I recently checked out my codes after getting an SES light. They were H72, H74, and H32. Since they were all H codes (indicating not current), I cleared them all and verified that they were gone. After driving for 5 or 6 days, the SES light came back on. I checked the codes again within an hour of the new SES light, and guess what: H72, H74, and H32. That doesn't make sense to me, because at the least they should indicate current codes rather than historical codes IMHO. It is almost like the old historical codes were somehow retained somewhere in the system and regurgitated when the car was in the mood. Does anyone know the answer to this problem? I did verify that the previous codes were gone by reading the CCM and getting the appropriate C12 code and blanks for the ECM and EBCM.

 

I can understand your confusion and will take a stab at explanation according to my understanding, more knowledgeable members please feel free to correct me if I am wrong.
The term History simply relates to key cycles and since you read the codes after turning the engine off and turning the key back to run position you entered a new key cycle.
Yes I know, you are supposed to check for check for codes with engine off, the ECM enters "field service" mode if you jump the ALDL with the engine running. Logic dictates then that ALL codes will be history codes since they occurred on the previous key cycle!
As an experiment I even tried this: with key off I disconnected various sensors, (map, iat) jumped the ALDL pin A to G then turned the key on. No code was recorded even though the sensor connector was off!
If I started the engine with the sensor disconnected the SES light would illuminate, I then turned the key off, jumped pin A to G and the appropriate sensor code was displayed as HISTORY because it was the previous key cycle.
I believe the ECM's reference to historic rather than current (present key cycle) is there to help technicians using the Tech 1 scan tool, the "H" display when checking codes by jumping the ALDL just ads to the confusion, hope that helps.

 

That explanation makes sense and would explain what I am getting. I will just assume you are correct. Evidently I need to check out the EGR valve, and probably just replace it. The H72 and H74 refer to display problems that don't bother me. Thanks for your comments. It is very helpful.

 

Best practice would be to follow the diagnostic sequence in the FSM, an egr related code could be caused by vacuum or electrical problems rather than the valve itself.

 

I'm not sure I have that info. Could you recommend a source to learn about that diagnostic procedure?
Thanks a bunch!!

 

www.c4guru.com in the tech section. Join up while you are there. This is Bogus' baby and a fine site.

 

I'll check it out. Thanks!!