Trouble Code 42 On My 81
Thread Starter
Burning Brakes
Joined: Oct 2009
Posts: 838
Likes: 2
From: Baton Rouge Louisiana
Trouble Code 42 On My 81
For about a year now I was getting an intermittent "check engine' light on my 81, so I have been driving with my system diagnostics meter hooked up while I was driving trying to get the code number. It never stayed on long enough to show the error code. Finally Code 42 started going on-off continuously, thought I wasn't going to make it home the car ran so rough. The shop service manual says it's "EST By-Pass Voltage Low. After doing all the wiring tests the manual says to replace the HEI Module (thermal module). The existing module didn't have any thermal grease under it. I re-installed a new module with dielectric grease, the car runs great now with no check-engine light problems. I find it hard to believe the absence of the grease caused so much trouble.
Instructor
Joined: Aug 2010
Posts: 233
Likes: 18
From: Monrovia Md
It could be that allowing it to get to hot (no thermal grease) was causing it to misbehave. It is possible that getting it cooler has it working properly. I would still guess something is marginal and will probably return. A new hei module might be relatively inexpensive insurance
Le Mans Master
Joined: May 1999
Posts: 6,692
Likes: 29
From: Texas
I chased a code 42 for several years. Several ignition module changeouts. Re-wired entire distributor - you can buy a complete harness, both ends. Nothing worked. Finally bought a new ecm and problem went away. New ecm costs only about what two ignition modules cost. Have two spare modules if needed. Oh, also replaced tps twice. Have spares of that also. Only problem is I put my hypertech chip in backwards - really stupid thing to do - and it fried it. Still had the original and running that now.
Thread Starter
Burning Brakes
Joined: Oct 2009
Posts: 838
Likes: 2
From: Baton Rouge Louisiana
Well my problems are continuing now, trouble code 42 cleared up after replacing the HEI module. Did some test driving and got it up to normal operating temperature and here we go again. Now getting trouble code 23 (MC Solenoid Circuit), and it stayed locked in. After following all the volt checks per the service manual it states to replace the MC solenoid and ECM. Wonder will go wrong next?
Le Mans Master
Joined: Oct 2002
Posts: 7,098
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From: Las Vegas - Just stop perpetuating myths please.
I dont know which service manual u are gettin your code descriptions from but my GM code book says code 23 is "IAT Sensor low" meaning intake air temp sensor reading is low. Did u modify your intake at all? How is the sensor and wiring looking?
BTW my code book says code 42 is "EST Circuit grounded" meaning electronic spark timing which consists of the ign module, computer (ECM) and the connecting wiring. It reads to since u have replaced the ign module either the wiring or the ECM have a problem. Here is a good link for that: http://batauto.com/articles/gmmsundrstd42.html
Hope this helps.
BTW my code book says code 42 is "EST Circuit grounded" meaning electronic spark timing which consists of the ign module, computer (ECM) and the connecting wiring. It reads to since u have replaced the ign module either the wiring or the ECM have a problem. Here is a good link for that: http://batauto.com/articles/gmmsundrstd42.html
Hope this helps.
Le Mans Master
Joined: Dec 2006
Posts: 8,837
Likes: 1,028
I dont know which service manual u are gettin your code descriptions from but my GM code book says code 23 is "IAT Sensor low" meaning intake air temp sensor reading is low. Did u modify your intake at all? How is the sensor and wiring looking?
BTW my code book says code 42 is "EST Circuit grounded" meaning electronic spark timing which consists of the ign module, computer (ECM) and the connecting wiring. It reads to since u have replaced the ign module either the wiring or the ECM have a problem. Here is a good link for that: http://batauto.com/articles/gmmsundrstd42.html
Hope this helps.
BTW my code book says code 42 is "EST Circuit grounded" meaning electronic spark timing which consists of the ign module, computer (ECM) and the connecting wiring. It reads to since u have replaced the ign module either the wiring or the ECM have a problem. Here is a good link for that: http://batauto.com/articles/gmmsundrstd42.html
Hope this helps.
Code 42 is EST By-Pass voltage low.
I have the 81 ECM factory service manual supplement book.
Le Mans Master
Joined: Dec 2006
Posts: 8,837
Likes: 1,028
Well my problems are continuing now, trouble code 42 cleared up after replacing the HEI module. Did some test driving and got it up to normal operating temperature and here we go again. Now getting trouble code 23 (MC Solenoid Circuit), and it stayed locked in. After following all the volt checks per the service manual it states to replace the MC solenoid and ECM. Wonder will go wrong next?
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Thread Starter
Burning Brakes
Joined: Oct 2009
Posts: 838
Likes: 2
From: Baton Rouge Louisiana
Le Mans Master
Joined: Dec 2006
Posts: 8,837
Likes: 1,028
Thread Starter
Burning Brakes
Joined: Oct 2009
Posts: 838
Likes: 2
From: Baton Rouge Louisiana
Team Owner
Joined: Jan 2006
Posts: 37,637
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From: Crossville TN
You may also have the dreaded 'loose and/or corroded connector' syndrome on that old ECM. After years of use, the contacts in the plugs AND the power and grounding connections can become 'iffy', because of the sensitive electrical needs of the ECM, etc.
Pull all the plugs and clean all contacts (both ends) with alcohol and Q-tips. Then put a coating of contact grease to keep them from corroding in the future. Also, remove any direct grounding connections, clean all surfaces/connectors/washers involved and reconnect them to frame, etc.
You need to eliminate those issues first, if you want to be sure that what your reader shows is valid.
Pull all the plugs and clean all contacts (both ends) with alcohol and Q-tips. Then put a coating of contact grease to keep them from corroding in the future. Also, remove any direct grounding connections, clean all surfaces/connectors/washers involved and reconnect them to frame, etc.
You need to eliminate those issues first, if you want to be sure that what your reader shows is valid.
Last edited by 7T1vette; Sep 15, 2015 at 01:36 AM.
Heel & Toe
Joined: Aug 2014
Posts: 15
Likes: 2
From: Rehoboth MA
I agree with 7T1vette. I was chasing that code 42 for most of the summer. It all started after I had performed a tune up on the car( plug,cap,rotor,and wires). After changing modules and scratching my head, I went ahead and clean all of the connectors between the dizzy and the ecm. So far I haven't had any issues with code 42 popping up.
The big headache with these later C3 is the computer, it's real basic. When it pops a code 42 you have to scrutinize every component of the system between A-Z. Unlike modern cars which tell you what sensor is in trouble, our cars throw kinda general codes.
The big headache with these later C3 is the computer, it's real basic. When it pops a code 42 you have to scrutinize every component of the system between A-Z. Unlike modern cars which tell you what sensor is in trouble, our cars throw kinda general codes.
Thread Starter
Burning Brakes
Joined: Oct 2009
Posts: 838
Likes: 2
From: Baton Rouge Louisiana
Thanks a ton guys for the info, I'm fixing to dig into this much more than I thought I needed to. Will start cleaning the contacts and grounds today (glad I'm retired) I have dielectric grease also.
I did check resistance across my MC solenoid as stated by Big2Bird, the meter read well over 10. As of now the only trouble code I have is 23 (problem in the M/C Solenoid Circuit)
I did check resistance across my MC solenoid as stated by Big2Bird, the meter read well over 10. As of now the only trouble code I have is 23 (problem in the M/C Solenoid Circuit)
Last edited by golfradict; Sep 15, 2015 at 08:45 AM.
Thread Starter
Burning Brakes
Joined: Oct 2009
Posts: 838
Likes: 2
From: Baton Rouge Louisiana
A rather disappointing follow up here if interested. Today I cleaned all the contacts I could get my hands on, afterward started the car and as soon as it got up to normal temp and went into closed loop, all the trouble codes reappeared, at least 7 different codes. Got a new ECM on order
Thread Starter
Burning Brakes
Joined: Oct 2009
Posts: 838
Likes: 2
From: Baton Rouge Louisiana
Thanks 7T1vette, this isn't my first time to have ECM problems, I had one go out a couple of years back. I will give it one more chance. When the next fails I'm putting in a vacuum advance distributor and Holley carb. I like my stock engine compartment look, just don't like the computer problems, should have bought me a good 1980. I'm running true duals with no cats, the air pump isn't functional either. No problems here with smog regulations
Le Mans Master
Joined: Dec 2006
Posts: 8,837
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Thanks 7T1vette, this isn't my first time to have ECM problems, I had one go out a couple of years back. I will give it one more chance. When the next fails I'm putting in a vacuum advance distributor and Holley carb. I like my stock engine compartment look, just don't like the computer problems, should have bought me a good 1980. I'm running true duals with no cats, the air pump isn't functional either. No problems here with smog regulations
The ECM has been in my car 34 years, and knock on wood, I have yet to have a trouble code.(Except for when I unplug something).
Thread Starter
Burning Brakes
Joined: Oct 2009
Posts: 838
Likes: 2
From: Baton Rouge Louisiana
Wish I could say the same, my car was restored after a bad front end wreck, still only has 25K original miles. I will surely continue to dig for the problem. Thanks Big2Bird