g_murray

Am _convinced_ that there's a distinct difference between whether you clear a
CEL (check engine light) by yanking the battery cables as against using an OBD-ii code reader/scanner.

Using _either_ method WILL

i) clear the CEL (permanent and pending) codes
AND
ii) will cause the 'readiness monitors' to become 'NOT ready'.

**HOWEVER** -- I've recently discovered (coz it happened to me!) -- if you
clear a CEL using a code scanner that there appears to be 'some sort of
residual memory' that ISN'T totally cleared. The 'readiness monitors' work off
of internal 'counters' which are, themselves, 'incremented' every time you go
thru' a *successful* drive cycle. When a particular counter-value is reached
THEN the particular readiness monitor is 'set' -- be it the cataclysmic
monitor or the secondary air, etc. etc.

When the PO (previous owner) let the battery go dead on my '96 993 it took, literally, 700+ miles of 'various types of driving' before all 8 readiness monitors would set. (Yes, I tried all the web-forum-based drive cycles too!).

Recently I had to clear a CEL (misfire on #4 cylinder due to my 'Seafoam Fandango') and it took only 4 separate 'drives' -- some stop-go-traffic but mostly highway stuff before all 8 set again. Total mileage ...approx 200.

SOOOOOOO, boyz n girlz.... I conclude that if you have to get a CEL cleared -- by a code scanner -- then you **should** perform the 'drive cyles' FIRST -- because, doing so, should 'build on' / cause the **existing** counters (which STILL contain values) to increment further --- and thus, cause you to obtain a 'readiness set' condition ...earlier than advertised (and a LOT earlier than if you yank the battery!)

G.

(Maybe this explains why some of our fellow posters have been able to get the readiness monitors to set 'quite easily' -- after 'scanner/code-clearing activities'' - ...and for others ...it's been a nightmare and a case of 'get thee to the $tealership' (and/or buddy with the Bosch Hammer!) ...if the battery has been removed/changed or gone dead.)


******> And, all the more reason, now that I think of it, for those of us in the North East (enduring these sub-zero COLD spells) ...to put your battery on a trickle charger OR religiously charge it once a week -- otherwise you could find yerself in a bit of a bind if the battery goes dead and your state inspection is due in the springtime.

This could very well happen QUICKER than you realize because the *combination* of the cold temps AND the fact that the immobilizer is on all the time - is a sure battery killer. You may get away with not driving the car for a week or two, during the summer (when the battery capacity isn't being 'compromised') ...but nowadays that's NOT the case!

Lorenfb

" I conclude that if you have to get a CEL cleared -- by a code scanner -- then you **should** perform the 'drive cyles' FIRST -- because, doing so, should 'build on' / cause the **existing** counters (which STILL contain values) to increment further --- and thus, cause you to obtain a 'readiness set' condition ...earlier than advertised (and a LOT earlier than if you yank the battery!)" - g_murray -

Once the an OBDII CEL reset via a scanner is performed, the readiness codes/states are reset
which requires the typical multiple drive cycles.

Check here ( www.systemsc.com/codes.htm ) for more info.

g_murray

Yep, Yep, that's what I said. Using a code scanner OR disconnecting the battery / fuses WILL 'clear' a CEL AND will also reset all the readiness monitors.

It's the "internal counters" that DON'T get completely cleared (seemingly!) when you i) _keep the battery connected and merely clear the code via a code-scanner_.

The 'act of clearing the CEL' (via scanner) apparently has the effect of also setting all the readiness monitors to off (default) yet NOT totally destroying the 'values' that are held in 'counter' KAM (keep alive memory).

These 'internal counters' get 'incremented' when you _complete_ a drive cycle.

When they've reached a predetermined 'value' - THEN the triggering occurs -- i.e. it's THEN that the particular 'readiness monitor' gets 'set'.

So, again, my conclusion - based on the 'rapidity' at which my readiness monitors were all set -- is that it's "better" to keep your battery in tip-top condition than to let it die AND that it's better to clear a CEL via a scanner than to pull the power.

G.

sojglenn

I *think* i agree. At least in regard to the importance of these "counters" when it comes to readiness codes. I drove my new to me 97 C2 around for months trying to get those dumb readiness codes to set and they just wouldn't. So i brought it to the dealer and they told me they needed to clear some of these "counters", and low-and-behold two days later the readiness codes had set.

However, what i am wondering is if the act of removing the power causes some problem with the counters so that they won't increment correctly. Or the counters get out of sync somehow with what the rest of the OBD system is expecting. And that is why disconnecting the power has a different effect than clearing the codes with a scanner. But that is only speculation.

-S

g_murray

I think there definitely be some truth in them thar words.

Lorenfb

Summary:

A. DME or battery removal

1. CEL reset
2. readiness codes reset
3. OBDII diagnostic faults remain

B. CEL reset via scanner

1. readiness codes reset
2. OBDII diagnostic faults reset

An engine shut down after driving to fully set readiness codes results in a reset
of the non-continuous readiness codes.

g_murray

Yep, Yep, I agree.

What we were mulling over, tho', is whether the 'counters' get _completely_ reset to zero -- when you clear a code via scanner. Conventional wisdom/driving experience seems to suggest that, unlike yanking the battery -- that clearing a CEL via scanner does NOT completely reset the 'counters'.

Net result -- as you drive the car and _complete_ various drive cycle tests -- that the counters (which are non-zero because all u did was clear the CEL -- *NOT* yank the power) are now _further_ incremented. Now, since they were already, 'half way there', (so to speak i.e. non-zero), then a few more increments causes them to 'reach their golden values' whereby the READINESS MONITORS are now triggered to 'ON'.

*****'Counters' reaching pre-set values triggers the readiness monitors to THEN go to 'set'.*****

If the counters are zero'd (seemingly by u yanking the power or letting the battery die) -- then it's gonna take a _long time_ plus multiple 'complex drive cycles' for them to reach their golden values. Then and only then will the readiness monitors go to 'set'.

Conclusion - if u don't drive the car often -- then keep your battery in good condition via "trickle charger" -- otherwise you might have a 'surprise' waiting for you next time u try and get it state-inspected !

Phew!

G.

Lorenfb

"What we were mulling over, tho', is whether the 'counters' get _completely_ reset to zero -- when you clear a code via scanner. Conventional wisdom/driving experience seems to suggest that, unlike yanking the battery -- that clearing a CEL via scanner does NOT completely reset the 'counters'."

Bottom line: There're NO counters. There're just states, i.e. a go or no go.
When a CEL exists, there's a fault with the emission system or engine operation
which requires a corrective action and a subsequent reset of the CEL. Because
of the fault, ALL the readiness conditions (emission conditions) MUST be re-run.
Therefore, the CEL resets ALL readiness states requiring NEW drive cycles.

That's the logic of OBDII! Read why and how OBDII was written.
Buy an OBDII scanner which can view the readiness states and
reset the CEL and see what happens. No guessing allowed!