Oxygen sensor response times

[hjb981]

As some of you know, I am about to change my O2 sensors (the two in the manifolds), since I occasionally get codes telling me that the heating is not working properly (PCM 0135 and 0155). I just checked some readings with my OBD link computer interface and read the following values:

O2 Response Lean/Rich Transitions Time Bank 1 Sensor 1: 25 ms.

O2 Response Lean/Rich Transitions Time Bank 2 Sensor 1: 83 ms.

O2 Response Rich/Lean Transitions Time Bank 2 Sensor 1: 94 ms.

I also meant to record the B1S1 R/L Transitions, but accidentally recorded the following isntead:

O2 Response Rich/Lean Switches Bank 1 Sensor 1: 145.

Does anyone know what that number means? If it means switches per second, then one switch would be less than 7 ms.

Anyways, comparing Lean/Rich for B1S1 and B2S1 above it is clear that the Bank 2 sensor is slower than it could be.

I also got the following reading, which I suppose are for the O2 sensor in front of the cat. conv.

O2 Response Lean/Rich Transitions Time Bank 1 Sensor 2: 30 ms.

O2 Response Rich/Lean Transitions Time Bank 1 Sensor 2: 44 ms.

I do not worry very much about the last to values, since as far as I know that sensor is only involved in monitoring the efficiency of the catalytic converter. Since it is hence not involved in the feedback loop for engine control, its response time should not matter (other than probably setting a code if it got very slow). Is that correct? Is the O2 sensor just in front of the cat involved in the engine control in any way?

Does anyone know what the normal switch times are for new O2 sensors?

I will change the two sensors in the manifold/crossover either way, so I will find the answer to the last question, but I am unsure if there is any point in replacing the "third" sensor in front of the catalytic converter.

/Jonas

[Cadillac Jim]

All four oxygen sensors used in Cadillac Northstar DFI are "broadband" types which means that they can record waveforms of rich/lean transitions, not just

"slowly" varying overall signals. The practical meaning is that the DFI can work for each individual cylinder. Slowly varying oxygen sensors can monitor mixture for a TBI, and for a port-injection DFI which varies overall mixture, but unless you have one for each cylinder you need broadband oxygen sensors to tune the mixture for each cylinder. Note that the exhaust reading obtained through an oxygen sensor is the exhaust mixture, not the intake mixture.

The transition time from rich/lean is the measured time, which is not necessarily the fastest that the oxygen sensor is capable of. As the exhaust stroke begins for a given cylinder, with both valves open during the overlap, the exhaust mixture will have its peak rich reading, which quickly declines as the fully-burned gases dominate the flow. Thus the mixture waveform, or shape of the oxygen sensor reading graphed versus time on a scan tool, is a series of peaks on a much more slowly varying pedestal; the overall average is what you would see without a broadband oxygen sensor. A transition time of 1 ms would work for an exhaust rate of up to a cylinder firing every 2 ms, 6000 RPM corresponds to a firing rate of one every 5 ms on a V8. You need that kind of speed on each bank of a V8 with a two-plane crankshaft because the firing order makes two cylinders on each bank fire in sequence, rather than the firing switching banks every time, so you need the same oxygen sensor speed as a straight-eight for both banks. The times of 2 ms probably are for an idling engine at about 675 RPM, which fires about every 45 ms.

The rich/lean ratio of 1:145 probably is the ratio of the time the reading is rich versus the time the reading is lean. The waveform spikes are high about 1/146 of the time, or 0,7%. If this number is computed by your PCM, it's something to record and watch for changes. If it's computed by your scan tool, you also should record it and watch for changes, but don't count on this number being the same for every scan tool, even the same one after software upgrades or patches.

You should not see a significant change in response times for new O2 sensors, since the times you are seeing are probably slower than the sensors can operate. Note that the OBD codes

P0135 Heated Oxygen Sensor (HO2S) Heater Performance Bank 1 Sensor 1

P0155 Heated Oxygen Sensor (HO2S) Heater Performance Bank 2 Sensor 1

are for the heaters, not the sensors themselves. A slow oxygen sensor would throw different codes, such as

P0133 Heated Oxygen Sensor (HO2S) Slow Response Bank 1 Sensor 1

P0153 Heated Oxygen Sensor (HO2S) Slow Response Bank 2 Sensor 1

The sensors on the cat watch the waveforms into and out of the cat. The input is that of the two banks added together, with a little delay for the front bank because of the time of transit through the crossover pipe, but it shows peaks for all eight cylinders. Also, the rich peaks are spread out and lower because of the mixing between the first oxygen sensor and the cat input, and the rich/lean times are longer; your scan tool reports 2.3 to 2.45 ms. If the output mimics the input too closely, or of the output is too rich, the cat isn't working properly, and the PCM throws a code such as

P0420 Catalyst System Low Efficiency

I got a P0420 once because I had a cracked cat. I cleared it and it didn't come back, but my mechanic's scan tool read the two waveforms that told the tale, and indeed the cat was cracked when we pulled it.

Since the oxygen sensors on the cat aren't throwing codes, and since they are relatively easy to change when they do, I would leave them alone until I had a reason to change them.

[hjb981]

Thank you Jim for that informative answer!

You actually answered another question I had - after reading some information about the operation of O2 sensors, I got the impression that the PCM varied the mixture slightly back and forth to create a waveform, which I thought seemed sub-optimal. Your explanation makes more sense to me.

As some additional information, I found this in the 1997 E/K FSM (p. 6-778):

"In addition to catalyst monitoring, the Bank 1 HO2S 3 heated oxygen sensor has a limited role in controlling fuel delivery. If the Bank 1 HO2S signal indicates a high or low oxygen content for an extended period of time while in a closed loop, the Control Module adjusts the fuel delivery slightly in order to compensate."

I guess that there is a typo there, since that sensor is the one after the catalyst, meaning that it is supposed to give an almost flat signal that indicates no oxygen present (which is depicted next to the text), and hence should not be able to detect a (too) low oxygen content. It think that it would make more sens if it was Bank 1 HO2S 2 (the sensor right before the catalyst).

[hjb981]

The slow response time DTC:s set if the average is over 150 ms for R/L or L/R transitions. The measured times that my scan tool read did not change with engine speed, however, they changed once during my 15-minute drive (one changing from 20 to 25 ms, for example), so it cannot be as simple as them just depending on the firing frequency of the engine (which should lead to identical values for both banks as well). I was looking at a graph that explains it in the ScanXL software, and according to that the lean to rich (or rich to lean) switch time is the time it takes for the signal to go from the low to high (or vice versa) threshold. In other words, the more the oxygen sensor signal would resemble a square wave, the shorter the time, and the more the signal would resemble a triangular wave (or sine wave for that matter), the longer the time. The only thing that does not make sense is that the times I recorded are too long - It is not possible to keep up with the firing of the cylinders with those long times (just like you indicated, Jim). Thus I am still a bit unsure how I should interpret those times. 6000 rpm = 100 Hz. 100 Hz * 8/2 = 400 Hz, or 2.5 ms. So the period should be 2.5 seconds at max rpm, leading to lean to rich or rich to lean time of about 1 ms according to the definition above. Hence the time that is reported by the PCM has to be something else (especially given that the fault code sets when it exceeds 150 ms -- 15 times to slow even for idle). I am confused ...

[Cadillac Jim]

I don't think that the FSM has a typo over which oxygen sensor used to set the long-term calibration on the mixture. There should be near zero, just a trace of oxygen left, for optimal combustion - and emissions - and best engine life.

The reason that I mentioned the response times was to put the numbers into perspective. The response time requirement of the oxygen sensor is determined by the rate of change of the mixture that it measures and the requirements of the data processor that uses it. The rate of change is reduced by locating the oxygen sensors away from the port, and they are several inches away in the collector just before the junction to the Y-tube and crossover; I don't know how the measured rate would change with engine RPM. If it is completely constant, then it probably represents the rate limit of the sensor. But, the OBD code is for the heater performance, not the speed. Read the speed numbers with the new sensors and see if it changes, or if it is a function of RPM with the new sensors.