Handling Multiple DTCs

I saw a post on a forum where somebody was seeking help for multiple DTCs . Another person responded stressing that you always handle the DTCs from lowest number to highest. This is not what I have learned at all. It doesn't even make sense to me. If you had a P1345 and a P0171, I'm handling the P1345 first. My understanding is that you handle the first DTC that pops up on the list. The other DTCs may help narrow down the direction for diagnosis. I even understand that some will handle the first one in the list, then clear the codes to see if the others return. Obviously this wouldn't be the case if one the first DTC is a "P1345" and another one is completely unrelated. So, what is the proper "rule of thumb" when addressing multiple DOCs? What do you do?

Good question! :silly: I dunno if there's a really good rule of thumb on this one. Every car (and customer) is different, so they all get treated slightly differently.

I think the customers complain and obvious symptoms would be a big factor for me. Say, the owner is complaining of a misfire? I see a P0300, but I also see a handful of U-codes stored in the PCM. Unless I see any other obvious network issues, I'll skip the U-codes. Fix what they're complaining about, and go after the U-codes if/when they come back.

I always handle the customer concern first. I also group dtc’s into two groups based on experience- cause & effect. There’s nothing wrong with starting down a path, realising it’s not right, efficient or the corrrct method or test and changing direction. Hope this helps. Steve.

This can be confusing especially if there are no running issues and the engine light is on with multiple codes relating to different areas. In this case, I would want to concentrate what is most important to keep the engine /transmission running and them safe on the road. I also look to see what is in "History" and what is "Current" and make a priority list. If it doesn't have anything to do with the engine or trans then what accessories don't work and what do. U-codes are a PITA most times for me. Unless there is a symptom, the customer usually doesn't want to spend the money for the diag.

If there is a symptom, then I would skip all other codes not relating to the initial concern and attack those later.

My logic is first action check battery voltage then:
Coms failures, circuit failures or low supply voltages should be considered first, then look for a pattern in the other codes and decide what live data to look at.