Resetting the computer: Myth or reality?

[Racerjk]

I changed out the 02 sensors on my Formula yesterday for the first time. I was getting a black sooty build-up on the right side of my bumper, and the check engine light would come on if you drove below 70 in sixth gear. My 02 sensors had never been changed, so I decided to try that first.

The problem seemed to be fixed. However, I wonder if I should've left the battery disconnected longer to "reset" the computer.

I've heard some say this is a myth, and then other folks swear you need to do this. Which is true? Does the computer need to be "reset" or will it do this on it's own after I put the O2 sensors in?

 

[nobadges]

Is the check engine light still on?

 

[Racerjk]

Nope.

The check engine light would only come on in certain situations as I mentioned, but not every time. Sometimes the light would go off after a bit, other times it would stay on. After you shut the car down a for a while, the light would go off. The light didn't come on last night at all.

 

[Tosh]

Score one for the guys who win at the 'wishfully throw money at the problem without proper diagnosis' lottery. @*%# Yeah!
(Can we also have more info about the car?)

 

[cousincletus]

Yep, the O2 sensor is a good guess. Most of the time you can throw a couple parts or so at the problem and still be cheaper than going to a repair shop.

[ December 31, 2005, 05:31 AM: Message edited by: cousincletus ]

 

[rjundi]

A better idea is visit an autozone to read the stored codes rather than throwing parts ($$$). Our local ones read the codes for free.

 

[Dark Jedi]

I will also say go to Auto Zone and get the code read. I believe even with the ckeck engine light is off it will still store the codes unless you disconnect the neg batt term. I dont know how many 02 sensors your car has but I think my truck has 4. so its better to get it checked to see which bank is bad even if bad. your cat could be bad to many things can be wrong. so get the codes checked.

 

[Gary Allan]

Well, look at it this way ..he didn't go whole hawg into parts replacing ..he just did what he figured that he'd been screwing off about doing for a long time. The light was just a prompt that he didn't want to ignore.

Yes, hit up AA, AZ, or any of the other fine places that will perform this service free.

 

[yooper]

I'm still interested in the original question. Does disconnecting the battery for a certain amount of time reset anything in the cars computer?

 

[Gary Allan]

It resets the codes. The long term adaptives and other things are usually retained. There are techniques for some PCM/ECUs to reset them as well. They're self correcting after so many "events" by themselves. That's why the feature length advertisments ..err..I mean enthusist mags feature installation articles say that they don't get the dyno output that they should ..since the computer has to relearn the new part ..and naturally they ran out of dyno time before the deadline for publication.

 

[DJ]

On most vehicles disconnecting the battery can erase trouble codes and the learned fueling trims. There are safeguards in place though so the OBD2 scan only emmisions tested vehicles can not pass like this because after a clearing like that the computer has to perform a series of checks as you drive and it usually takes a couple days worth of driving to let it get through all the "readyness tests" and verify everything is working.
O2s are failing LONG before they set a code so if they are old and the car is giving some indication they might be an issue replacing them is not all bad. Yes AZ scans but only OBD2 cars.

As far as which O2 sensors to replace the ones behind the cats on an OBD2 vehicle are only tattle tails and do not effect engine operation they only check for catalyst efficiency so my advise would be replaxce the fronts every 50K or so and rotate the fronts you pulled out to the rear positions. If you get a rear O2 code I would put the new sensor in front and put the used one in the rear again because they don't affect engine operation.

yooper, I lived up past Houghton for a few years and was up over xmas was shocked at how little snow was left

.

 

[brianl703]

The check engine light will go off within one drive cycle where the test that triggered the light (which is dependent upon which code has been set) in the first place completes without error.

"Drive cycle" means a drive where the proper conditions exist for allowing the test to run. Some, especially the catalyst monitor test, are very picky (at least on Ford vehicles) about when they'll run, and if you want to make the test run you have to know what sort of driving will allow that to happen.

In the case of the Ford catalyst monitor, I have found that about 5 miles of driving with the engine fully warmed up at a constant speed of no more than 60MPH without stops will usually do it.

Another little bit of trivia is that the check engine light usually won't come on until the test fails twice. The first failure causes these to show up as "pending" codes with a scantool. The second failure turns on the light.

There are some tests, such as the misfire test, that will turn the light on immediately if they fail. Misfire severe enough to result in catalyst damage will cause the light to blink.

 

[Racerjk]

Thanks Gary Allan,DJ, and brianl703. Those were some very informative posts!

My 02 sensors were the originals that came with the car. I changed the fronts because I felt it was time anyway regardless of the check engine light problem. My Formula is a '95 model with 58k original miles on it. If this doesn't fix the problem, of course I will have the code read by AZ or somewhere else. Under mid to wide open throttle conditions, a cloud of "soot" would come out of the pipes. I didn't notice this quite as much if at all last night. I've heard the LT1 is known to do this anyway, but mine seemed extreme.

I can't believe nobody said "don't drive below 70 in sixth gear."

 

[Mike_dup1]

quote:

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It resets the codes.

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You mean it clears any set codes?

Its like rebooting your PC.

 

[Gary Allan]

DJ - my 99 FSM for my jeep says that the front and rear O2 look identical ..but are not. I imagine that this is not the case with all setups ..but just in case a DC owner reads this and starts swapping O2 sensors and gets unfavorable results ..I figured that I'd throw this out there.

 

[brianl703]

Sometimes the front and rear O2 sensors have different connectors, but are otherwise identical.

 

[wytstang]

I'd like to add something here.....as of 12/30/05 Advance Auto is no longer allowed to run codes on OBD 2 cars. We (I work at one) can only run codes for OBD1 cars. Why???Have no clue. Are other AA doing the same? I would assume so if not they will.

Also disconnecting the neg batt will erase all codes. Failure to do so will result in a stored KAM (Keep Alive Memory) and may pop up later on during a code search. This KAM memory is always stored as long as the EEC has 12v, disconecting the neg batt will erase them.
"The EEC has three different kinds of memory, ROM KAM & RAM. Read Only Memory or ROM; this is the long-term memory where the master program is kept. The ROM cannot be altered and lasts 20 years after the EEC is disconnected from voltage. Keep-Alive Memory or KAM; this is the complex mid term memory; this memory lasts for as long as EEC is receiving 12 volts. The KAM is where the trouble codes are stored, and where the sensor baselines are memorized. Random Access Memory or RAM is the short-term memory; this is where data is stored during a trip. Once the key is turn off the RAM is deleted."
http://fordfuelinjection.com/index.php?s=Keep+Alive+Memory&submit=SEARCH

 

[Gary Allan]

quote:

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Sometimes the front and rear O2 sensors have different connectors, but are otherwise identical.

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Chapter 14 - Page 30 of the 1999 Jeep Wrangler Factory Service Manual:

Downstream Heated Oxygen Sensor


(last paragraph)
The upstream and downstream sensors may appear exactly alike, but are not interchangeable.

Are you absolutely sure?

 

[rg144]

Quote:
On most vehicles disconnecting the battery can erase trouble codes and the learned fueling trims. There are safeguards in place though so the OBD2 scan only emmisions tested vehicles can not pass like this because after a clearing like that the computer has to perform a series of checks as you drive and it usually takes a couple days worth of driving to let it get through all the "readyness tests" and verify everything is working.

I just went through this for emmisions tests. Cleared the code (after doing plugs and wires for cylinder 6 misfire). Took it the shop. Was told 3 tests had not finished and to drive for a day (he said by 100 miles it would definitely be OK). The explanation was '96 models, up to 2 unfinished tests are allowed. '98 it is 1 test - so it get stricter (in PA anyway).

 

[brianl703]

The same Walker universal sensor is specified for the downstream and upstream sensors in the 1996 Ford Contour.

(EDIT: The same is true for the 1999 Jeep Wrangler with the 2.5L).

The difference is the connector. The upstream sensor uses an square AMP connector I've never seen on any other non-CDW27 body Ford vehicle. The downstream sensor uses the same style circular connector Ford has been using for years, probably just keyed differently. (It's greenish blue, I believe the color indicates the keying).

Maybe Daimler-Chrysler is referring to the connector. (EDIT: They very likely are; the connectors ARE keyed differently according to the diagrams on http://www.oxygensensors.com ) After all, a different connector renders them non-interchangeable, at least without cutting and splicing(or dremeling in the case of connector that is key differently). It also renders a bit of goof-proofness, though not much..someone could still install the downstream sensor in the upstream position and vice-versa, and plug them in wrong, assuming the wires reach.