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Multiple Codes, Where To Start The Repairs

In the article “How to Fix Multiple DTCs” it was pointed out that a single problem can cause multiple codes, and that these extra codes may be false. It was also pointed out that you should not clear the codes until you are sure a repair has been accomplished.

Before we get started it is important that you have performed a proper baseline procedure to confirm that no outside problems exist that could be contributing to the driveability problems.

It is also very important that you confirm that the PCM is functional. You must have a scan tool with a decent data retrieval rate for this testing; a code reader simply will not work. If the information coming from the PCM is not reliable then any information including codes should be considered false.

Why we don’t clear codes immediately.

Let’s discuss this from the perspective of ‘Misfire Codes’ (P03xx series) being present. If the vehicle you are servicing is new, you could probably clear the codes without any loss of important data. However, if it has 50,000 or more miles on it the computer has been adjusting its wear curve to compensate for the engine wear that takes place through normal use.

If you clear this ‘wear curve’ data, the computer thinks it is a new engine with zero wear and as such this may cause the MIL to return almost immediately. Even though you repaired the problem that it had previously.

What if it doesn’t have a misfire code?

Here it is best to use your own judgment, but I would still wait to clear the codes. Just because you do not have a misfire code now, does not mean that you would not cause one because of clearing the wear data. If after the repairs, a check of engine vacuum shows a steady reading of 18 inches or more at warmed up idle is safe enough for me to clear the codes.

I have personally observed an extremely limited number of cars that without clearing the codes, it still has a misfire even when it has been repaired. But it is so rare that it is almost non existent.

Yes, I know that the customer does not like that “Malfunction Indicator Lamp” glaring at them and they want it off. This is when a good service advisor takes the time to explain that you are trying to help the customer by not performing unnecessary repairs.

If you have codes from every major group, this is my personal recommended sequence derived from many years and thousands of vehicle diagnostics.

The P03xx Misfire Codes are the ones you should address first. The reason is simply that the manufacturers have more control in this area and it can affect the other systems easier.

If you do not have a proper running engine, how can the rest of the information be deemed reliable?

The sequence of testing the components by DTC group is as follows; Fuel and Air Metering controls, P00xx to P0299, then the P05xx Vehicle Speed and Idle Speed Controls, and P04xx Auxiliary Emission Controls. And lastly the P07xx and P08xx Transmission Codes.

I personally try to get the engine to run as best as possible quickly, then perform a quick test of the other system that has a code. Here is a simple scenario that I have seen repeated many times.

A vehicle has two codes stored, one a misfire code, and the other an evaporative system code.

The vehicle needs 18 inches of vacuum supplied by the engine to apply testing.

If the engine vacuum is low because of a misfiring cylinder, when the PCM commands the Vacuum Solenoid to open and the Vent Solenoid to close (applies solenoid grounds), it expects the sensor to report that 18 inches of vacuum. But it does not have 18 inches of vacuum and the PCM thinks it has a leak.  So it stores a false code.

Here is another scenario; A vehicle with a MAF code and a Catalyst code. Typically the PCM will substitute a default value for the MAF sensor into its calculations, however if the vehicle has a small unregulated air leak (think hole in the snorkel tube after sensor) then the engine will be receiving unmetered oxygen and most likely the catalyst will not be able to store it, thus a catalyst code.

Before you repair this unregulated air problem, you should establish a baseline of the post catalyst oxygen sensor. Here a good scan tool and software kit like the 2X80S makes establishing a performance baseline test a non event. Then perform your repair and take the vehicle on a good test drive to thoroughly warm up the catalyst. Upon returning retest the post catalyst oxygen sensor and compare it to your performance baseline for improvement in the oxygen storage capability of the catalyst.

In the article “How to Fix Multiple DTCs” you saw how an evaporative code can cause a transmission code, so take the time to fix the simple things first and retest.

As you can tell, simply replacing parts is a bad idea and can prove very costly. Before you jump into the repairs, stop and think about what is happening and what the PCM expects. Then you can proceed with your repairs.

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All above information is based on published information as of 01/2015 or products purchsed to confirm
 

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