Auto Repair - Roadshow forum


GMC Error Code 34(MAF sensor) & 42(EST circuit)

Vehicle: 1991 GMC Sierra 5.7L, 4000 miles on rebuilt engine, stock ignition system

As some of you may remember, my problem started off as being the Engine Service Light remained constantly lit. There were no error codes to be read (Or so I thought.) Managed to reset the system and the light remained off until the next test drive.
The light came back on and remained on. Found an owner's manual for the code reader and I had not been using it correctly. Was now able to read a Code 42, Electronic spark timing circuit - open or shorted.

Rerouted spark plug wires to insure no electrical interference. Checked connections, and continuity of the 423 EST wire, 424 By-pass wire coming from the Ignition Module going to the ECM. Also checked for possible shorts. Everything appears to be good, physically.

Following a trouble shooting flow chart, disconnected the ECM connectors. Turned the ignition on.
Probed the ECM harness connector CKT 423 with an Ohmmeter to ground. It should read less than 500 Ohms. It did at 356 but fluctuated with each try, though remaining below 500. The last try gave a reading of 0 Ohms, that I do not understand, but could be due to my inexperience/lack of ability.
Next probed ECM harness connector CKT 424 with a test light. The light remained off. No electrical current.
Following the flow chart, the next step was to connect the Ohmmeter to ECM harness CKT 423 and ground. Again probe ECM harness CKT 424 with the test light connected to 12 Volts. As the test light contacts CKT 424 the resistance should switch from under 500 to over 5000 Ohms. Resistance should have gone high (open circuit). It did NOT. IIRC, it remained at 0.

The flow chart stated this means:
CKT 424 open, faulty connections or faulty Ignition Module. (Wire has continuity, connections feel solid.)
CKT 423 shorted to ground. (Wire has continuity, does not appear to be shorted.)
Faulty Ignition Module?? When last test driven for 30 minutes, the engine did not exhibit any of the classic symptoms. It did not overheat. The engine started easily. The engine ran smoothly while being driven, but may have had a barely noticeable miss at idle.

Now it gets even better.
About one week later, when the weather improved (It wasn't snowing and the temperature was above 0 degrees F.), decided to do a retest.
The battery was dead! Something drained it. I believe it to be the small light in the glove box remaining on, when I left the door open after working on the wires going to the ECM. Maybe?

Ok, charged the battery and started over this morning.
Turning the ignition key to start, the console trouble lights came on as they should, but the starter motor did not/would not crank the engine. No clicking of the solenoid and the battery shows 12.94 Volts.
In addition, the code reader now shows a Code 34 (possible MAF sensor/MAP sensor problem) and then the original Code 42.

So now, on top of everything the engine will not even start.
Please, what do I do next?
Your assistance in this matter is greatly appreciated.
Best Wishes,

Answer This Ask For Clarification
Discussion is locked
You are posting a reply to: GMC Error Code 34(MAF sensor) & 42(EST circuit)
The posting of advertisements, profanity, or personal attacks is prohibited. Please refer to our CNET Forums policies for details. All submitted content is subject to our Terms of Use.
Track this discussion and email me when there are updates

If you're asking for technical help, please be sure to include all your system info, including operating system, model number, and any other specifics related to the problem. Also please exercise your best judgment when posting in the forums--revealing personal information such as your e-mail address, telephone number, and address is not recommended.

You are reporting the following post: GMC Error Code 34(MAF sensor) & 42(EST circuit)
This post has been flagged and will be reviewed by our staff. Thank you for helping us maintain CNET's great community.
Sorry, there was a problem flagging this post. Please try again now or at a later time.
If you believe this post is offensive or violates the CNET Forums' Usage policies, you can report it below (this will not automatically remove the post). Once reported, our moderators will be notified and the post will be reviewed.

All Answers

Collapse -
Additional Question

In reply to: GMC Error Code 34(MAF sensor) & 42(EST circuit)

Should have added, the distributor and Ignition Module were replaced when the rebuilt engine was installed. It is from CarQuest and just found out it was made in China.
Would the failure of the Ignition Module result in the engine not even cranking?
Best Wishes,

Collapse -
No start

In reply to: GMC Error Code 34(MAF sensor) & 42(EST circuit)

Hi, Gus; The battery can still be the culprit on the 'no crank' issue, even though it is fully charged. Those side post battery cables can get corroded and you won't see it until you take the bolt out of the cable. I've heard that those gel batteries handle the cold better than the wet cell batteries. Nearly, always, when I'd get an EST code, it would be a faulty ignition control module (inside the distributor cap), but I never had to deal with one in your climate. Most would take a dump when hot. Hmmmm.
None of those codes, however, would not cause a 'no crank' issue. GM MAP sensors of that vintage were fairly common problems, but would cause an over-fueling issue, not a cranking issue. Do you have to go all the way in to Fairbanks for parts?
Good luck, my friend.

Collapse -
Ignition Module

In reply to: No start

Howdy Loren,
Thank you for sharing your insight/experience. It started snowing again this morning, little over four inches so far. Subsequently, no working on the truck today. Probably plow the driveway tomorrow as there was already a couple of inches of fresh snow on the ground, before it started to snow today.
Will replace the Ignition Control Module and clean the battery terminals.
Only have to make a trip of 50 miles to purchase or order parts, if not in stock. Probably be end of next week before I can get the part in hand. Will keep you posted as events unfold.
Thanks again.
From the Ol' Curmudgeon up North
Best Wishes,

Popular Forums

Computer Newbies 10,686 discussions
Computer Help 54,365 discussions
Laptops 21,181 discussions
Networking & Wireless 16,313 discussions
Phones 17,137 discussions
Security 31,287 discussions
TVs & Home Theaters 22,101 discussions
Windows 7 8,164 discussions
Windows 10 2,657 discussions


Here's Everything to Know About the 2019 Grammys

Find out how to watch the Grammy Awards if you don't have cable and more.