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How To Fix Multiple DTCs

It is not uncommon to find multiple Diagnostic Trouble Codes in one vehicle when a simple code retrieval is performed. Especially when the malfunction indicator light has been on for a week or more.

Sometimes you retrieve what can be termed a ‘laundry list’ of codes. If you jump to the conclusion that all these things need repaired, you might be spending lots of money unnecessarily.

Many times when the customer drops the car off for service, they say things like “The light has been on for a while, but I haven’t noticed anything unusual.” Or “It drives just the same.”

The job of a mechanic is to be more analytical of a vehicle’s performance, and we find problems that the driver does not notice simply because of our training and experience.

We wanted to test a sampling of different vehicles with different communications protocols to see if there was any truth to these type statements from customers. We chose the following vehicles to test; Mercedes C class, Ford F series truck, Dodge Minivan, and an Audi A series.

On each vehicle we performed a thorough baseline testing including performance testing with a 2X80S scan tool and ScanMaster software to verify the vehicles were running properly. Then we drove them for a week to become accustomed to how each drove without any trouble codes in the PCM.

We chose to force a rather innocuous code into the PCM’s memory, so we loosened the gas cap and allowed the PCM to find a P0442 code and store it. Then we drove each vehicle for another week with the P0442 code stored.

The first couple days afterward, there was no change in the performance characteristics of any of them. Then we noticed performance issues that varied from vehicle to vehicle. The Dodge minivan developed transmission poor shift points and fuel consumption increased slightly. The others developed similar changes in performance characteristics; increased fuel consumption, sluggishness in acceleration, etc.

At the end of the week we retrieved the codes again. All had stored codes related to other emission systems in addition to the P0442. During performance testing observations most vehicles exhibited a rich bias from the Heated Oxygen Sensors. The Dodge had stored a P1698 which is a failure to communicate with the transmission.

What this demonstrates is upon testing any vehicle, you should be very cautious about your choice of repair procedures and which code or codes you are going to repair. You may want to repair only one of the trouble codes, and send the vehicle home for a week or so, then retest. This would allow you to confirm additional repairs are needed or not and possibly save the customer money.

You should not clear the codes until you establish if all of the codes are real. To do this requires a full baseline procedure be followed by a performance baseline and general observations of the data stream as well as checking manufacturer technical service bulletins.

Plus you should confirm that the PCM is functional. Just because the vehicle starts does not confirm that the PCM is functional.

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