May 14, 2011
1,996
4,866
Barrie Ontario
VetteCoins
76,036
Car
1991 Coupe
My Service Engine Soon Light comes on after about 10 minutes of driving at highway speeds. Car runs fine, but curiosity led me to try and check it out.
I used a jumper in the OBD connector and got 2 codes H74 and H32.
I cannot find either code in my Service Manual, but a search on line showed 32 as an EGR System Failure. ( I installed a new gas cap but still get the warning)
But there is nothing for 74.
So does anyone out there have a comprehensive list of the error codes and any ideas as to how to correct the situation? or is it the dreaded visit to the shop with my wallet out!
The vette is a 1991, base 5.7L
 
My Service Engine Soon Light comes on after about 10 minutes of driving at highway speeds. Car runs fine, but curiosity led me to try and check it out.
I used a jumper in the OBD connector and got 2 codes H74 and H32.
I cannot find either code in my Service Manual, but a search on line showed 32 as an EGR System Failure. ( I installed a new gas cap but still get the warning)
But there is nothing for 74.
So does anyone out there have a comprehensive list of the error codes and any ideas as to how to correct the situation? or is it the dreaded visit to the shop with my wallet out!
The vette is a 1991, base 5.7L
An H usually signifies "history". If you get a "C" or "P" it's "current" or "present".
For H74, has the radio been replaced, or taken out and been reinstalled?
If you buy or borrow an OBD2 reader, you'll get be able to clear the codes.
Some vehicles will auto reset (if the issue no longer exists) after a certain amount of ignition key cycles.
 
Thanks Axe. The radio has been dead since I've owned it, so that could be a vary old code.
If the EGR code is also history then maybe its nothing to concern about.
I believe I cleared the codes using the info computer, at least that's what the guy in the web video said could be done.
We'll see what happens.
 
  • Like
Reactions: Black 03 Z06
My bad....your car (91) is OBD not OBD2.

Not sure if it applies, but I found this:
CCM (Central Control Module) Codes 72 and 74 usually occur after the radio control head was replaced. The radio head LED and LCD open circuits are noticed by the CCM, setting the code. There is a simple fix while the radio head is being replaced. Resistors and one diode must be installed to alleviate the problem. A 1K 1/2 watt resistor is installed in the following manner: one side of the resistor must be powered by the radio head C2 connector in position 8 (pink wire). Connect one end of the resistor to this wire. This will send the correct voltage signal to the CCM once the other side of the resistor is connected. At radio head connector C1, locate the gray/black striped wire at position 3. Connect the other end of the resistor here. Be careful: the radio head has two gray/black striped wires in connector C1. The gray/black striped wire at position 4 is the incandescent lamp control. It will not set a code when left disconnected. To recap: the resistor will be installed at connector C2 position 8 on one end and connector C1 at position 3 for the other end.
 
So here we are. After about 10 minutes at highway speeds, I still get the Check Engine Soon.
When I did the code check before, based on google videos, I jumped across the upper right (A) to the lower left (G) of the OBD connector.
Now after looking at my Chiltons manual. I find 2 things. Thing 1 is a complete list of the diagnostics codes, which is good.
Thing 2, is a diagram of the OBD connector with direction to install a jumper from A to B being the 2 most upper right connectors.

I am leery of trying the A to B connection, as I don't want to blow it up and I've never seen it in any videos. Maybe it applies to the Full digital dash of the earlier models, my 91 has the analogue dash.

Anyone got any ideas on this?
 
  • Like
Reactions: Black 03 Z06
My Service Engine Soon Light comes on after about 10 minutes of driving at highway speeds. Car runs fine, but curiosity led me to try and check it out.
I used a jumper in the OBD connector and got 2 codes H74 and H32.
I cannot find either code in my Service Manual, but a search on line showed 32 as an EGR System Failure. ( I installed a new gas cap but still get the warning)
But there is nothing for 74.
So does anyone out there have a comprehensive list of the error codes and any ideas as to how to correct the situation? or is it the dreaded visit to the shop with my wallet out!
The vette is a 1991, base 5.7L
My 08 Coupe had the problem, it was P0449 emission vent solenoid system. >Replaced the solenoid and new harness was being recommended by GM, did both and the problem cleared immediatley. Trust you have solved your problem by now. Champion Motors in Scarborough solved my problem.
 
  • Like
Reactions: Black 03 Z06
My 08 Coupe had the problem, it was P0449 emission vent solenoid system. >Replaced the solenoid and new harness was being recommended by GM, did both and the problem cleared immediatley. Trust you have solved your problem by now. Champion Motors in Scarborough solved my problem.
Thanks for the info. I kept digging and it appears that the H32 error is the EGR system. On review the EGR does not do anything unless the vehicle is running for a long stretch at relatively steady throttle, hence light comes on after about 10 minutes on the highway.
There is a long list of things to check but most of the comments I found point to the EGR valve and if it doesn't work it will not affect the engine in any way, just gives an annoying Service Engine Light. To get to the valve (it can be cleaned) I need to remove the intake plenum. Just haven't done it yet.
 
Thanks for the info. I kept digging and it appears that the H32 error is the EGR system. On review the EGR does not do anything unless the vehicle is running for a long stretch at relatively steady throttle, hence light comes on after about 10 minutes on the highway.
There is a long list of things to check but most of the comments I found point to the EGR valve and if it doesn't work it will not affect the engine in any way, just gives an annoying Service Engine Light. To get to the valve (it can be cleaned) I need to remove the intake plenum. Just haven't done it yet.
Well, holy doodle, I finally fixed it! Only been 2 years but there was a pandemic in the middle. Purchased a new EGR valve and gaskets from Rock Auto, $107 CDN all in. I started at it following the shop manual and discovered that the plenum can be removed with no need to drain the rad or depressurize the fuel system, or remove the throttle body(just unbolt it from the plenum) which saves a lot of time. I also discovered that a furry little creature had chewed through the air line that runs below the plenum and connects to the EGR Valve, but you cannot see or check that line unless you remove the plenum. If I had known it was the air line I could have saved the EGR removal but seeing as I had the parts I replaced it anyway. Its all good, service engine light stays off, just glad I didn't have to take it to a shop
 
Old Thread: Hello . There have been no replies in this thread for 100 days.
Content in this thread may no longer be relevant.
Perhaps it would be better to start a new thread instead.

Users who are viewing this thread