Service Engine Soon light but no codes

[LT4John]

On my 96 LT4 the "Service Engine Soon" light is on, but when I short pins 4 and 12 on the OBDII connector all I get is a C12 code on the dash, which I believe means there are no codes to display. Anyone know why the light is on but there are no codes?
:confused

 

[Tims93]

John, Had a similar problem on my 93 recently, turned out to be a bad 10 amp fuse inside the interior panel. Drove me crazy for a month. Hope your fix is as easy. Tim

 

[LT4John]

Thanks, I will check my fuses.

 

[LT4John]

All fuses checked out Okay.
Not sure what to do next:confused

Help;help

 

[Rabbit]

I believe you will need a scan tool to retrieve codes on your 96 which is OBD II.

The grounding of pins 12 to 4 will work on OBD I, but unless I am mistaken will not work on yours.

 

[Skant]

I'm having the very same problem... or at least the same symptoms. My LT4 just started having the SES light come on and stay on. I can't feel any drivability problems though... seems to be fine except for the warning light.

I tried the 'shorting 4 to 12' thing, and it _does_ work. As in it does go into a diagnostic mode, but the only code it finds in any of the modules is the standard C12.

It looks like it shows module 1, 9, and A. And I think there are more than that?

Perhaps this method is only partially backward compatible on OBD II? Maybe it can only show some of the codes? OBD II seems to have its own set of codes (more of them), so maybe this mechanism can only show the ones that map to OBD I easily. Partial backwards compatability?

Do I need to buy a real scanner to get the information I'm after?

- Skant

 

[Tuna]

On my '92, when the SES comes on it has meant that the battery is going bad - low voltage and not accepting/holding a charge. It's been my first sign of needing a new battery.
Check you battery voltage and charging system.

 

[carguru]

For the 96, you need the scan tool. Shorting pins 4 and 12 works only for the '94 and '95 models because they were OBD I with and OBD II connector. All '96 are OBD II. The scanner will give you a lot more info on the problem.

 

[wwood]

I had that happen to me. The problem was simple to solve in my case because it was the electrical connection to the throttle body had worked loose. It looked tight, but was not. You may wish to check all your electrical connections to your motor. You may get lucky as I did.

 

[Nigel80031]

I had the engine service soon light come on last week for my 1995 and had code H31 listed for module 4. There is no such code listed anywhere in the manual. The light was only on for one trip.

Anyone know what might had caused this.

 

[Rabbit]

Module 4 relates to..PCM/ECM waiting for instructions.
H31 is History code for LCD Data Circuit Shorted to B+ or CCM internal open. Probably just a temporary glitch I hope..

 

[LT4John]

Here is the latest. On a OBDII, if you short pins 4 to 12 , according to Chris Killebrew (Gordon's wife) who was kind enough to answer my email;

"You can read the codes
EXCEPT: the PCM codes, that are in the CCM Cavity 12, Module #4, do require a scanner to read them.
You can read all the other codes for: Cavity 6: LTPWS
Cavity 12: CCM
Module #1, CCM
Module #7, RTD
Module #9, ABS/TCS
Module #A, SIR
Cavity 8: PKE"

I am not sure what that all means, but the fact is, I can’t pull all the codes with a shorting block. I ordered an interface tool for my PC from ScanTool, hooked it up to my PC, and was able to pull the failure code. I got a code “P0100”; which means a "mass or volume air flow circuit malfunction". In looking around the MAF I found that the wires going to the MAF were in bad shape, so I rewired the connector. So far so good, the light is out and there are no codes in memory.:dance

 

[Nigel80031]

If you need a cheap code reader, the one at this site might work for you.

www.harborfreight.com/cpi/ctaf/Displayitem.taf?itemnumber=46030

 

[wx briefer]

According to manual, something as simple as a misfire while sitting at a light can trip the SES. After about a hundred miles the light may go out by itself. Also, no particular codes will be set. However, if the light starts to BLINK, then you DO have a more pressing problem.

 

[Hib Halverson]

On my 96 LT4 the "Service Engine Soon" light is on, but when I short pins 4 and 12 on the OBDII connector all I get is a C12 code on the dash, which I believe means there are no codes to display. Anyone know why the light is on but there are no codes?
:confused

No OBD II PCM will display PCM/ ECM codes by shorting any pin of the DLC. You need a PC-based scan test application that supports OBD II or a dedicated-hardware scan tester that supports OBD II to read the codes. You can't read OBD II codes on the IP because it only supports a 2 digit display and OBD II codes are four digits.

For the 96, you need the scan tool. Shorting pins 4 and 12 works only for the '94 and '95 models because they were OBD I with and OBD II connector. All '96 are OBD II. The scanner will give you a lot more info on the problem.

I'm sorry, but the above is quite right.
most MY94 and all MY95 VIN P engines (LT1) were part of the early-roll out of OBD II by GM. Both have some level of OBD II capability but only OBD codes would turn on the light. Any OBDII faults were detected and stored in memory but would not turn on the light. For MY94 VIN P (see possible exception below) and 95 VIN P, you cannot ground anything and get PCM codes. An exception is any VIN J (LT5) engine which is, as "carguru" says, an OBD system that used the 16-pin OBD II, DLC connector. Another exception could be what the service manual claims: some 94 VIN P engines came with 12-pin DLCs and can be diagnosed by grounding the connector's diagnostic pin.

Bottom line: if it's a 94 or 95 LT1 and it has a 16-pin connector, you need a scan tester.