sensor with rather long leads back to the engine control computer. Are you certain that the noise is not being picked up by a failed portion of the shielding of the wires? Have you verified that the the output of the sensor is actually changing? If the system worked before, and if the engine is still running as it should, then I would suspect a wire problem of some kind, or perhaps there is an air injection valve problem, if the emissions system has that. If you are able to look at the signal with an oscilloscope you may see a noise signal, if there actually is one. It might even be a variation in the heating voltage causing the variation.
If you do install such a filter, be sure to keep it away from areas of heat, and put it in line closer to the engine control computer.

#### john2k

Joined Nov 14, 2019
87
So i've tried this as per my diagram above with a 4.7uF capacitor and a 1Mohm carbon film resistor. It has got rid of the catalytic convertor efficiency error but now it gives an error saying the O2 Sensor Circuit High Voltage. Any ideas what could be wrong? have I used too high a resistor or capacitor maybe?

#### MisterBill2

Joined Jan 23, 2018
6,696
OK, now it is clearer, that there is some sort of noise, or variation, on a sensor with rather long leads back to the engine control computer. Are you certain that the noise is not being picked up by a failed portion of the shielding of the wires? Have you verified that the the output of the sensor is actually changing? If the system worked before, and if the engine is still running as it should, then I would suspect a wire problem of some kind, or perhaps there is an air injection valve problem, if the emissions system has that. If you are able to look at the signal with an oscilloscope you may see a noise signal, if there actually is one. It might even be a variation in the heating voltage causing the variation.
If you do install such a filter, be sure to keep it away from areas of heat, and put it in line closer to the engine control computer.
Probably too high a resistor value.

Joined Jan 15, 2015
5,604
Back to earlier you have a 4.7 second time constant (R * C) and it will take about 5 time constants for the cap to charge. Now if I remember correctly an O2 sensor outputs approximately between 0.1 and 0.9 volt. I don't know haw quick the ECM looks at the sensor(s). You did make the RC network look like this correct? The cap to ground. I can't see how the output voltage could be high, that's strange. I don't know that I would have gone with such a long RC time constant but again I don't know how often the ECM looks and assuming a stable O2 sensor out there should be a nice clean voltage level. Do you have a way to measure the actual voltage to the ECM?

Ron

#### john2k

Joined Nov 14, 2019
87
Back to earlier you have a 4.7 second time constant (R * C) and it will take about 5 time constants for the cap to charge. Now if I remember correctly an O2 sensor outputs approximately between 0.1 and 0.9 volt. I don't know haw quick the ECM looks at the sensor(s). You did make the RC network look like this correct? The cap to ground. I can't see how the output voltage could be high, that's strange. I don't know that I would have gone with such a long RC time constant but again I don't know how often the ECM looks and assuming a stable O2 sensor out there should be a nice clean voltage level. Do you have a way to measure the actual voltage to the ECM?

Ron
Hi Ron,

Yes I did make the RC filter exactly like that but the only difference being that I used a 4.7uF capacitor instead of the 1uF capacitor that is suggested in that article because in another article I read someone saying the 1uF wasn't enough for him. Maybe I should swap over to a 1uF instead? The voltage reading I am getting from the ECU for that O2 sensor is now showing as 1.28V which is definately high. As you mentioned it should be under 0.9V

#### #12

Joined Nov 30, 2010
18,210
I picked up this circuit on a Ford forum. 1 meg and 10 uf. Neither of those parts creates voltage so the sensor seems like the bad guy.

#### john2k

Joined Nov 14, 2019
87
I picked up this circuit on a Ford forum. 1 meg and 10 uf. Neither of those parts creates voltage so the sensor seems like the bad guy.
Sensor is brand new. I changed the sensor out for brand new OEM one first to see if the efficiency error was related to the sensor. This didn't solve the issue and i've checked everything else the manufacturers documentation says and it's final outcome is that the cat needs replacing. Hence why I want to insall this filter.

#### #12

Joined Nov 30, 2010
18,210
Darn! Missed again.

Joined Jan 15, 2015
5,604
Hi Ron,

Yes I did make the RC filter exactly like that but the only difference being that I used a 4.7uF capacitor instead of the 1uF capacitor that is suggested in that article because in another article I read someone saying the 1uF wasn't enough for him. Maybe I should swap over to a 1uF instead? The voltage reading I am getting from the ECU for that O2 sensor is now showing as 1.28V which is definately high. As you mentioned it should be under 0.9V
That's what I would try next and I can't figure where the excessive voltage is from unless it is being generated by noise. Try a 1 uF and see if it gets us anything.

Ron

#### john2k

Joined Nov 14, 2019
87
That's what I would try next and I can't figure where the excessive voltage is from unless it is being generated by noise. Try a 1 uF and see if it gets us anything.

Ron
Thanks, i'll order a few 1uF capacitors and see if that helps and update you.

#### Wolframore

Joined Jan 21, 2019
1,748
It could be issues with your upstream sensor, bad wiring, bad ignition, bad cat, stuck injector.... number of issues that can cause this. While it doesn’t actually affect drivability it can be pointing to an issue other than a cat.

If you use two connectors and it can be added or removed between the cat and harness.

I have heard of some success putting in a offset to where the sensor mount.

#### MisterBill2

Joined Jan 23, 2018
6,696
If the MCU is complaining that the voltage from a heated oxygen sensor is excessively high, I would suspect that there is a resistance in the heater return power wire, if that ground is shared with the oxygen sensor ground path. Or a bad ground of the sensor itself. Any resistance in the heating current return path that could cause a voltage drop added to the sensor voltage will cause a problem. But this is a guess because I do not know the physical arrangement.

Is the intention to restore the system to original performance? Or to avoid having to replace a catalytic converter?