How many trips for ECM to self clear a P0156 after replacing O2 sensor?

[cdq109]

Hello,

Does anyone know how many trips the ECM requires before it will self-clear a P0156 error code, after the O2 sensor is replaced?

I do not want to reset the ECM, as I don't want to drive the test pattern, for quite literally at least the 10th time in the last few months.

I've put like 1,300 miles on the damned thing, mostly just driving the test pattern.

Moved from the San Diego are to the San Francisco Bay area, and am now in an "enhanced" testing area.

It has been like playing a nightmare version of "whack-a-mole". Fix one code another pops up.

So far the following items have been replaced (some only after various vague codes only intermittently came on and multiple relearn trips had to be performed) just to get the OBD-II ready paramaters set such that my MIL light was off, and all the required test patterns had passed without any codes being thrown (oh and like 7 Smog re-tests done, none "gross poluters" and hte last one only failing by 1 part per million for Hydrocarbons for the ASM 25 MPH test, everything else was FINE:

Distributor (Crank position sensor died after ~180,000 miles. Word to the wise NAPA or OEM ONLY! Richporter NS-60 gave me two BAD parts)
Driver Side Exhaust Manifold (was cracked)
Driver Side Catalytic Converter (honeycomb was melted, found after cracks in manifold, replacing various 02 sensors--extra O2 from cracks, and extra fuel from ECM dumping more fuel to compensate for a false lean condition = super heated cats and melted substrate for bank 2 (Doh!!!!)
Driver Upstream O2
Passenger Upstream O2
Passenger Downstream O2
Knock Sensor
New Plugs
New Plug Wires
Tightened Ground Screws in Speedometer Head Unit (after getting a code for that early on)
195 Degree Thermostat Installed
Ignition Timing Set, tweaked, re-set, re-tweaked etc...
MAF sensor cleaned (CRC MAF cleaner)
So today I finally did the Seafoam into the break booster vacuum line treatment.

After that cleared, took it on a road test.

50 miles later the Driver Side Downs Stream O2 Sensor (the only one I had not replaced yet) threw a code and my MIL check engine light came on.

So anyway I am 1 ppm away from passing the damned smog on this thing (CRC Garunteed to Pass tried way before the crack in exhaust manfold was found, so fuel system should be good).

So tomorrow I dutifully plan on replacing that sensor (I don't doubt that the Seafoam treatment is tied to the code being thrown, but only because the O2 sensor was in need of replacement anyway and had reported borderline passing voltages previously). It's is ANCIENT (bought the truck in like 2001 and I don't think I've had to replace O2 sensors until 2016, so I can't complain there...)

But the thought of me having to reset the ECM for it to learn the fuel/air trims again and go through all the test patterns, well it's enough to make me want to start drinking....HEAVILY...(kidding, mostly...a guy can only take so much...).

The only thing worse than resetting the ECM and having to drive the test pattern again (2x) would be to not reset it, only to learn that the ECM has to go through like 200 full trips before it will self clear the code.

I know the FSM says the "Two Trip Logic" requires that the same code be present during two consecutive trips, but it does not say how many consecutive trips might allow the ECM to clear itself of the code.

I know the normal process is for people to just reset the codes on the ECM via scantool, I just can't bare the thought of doing that again.

[sroberts]

You may want one of these, they can be had for ~ $10 and will clear selective codes (via the Torque app). Plus they'll show you many measurement levels.

[nickelghandi]

I once replaced my O2 sensor (don't remember which one) and the light stayed on. I hadn't yet bought my ScanGage so I disconnected my battery overnight and the light was gone the next day when I reconnected and started the car. It didn't drive any differently or act like it was learning anything new. I know that an ECU reset will cause it to relearn all the trim values, but that should take only a day or two of normal driving tops whereas it is probably more conservative in allowing a trouble code to go away to prevent damage from intermittently working components.

I tried this just now... If I disconnect my MAF sensor in my Volvo (my Pathfinder is having the transmission rebuilt) and let the ECU think that there is no voltage from the sensor, it will throw a code it will run rough until I power off the car and plug it back in. On restart it runs normally, but mine keeps the code until I clear it with my ScanGage. I only let it run for a few minutes and drove it around the block to test that it was working, but you get the idea. It would probably clear it on its own eventually (probably sooner with the MAF sensor because many things depend on that reading), but I was able to just clear the code with my tool. For something like an O2 sensor, it could take a significantly longer amount of time. Since the ECU is constantly learning and adjusting values based on polling, it could take months for all the conditions to be met for it to clear the light.

Maybe I am misunderstanding what you mean by it relearning everything and driving the test pattern again. In KY we don't have any emissions testing, so I have nothing to compare to, nor have I ever monitored that closely all the different systems in my Pathfinder. Do you mean that you would have to go through ALL the "enhanced" testing again because your sensors would have different readings due to the new O2 Sensor and ECU code clear? If so, and you are on a time constraint, that might be unavoidable. Given enough time, it will probably clear the code itself, but I think it will be more conservative in doing so versus relearning trim values after clearing the code from memory by any means you choose.

Here's one more example. Running premium gasoline in a high compression engine for a long time will allow the ECU to advance the timing and take advantage of the higher octane gas. Try running a lower octane gasoline in that same car and the knock sensor will almost immediately retard the timing to prevent premature detonation and engine damage. Put premium in the next tank and it won't advance the timing much, and you will see poor gas mileage from incomplete combustion. It would take several more tanks of premium to get it to advance the timing than it does regular gas to get it to retard it. This is also to prevent damage to the engine. I realize I am rambling now, but I am trying to reinforce that the time taken for it to reset its trim values probably pales in comparison to how long it will take it to eliminate the code on its own.

Don't rule out the possibility that your issue isn't totally cured either. Good luck with this and keep us updated.