Trouble Codes for a '93

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Mar 27, 2003
Georgia Tech
Trouble Code help with a '93

I checked the codes with the CEL-flashing method on my '93 today.

I found that the numbers I pulled do not match the numbers in my '97 manual. (I do not have an earlier manual, nor is it normally necessary with the few discrepancies)

I was hoping that someone could let me know what the actual codes are, or post a scan of the page in the earlier manual.

the numbers I counted were, 21 28 42 83 838 485 (repeats)

I am not confident of the spacing, either.

Any help would be great, need to get this truck emissions inspected very soon.
Last edited:
21 #1 oxygen sensor
28 #2 oxygen sensor
42 vehicle speed sensor
83, 84, 85 TCM/ECM communications.

O2 sensors will likely take care of the 21, 28. The 42 will need to be diagnosed. The 83, 84,85 will need a new engine ECU.

CruisinGA said:
I checked the codes with the CEL-flashing method on my '93 today.

the numbers I counted were, 21 28 42 83 84 85 (repeats)

Join the club. I just read my codes a week or so ago. I too have the rather infamous (mostly for early '93 models) [83, 84, and 85] CEL codes. Following that bit of disappointing news, I did a bit of research in this forum thanks to a post from Cruiserdan earlier this year.

I'll see if I can copy the links he posted -

Good luck with this. Speaking for myself, I sure would like to hear what you end up doing.

I don't have any time ATM, but before buying a replacement ECM, I plan on pursuing the TPS scenario some of the links above referenced.
I too have the 83,84,85 codes on my 1993.
Does anyone have a link or can provide details on how to adust the throttle position sensor (TPS)? I've seen comments that if the TPS is out of adjustment, it will trigger these codes. I just don't have a clue on how to adjust it and what to adjust it to?

When mine popped up in October 2003 I ended up picking a used ECU (with the correct pn) from I havent had the codes return since in 40k miles...
Thanks Dan, I'll call you tomorrow. Hope your dogs are hungry.

Been cruisin those threads, thanks for the links.

I'm going to fix the first three codes, reset the ECU and try to get the emissions test done, and then try and locate a used computer.

Thanks for the help.

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