OBD-II Codes

[enjr44]

I need an education on OBD II codes and how they can be used by a tech with a scanner or the Check Engine Light. I looked at the list of codes in the tech section and there are about a million of them.

Do all codes light the CEL? My guess is no.

How does the tech use those that just show up on a scanner?

For instance what would a Tech do with these codes?
P0505 - Idle Control System Malfunction
P0506 - Idle Control System RPM Lower Than Expected
P0507 - Idle Control System RPM Higher Than Expected

If you were looking at the tachometer, you wouldn't see the same thing, so how would these codes help troubleshoot the idle problem.

So can someone please educate me?

Thanks, Ed

 

[RonD]

The computer(PCM or EEC) is what monitors the sensors and generates the code, it will turn on the CEL if it determines there is an on going issue, it will flash the CEL if engine may get damaged or stop working.

Computer stores all codes in it's memory, even a one off, a code is generated when a sensor is not reading "normal", "normal" could be a preset range set at the factory, or in many cases "normal" is a Learned range, the computer learns the operating range of a sensor and that becomes the "normal" range.

In the codes you posted, I would suspect the IAC(idle air control) valve or circuit to be at fault.
The IAC valve is a controlled vacuum leak, it opens a little and idle goes up, it closes a little and idle goes down.
The computer controls this valve by sending it a specific voltage.
And this is a Learned range.
So, as an example, if the computer has been sending the IAC valve 3volts to get the engine to idle at 750RPM, and it now has to send 3.5volts to get idle at 750RPM it would set a code P0506, because at 3volts, RPM is lower than expected, so it had to raise the voltage.
You wouldn't see this on the tach because computer has raised the voltage to get 750RPM, that is its preset RPM target.

 

[adsm08]

In your specific example the computer knows that a certain pluse put to the iac should produce a certain engine speed. If it sees the rpm higher or lower outside a preset range it sets a code. As far as how we use the codes, we have these big books of tests for each code.

Postin' from teh Galaxy

 

[enjr44]

Thanks, RonD,
Ok, I understand what you have said. Now to a specific case. In drive or reverse, the PCM and IAC (new one) holds the rpm at 750 at all times. It doesn't matter what you do, it stays there (lights, AC, fan on high, etc.).

If you shift to neutral or park, it drops to 500 rpm and may or may not slowly recover to 700 or so rpm. IF you shift back to drive or park it immediately jumps back to 750. Each and every time. Back to park, it drops to 500. Over and over and over.

If you disconnect the battery and clear the memory, it idles a 750 in all transmission positions until the PCM relearns the settings and then it goes back to the low idle in park/neutral. This started a couple of months ago on a road trip (normal at one stop and low idle at the next) and is driving me nuts. Not so much because it is a real drive ability problem; but, because I can’t figure out what caused this (replaced the IAC on the off chance). The truck runs great otherwise.

Ed

 

[enjr44]

In your specific example the computer knows that a certain pluse put to the iac should produce a certain engine speed. If it sees the rpm higher or lower outside a preset range it sets a code. As far as how we use the codes, we have these big books of tests for each code.

Postin' from teh Galaxy

I am beginning to understand. For each code set you have a look up table that gives you additional tests or parts to test (in order of most likely) to eliminate the problem (or code). So, if the idle is low (outside the range) the tech data says look here and then here and then .......

So even if you have the code(s), without the tech data you are probably up the creek!!! Also, these scanners you borrow, from say Auto Zone, are they good enough to show the codes that do not trigger the ECL?

Ed

 

[cvar]

http://www.obd-codes.com

http://www.obd-codes.com/p0505
http://www.obd-codes.com/p0506
http://www.obd-codes.com/p0507

To read (or clear) OBD-II codes...
You can buy an ELM327 wireless gadget (Amazon/ebay/etc.) for $15 that talks to a freebie Android app, named TORQUE.
Download TORQUE from the GooglePlay store, and run it on your Droid smartphone. Very handy.

 

[RonD]

Yes, any basic scanner will show the "history" codes, along with "current" codes.

Some things I would check would be the TPS(throttle position sensor), and a small vacuum leak, one that opens and closes during engine operation.

The TPS is the same type of circuit as a volume control or light dimmer, so easy to test with an OHM meter, OHMs go up when rotated one way and then OHMs go down when rotated the other way.
The PCM sends voltage to the TPS, the return voltage from the TPS tells the PCM what position the gas pedal is in.
If the TPS doesn't have stable OHMs when it is closed, foot off the gas, RPMs could fluctuate.
When you drive the truck you use the gas pedal/TPS so when you stop and put into park it might fall to lower setting.

You can also test these types of sensors and valves live with a volt meter.
Use sewing needles to penetrate the wires, and test voltage live.
I would suggest testing TPS and IAC live, just to see it might be a wire issue vs an actual sensor issue.

A long time small vacuum leak would cause the PCM to learn incorrect IAC setting for 750RPM, when that vacuum leak stops the RPMs would drop down because extra air is cut off.
It is a small crack, probably on an intake vacuum hose, so close to the vacuum source.

Those are the "good" things, if just shifting from Park to Drive and then back to Park, without touching the gas pedal, causes the idle to drop then it is probably a PCM issue, which gets expensive quick.

One thing you can do which doesn't "fix" the problem but does make it a non-issue, is to set the base idle higher, say set it at 650-700RPM.
Usually there is a low idle adjustment on the intakes throttle linkage, underside.
With engine all warmed up, disconnect the IAC, idle should drop to current low idle setting, adjust idle higher.

 

[enjr44]

OKAY!!!

I think a light bulb just went on!!! I may not be the star on the Christmas tree; but, eventually…...

If I have a switched vacuum leak. A vacuum circuit that is switched on only in park or neutral would explain my symptoms. Idle is great in drive or reverse. Put it in neutral or park and the idle drops. Let it sit there and the sensors (idle speed and exhaust gases?) detect a lean condition and the PCM corrects for it. That is why it will correct itself after about 15/20 seconds.

Now the $64,000 question. Where can I find a vacuum diagram that has enough detail to find the leak?

Ed