Dealing with a P0421

May 1, 2014
Do you go for the “fast fix,” or do the homework and go for a permanent one?

This Mazda is suffering from a poorly working converter.

A 2005 Mazda 6 3.0L V6 came in with a misfire that was noticeable, but had no Check Engine light on or any DTCs for misfires. Instead it had two other DTCs: a P0421 (Warm Up Catalyst Below Threshold Bank 1) and a U1055 (Lost of Communication with Instrument Panel ICP Control Module).

The engine had three new coils installed recently, most likely to take care of the misfire. As you know, a misfire will cause a converter to cook because of the amount of unburned fuel passing through it. Well, the misfires did their job, as you can see from the DTC P0421 that was set.

Let’s take a look why this engine was misdiagnosed. Sometimes techs go with the easy, fast fix, and throw parts at the problem rather than performing a proper diagnosis. The tech who previously worked on the vehicle seemed to have just this problem, because he did not find the root cause of the problem.

Be careful on which converter you are troubleshooting. Some vehicles have more than one, and may have two serving one bank.

When I come across a vehicle that has a problem such as this one, I realize that there was not a proper diagnosis done on the vehicle. The tech should have looked up DTC and TSB information on his service information source to come up with a starting point for his diagnosis. Checking basics like engine mechanical condition, fuel trim figures under different engine operating scenarios, and reviewing the data PIDs for sensors that were outside the norm would have been a good place for this tech to start.

In addition, it’s never a bad idea to see whether the vehicle required a Reflash update. Start with the basics first before throwing parts at the problem.

While performing my diagnosis on the engine that had 164K on it, I found that it had a mechanical problem that was overlooked. Take a look at the Relative Compression test that I performed on the engine. Even though the test was performed on an old labscope, the information is still useful, especially when printed and presented to the owner.

This may be an older tool, taking an even older test, but it is as valid today as it ever was.

Scan tool data PIDS that need to be viewed can be numerous if you don’t go into Generic/Global OBD II data. The first areas to take a look at besides the DTCs are Pending DTCs, Freeze Frame, Live Data and then Mode 6. Each one of the areas are important for different reasons, so let’s start with Pending DTCs. If there is a Pending DTC, this information provides you with a closer look on what is going to fail next and possibly illuminate the Malfunction Indicator Lamp (MIL) when it matures into a full-blown DTC.

With Freeze Frame, we have a snapshot of the data when the DTC was set, providing us information on what the important parameters were when it occurred. Moving on to Live Data provides us with a view of what is happening now, such as Fuel Trim, O2 and a baseline of values that we can use as a comparison to the Freeze Frame data.

Checking the efficiency of two cats at once. Notice how the two 02 sensor patterns on the left mirror each other (bad), while the two on the right are an example of a properly working converter.

Checking in with Mode 6 is an excellent idea, because if it’s failing here we could anticipate a problem on the horizon. Remember, if it fails enough in Mode 6, it becomes elevated to a Pending DTC and then on to a DTC that turns the MIL on.

Let’s take a closer look at the converter test that I ran on the EScan tool. Take note of the test I performed at idle shows Bank 1 as a failure and Bank 2 as a pass. Also notice that the O2 sensor Bank 1 Sensor 1 is switching good, and the rear that should not be switching is a mirror image of the front sensor. The result shows us that the converter is not working, so now take a look at Bank 2 where it’s a totally different story. The Bank 2 Sensor 1 is switching as it should, and the rear sensor Bank 1 sensor 2 is a nice straight line that indicates the converter is working.

Top: This thermal image of the bad cat shows little heat being generated, indicating little to no catalytic reaction taking place. Bottom: A good cat, on the other hand, is seen in this thermal image.

This can be done on any scan tool that graphs. Granted, you won’t get the percentage of how bad or good, but you will see whether the rear O2 is moving. Remember to test at a steady rpm — I recommend testing at idle and at a higher rpm because some converters will fail at idle, high rpm, or both. To back up the diagnosis of a bad converter, I used the General Thermal Imager and compared Bank 1 (bad) and Bank 2 (acceptable).

When I get problem vehicles in, I get my camera, scan tool, labscope and thermal imager ready. I do this to capture the information so I can use it as a teaching tool in my classes and here in these articles. The Relative Compression test took about two minutes; the scan data captures from two scan tools, OTC and EScan that included running a cat efficiency test took about 25 minutes; and the thermal imager captures took another two minutes. Always use a test that you are comfortable with and that is easy to use. Try it on a known good vehicle, then on a bad one. Having the right equipment and a good game plan is an easy way to diagnosis converter codes. The complete process, including printing the information, took a little over an hour.

Try saving or printing information, including the comparison of good and bad, so it’s easier for the owner to understand what you had to do to repair the problem. Having information to review with the customer provides a relationship-building experience that provides the owner with confidence in your diagnosis and repair. This is something that the vehicle owner will share with his friends, and that’s advertising for your shop you can’t put a price on. 

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