My 96 stock LT4's service engine light came on so my first thought is to take it up Auto Zone and have them run a scan to tell me what code is causing it. Good or bad idea?

 

AutoZone slackers will most likely have a code reader that will popup a number/code. They will pull out a card and tell you what the code means, then try to sell you that part, not knowing if that is your problem at all...

a Napa Service center will likely have a scan tool that gives the technician loads of info as to what could cause the light/code to set. We use a Genysis 2 I think, good little unit...runs about 5K...so it better do more than pull codes.

For instance, an EGR code will promt autozone to want to sell you a new EGR valve...when a number of things could set such a code.

 

I agree with the above post. What you need to understand is that the DTC ( Data Trouble Code ) is only the starting point for solving the issue. Also, as you can see there is a difference between a code reader and a scanner. The scanner reads & displays the real time data that the ECM is getting from all the sensors and monitored systems as the motor is at idle and in closed loop.

Do you have a FSM ( Factory Service Manual ) for your car? The FSM has a section devoted to the engine electronics and has a page displaying what the normal data should look like on a scan. Using the DTC you are given a page for the diagnostics' on the particular DTC your car displays. That page is where the trouble shooting starts so you can determine if in fact you need a part, or if there is another issue, and what that other issue could be, and how to resolve the issue. The scan data maybe useful and or necessary in order to complete the diagnostic tree displayed for the DTC you are seeing, but not all the time. Mostly a test light and a DVOM will get you thru most of the diagnostic flow charts. It's not something the average back yard guy can't learn to do well. It just takes some time and reading of the FSM.


Tom

 

Good idea! Get the free scan, but don't buy anything until you are positive you need it.

 

Thanks for the feedback.....I did already go to Auto Zone and there report showed a
P0100 and listed a few possible causes such as MAF,EGR, and few others. That said I intend to take it to NAPA to have them run what sounds like from the above comments a more complete accurate scan.

 

All good information so far in posts.

Well here is a little more fuel for the fire. My DTC library does not show any P0100 code as listed for “GM”. However is does show under generic codes. It says “MAF or VAF”. The vette obviously has a MAF but I don’t believe the vette as an acronym for VAF, but it is an generic code.

I would use the on board diagnostics to see what kind of code the car itself displays. I would then clear the code and see if it re-appears as the same and not a false code. I think the code from the on board read out might be more direct than a generic code.

** What you would be looking for if it was the MAF would be DTC 48 (in module 4).

Having scanners are great (and expensive), but the built in diagnostics work great.

If you need help using the on board diagnostics, do a search and ask for help.

 

I don't know of anything "onboard" for OBD2 cars.

P100 is a short/open in the MAF circuit (usually a voltage return below .2 or above 4 volts). You need a scan tool and the Trouble Tree to figure it out and I wouldn't throw any parts at it until you went through the diagnostics. You could start, however, by clearing the Code and then drive it to see if it comes back.

 

This is from cashthe corvetteman from down under. Don't know if it works. Give it a try.


I am posting this to clear some things up about the "paperclip" trick as it applies to later model C4's ('94-'96), and I just want to make sure nobody is in the dark about all the various features of their car's built-in diagnostic mode. I will show that it does a lot more than display codes (it's got a menu!), and I will show you what the "SYS" message means, and I also hope to clear up some major misinformation.

Specifically, there have been numerous rumors that the 1996 cars have no diagnostic mode. You can enter the '96 diagnostic mode, but the rumors say it's only a holdover from previous years and that everything shown on the screen is garbage. This is false. The 1996 cars DO have an elaborate and functional diagnostic mode, with only one major change.

Our cars have numerous computers (it calls them "modules") networked together, one of which is the PCM, which controls the engine. The root of the rumors lies in the fact that in '96, new laws mandated the PCM's protocol to change to OBD-II, which used a standardized four-digit code numbering scheme, too big to fit on the LCD. Rather than find some other way to display the numbers, GM decided to say screw it and deleted the functions that read/clear PCM codes from the diagnostic mode. The PCM simply no longer shows up at all among the modules listed. They forgot to mention this little tidbit in the service manual, which confused a hell of a lot of people into thinking that whole chapter does not apply to '96 cars; other than that one detail, however, it does. GM did not change any of the functions involving other modules (CCM, EBTCM, etc), nor did those functions get corrupted by the switch to OBD-II; they all still show up and function correctly. Thus the majority of the diagnostic mode's power is still there in '96. I know because I checked.

-----Using diagnostic mode-----

To enter the diagnostic mode, you simply ground the diagnostic pin on your car's diagnostic connector (located near the driver's knees). Various DLC connectors were used throughout the C4's life, all with different pinouts; on '94-'96 cars, you connect the fourth pin from the left in each of the two rows of pins to one another, with a paperclip or some other conductor.

When you turn the ignition to "run," you will now be in diagnostic mode, which uses the LCD screen and the DIC (driver information center) buttons for input and output. You may start and drive the car while in diagnostic mode; in fact, the car has to be running for some of the functions. The only disadvantage is you will not be able to see your normal gauges while in this mode since the LCD screen will be in use.

When you start diagnostic mode, the LCD will show you any codes stored in the car's various computers. In '96, Module 1 is the CCM, 4 is the PCM (doesn't show up in '96), 7 is the RTD, 9 is the EBTCM, and A is the DERM. These are cycled through one at a time, with dashes indicating the end of codes for that module. A "C" prefix means the code is currently active, meaning that the relevant parameters are out of range right now; an "H" means the code has been stored in history but is not currently active.

Unlike other vehicle electronic systems, such as PCM, ABS, air bags, etc., there is no specific "CCM" lamp to indicate a malfunction when one is present. Instead, the CCM flashes the word "SYS" three times every once in a while on the LCD whenever there are active CCM codes present. If the code thrown is one that involves only the security system, the "SECURITY" lamp will illuminate instead, or they will both flash together if it involves the Fuel Enable Data Stream.

Afterwards you will enter the menu (or you can press any information center buttons to skip to the menu). Did you know there was a menu? I didn't, until I read the FSM.

You'll know you're in the menu when you see the number "1.0". The first digit, before the decimal point, refers to which module you are talking to at the moment, as numbered above. The second digit indicates the menu item. Item .0 means that computer is awaiting instructions. Item .1 displays any codes for that module and .7 clears them. This applies to all of the car's computers/modules. Note that any non-CCM codes will always show up with the "H" as in history, since the CCM does not know whether or not they are currently active.

The CCM has additional options on top of these; .2 and .3 let you cycle through the CCM's stored data and inputs, respectively, and option .4 lets you toggle each of its outputs manually (this part is fun).

To navigate the menu items, use the "gauges" and "trip odo" button. To navigate to the next or previous module, hold down the "fuel" or "trip" button for a second. To select a menu item, press "ENG MET."

One thing that I recommend doing at this point, is see if you have any EBTCM codes stored (module 9 in '96), and then delete them. You will hear the brake pressure modulator valves near the EBTCM click as it resets, and you might suddenly feel that your brakes feel and work much better than before. This was the case for me; I had a lateral accelerometer code, as well as a code from back when my Opti died making the tachometer go crazy. Neither code has come back since I cleared them, meaning that the presence of codes alone was enough to severely effect how my brake functions.

Now for the fun part: The CCM data, input, and outputs. These are options .2, .3, and .4 in the CCM module menu, respectively. After selecting any of these, press "eng met" to go to the next value and "fuel info" to go to the previous. The selections are:


Display CCM Data (Mode 1.2):
01 - Fuel Level (Gallons, tenths)
02 - Dimming Potentiometer
03 - Ambient Light Sensor
04 - Rear Defogger Timer
05 - Vehicle Speed
06 - Pass-Key
07 - Ignition Voltage
08 - Switched Voltage
09 - Cluster dimming
10 - LCD backlight dimming
11 - Radio & Climate dimming
12 - LED dimming
13 & 14 - Vehicle configuration
15 - Oil monitor count
16 - CCM version

Display CCM input status (Mode 1.3):

1 - PassKey fuel
2 - English/metric status
3 - Door key switch
4 - Right Door ajar
5 - Left door ajar
6 - key in ignition but in "off" or "acc" position
7 - hatch ajar
8 - Power door unlock
9 - Power door lock
10 - Parking lights
11 - Rear defogger input (car must be running)
12 - Seat belt switch
13 - High beam switch
14 - Low oil level switch

These values are either "1" or "0," and when toggled, the new value will be appended on to the old value (01 means it was 0 but is now 1).

Cycle CCM Outputs (Mode 1.4):

Selecting these options will manually activate the lights and sounds the CCM produces.

1 - Change oil light
2 - Check gauges indicator
3 - Fasten seatbelt indicator
4 - Security lamp
5 - High beam indicator
6 & 7 - chimes
8 - LCD blanking
9 - Defogger relay
10 - Courtesy lamp
11 - Low oil lamp
12 - Theft relay
13 - DAB relay
14 - door ajar light
15 - Horns (This will actually beep the horns, watch out!)

So there you have it; you can now dazzle and impress your friends with your car's diagnostic features, as well as test any of the circuits and systems in the interior. Please correct any factual errors I have made in this post, and if you know of any information that would be useful to add to this thread, so as to make it a centralized source for information about the late model CCM, let me know.

 

OK then.

I have been using these functions for years on my 94. Having the FSM is definitely a must or at least an asset to work the diagnostics and interpret the results. I would add that some of this take a bit of practice to see what each feature does and how it is displayed.

But what is new to me from this information is there is no codes displayed for module 4, the PCM (if I read this post correctly). The codes that are most talked about are the ones from the PCM/ECM and it sounds like you can’t get those from a 96 for the PCM. So my list of codes for the PCM for the 96 are useless.

And therefore some of the information in my previous post #6 is wrong as far as pulling codes.

 

Ok info but there is nothing in the CCM that has anything to do with a P100. Yes, there are Body Codes and other Modules which you can get out of some cars (including the Vette) but those systems have nothing to do with engine management which is the Poster's problem. If you have one of these Years, at least buy a decent Code Reader (meaning one that can clear Codes) or a Scanner that will give you the Sensor Value. Boneup too on how the information - Sensor Value - is displayed as it's a tad different from OBD1. A plus is that once you learn how it works, you'll also know how to fix your Toyota.

 

Well, yes P0100 is MAF. I have had mine do this. I recommend clearing the code and see if it comes back. Alot of things can cause that code to come up. It could be anything and it could be nothing. May not be bad idea to check your air filter, check the wires/connections to the MAF.