When will a CEL/MIL turn off by itself?

[Slow Car Sport Mode]

This is a general question; in my case, could be specific to the 2004 Ford in the sig (3.0L Vulcan.) Pretty simple question, if I was a betting man, the light is more than likely on for both P0420 and P0430 at the same time. If I look at the length of time it takes to set until it clears, I'd say it is marginal (takes awhile to set vs setting very quickly.)

I am continually bewildered by good ol' YouTube. You'll see the occasional video, "Hey, I used this and my light went off!" Usually in relation to emissions . So, while it is a whole different thread to explore the relationship between HO2S/O2 sensor (not all are heated?) values and range of setting a P0420 and P0430 code, my specific question is the parameters in which a car's computer or ECU/ECM will actually un set the light once it goes on.

Also not sure if it would ever do that when both P0420 AND P0430 set; maybe one or the other? This thread is not specifically about the proper fix for P0420 and P0430; it is about when a MIL will turn itself off by itself just regular driving not attempting to fix anything. I think I've had that happen on OBD1 cars. I know it CAN happen on OBD2, or MY1996 and later, cars.

Experts about how this goes (when a MIL will sometimes turn itself off,) question of this thread is how/why!

Here's from our outing over the weekend when it came on. Again.. answers don't have to be specifically about "SCSM's Ford," How does this go. Thank you

 

[MParr]

With your mileage, it could be oxygen sensors, catalytic converters, or both.

 

[Drew7a]

Some cars go by set # of drive cycles and if the ECU does not detect the fault after a given number of drives cycles it will reset.

 

[atikovi]

The CEL light will turn off after so many drive cycles where the fault doesn't appear any more. So unless your O2 sensors can miraculously heal themselves, that's not going to happen.

 

[MrHorspwer]

Diagnostic trouble codes set and store after a diagnostic routine within the ECU runs and fails.

Some diagnostic routines run continuously. Oxygen sensor voltage is a great example. The ECU is always monitoring the PID and expects it to operate within a specified range and in a specific way. When it falls out of that expectation, the ECU identifies it immediately (but may not immediately set a DTC or turn on the MIL, more on that later). Some diagnostic routines run only once during a drive cycle, not continuously.

Other diagnostic routines are only run under specific conditions. The EVAP system is a great example. Because very specific things must happen to check the EVAP system for leaks, the ECU may not run the diagnostic routine during every drive cycle.

OBD emission DTCs are bucketed into three "types":

• Type A - Immediately sets a DTC, stores a failure record, and illuminates the MIL
• Type B - Will set a "pending" DTC upon the first instance of a diagnostic routine running and failing, but will only illuminate the MIL, set a current DTC, and store a failure record on the second consecutive time the diagnostic runs and fails
• Type C - There are non-emission DTCs and usually manufacturer specific on their setting criteria. They will not illuminate a MIL

DTCs will clear on their own after the diagnostic routine runs and passes a certain number of times during consecutive drive cycles (four drive cycles is pretty common).

So, yeah, if you have something like a marginal sensor that is failing intermittently, it is totally possible to set a DTC set and then clear itself over a period of drive cycles. A fault that is brought about by ambient conditions is another example, like you have a sensor that has a fault when it gets wet. You can live in So Cal for most of the year with no MIL but as soon as it rains, a light comes on. A week after the rain stops, the light goes out again. Totally possible.

 

[SC Maintenance]

As mentioned, it depends on drive cycles, and seeing a certain number of drive cycles with no out of spec parameters.

If you don't actually know what the code is - you should read it or have it read. Could be something completely different.

If it is P0420 or P0430 - those are usually pretty simple. Its measuring the efficiency of the cat, and will stay on until it sees enough cycles where the final 02 sensor sees enough improvement in the emissions to give the all clear. It could be the 02 sensor, or the Cat - or something upstream causing to run poorly but not enough to set a code - like a dirty MAF sensor or sticking injector or whatever.

Read the code - go from there.

 

[slacktide_bitog]

It can turn off after a few cycles of not seeing the problem anymore, but the catalyst monitor test takes a long time, so it can be awhile before the CEL turns off again. Some failures are intermittent.

The last time I had a P0420 code that turned itself off, it was probably 1-2 weeks before it turned off, but I don't remember exactly

P0420 and P0430 work the same way since they are both the same error, just for different cats (first and second, respectively). However, a lot of cars with multiple cats only monitor the first cat.

Do you have a code reader? Have you scanned it yet? Scan the codes first and go from there. Do you have emissions testing where you are? How much longer do you plan on keeping the car?

 

[brages]

P0420 and P0430 work the same way since they are both the same error, just for different cats (first and second, respectively). However, a lot of cars with multiple cats only monitor the first cat.

I believe P0430 is only found on vee engines, where each bank of the engine has its own catalytic converter. When there are multiple cats in series, I think only the efficiency of the pre-cats is measured, at least that is how my cars are.

 

[dumkid]

i get a p0420/430 every time I go offroading or drive at low speeds for a long time. i used to also have a code for fuel trim, an 02 sensor was dying and reading lean sometimes. i replaced the 02 sensor and fuel trim code is gone. still have the cat efficiency codes, but as long as i don't drop below 40mph for long amounts of time (about a hour) then I have no codes

 

[SwampSurvivor]

The solution to get the check engine light off of my Grand Marquis was changing the engine.

 

[Aredeeem]

Recently had a P0101 on the Versa. After cleaning the MAF sensor the check engine light turned off after 3 days of moderate driving. About 75 miles.

 

[slacktide_bitog]

The solution to get the check engine light off of my Grand Marquis was changing the engine.

I hope the new engine is a Godzilla

 

[JMJNet]

Theoretically, if the condition does not exist then it should go off by itself.
Unfortunately, in some cases, the fix that you do may not fix the issue, then that is why DTC keeps turning on and never off.

I am battling similar case with P0171.
Initially, I can turn it off by replacing the gas cap.
Now, it is permanent and I have done a lot to it like 1st O2 (Fuel Ratio sensor), MAF Sensor, fuel pump has been replaced.

 

[SwampSurvivor]

I hope the new engine is a Godzilla

Physically it's smaller than both my 4.6 and 5.4! No questions on fitment. But I think 450 ft/lb would make very quick work of the 4R70w.

If my truck wasn't a rusted out junker, a Godzilla would have been strongly considered instead of a head gasket.

 

[wpod]

It looks like the engine running light after a while.

 

[PF52]

The solution to get the check engine light off of my Grand Marquis was changing the engine.

I think you're referring to a different light - the "change engine" light.

 

[PF52]

I used to get a CEL only in very certain conditions; a good long drive on the highway would do it every time but around town it would stay off for months (it was a code for intermittant lean condition, if I remember right). But a couple days of cold start cycles and it would go right back off.

 

[Slow Car Sport Mode]

I believe P0430 is only found on vee engines, where each bank of the engine has its own catalytic converter. When there are multiple cats in series, I think only the efficiency of the pre-cats is measured, at least that is how my cars are.

i get a p0420/430 every time I go offroading or drive at low speeds for a long time. i used to also have a code for fuel trim, an 02 sensor was dying and reading lean sometimes. i replaced the 02 sensor and fuel trim code is gone. still have the cat efficiency codes, but as long as i don't drop below 40mph for long amounts of time (about a hour) then I have no codes

Yeah I'm noticing patterns too. Seems to set with regularity.

In my case, it really is a P0420 and P0430 (yet P1000 remains? Weird.) Probably O2 sensors will change and I wonder if it is gonna need that new gas cap as well.

Or. If it will eventually turn itself off.. (voltage code is because I threw the battery on the charger, most likely.)

Just cleared them again...

After you clear them, you see this

Fords have P1000 until the monitors set.. I am not sure if the manufacturers "keep secret" how many run cycles it takes for the computer to clear a MIL, if it decides to do so? O2s do just get lazy sometimes..

What codes have you all had turn off by themselves?

 

[Slow Car Sport Mode]

What about "Historic" codes?

And are "Historic" codes the same as "Permanent" codes?

How do those clear ?

 

[John105]

The solution to get the check engine light off of my Grand Marquis was changing the engine.

I remember on my 1998 Maxima, actually turning a screw on the ECU to do a reset. When that stopped working, the forum said to turn the other 4 screws counter clockwise, remove the ECU from its mounting, and call 866-cash4cars