What I Learned While Diagnosing OBD Fault Codes P0421 and P0431 – Part 1

You have probably landed on this article after doing a search on OBD Fault Codes P0421 or P0431. I’m going to start out at a high level (Part 1) then take a deep dive (Part 2). The deep dive will probably make your eyes glaze over but trust me, information is power if you are negotiating with a dealer or independent mechanic. I will try to keep it as simple as possible.

My adventure was with a 2007 Porsche Cayman S. But understand that these codes are “generic” and may result from any car qualified to be sold in the US.

Very high level- All cars sold in the US have on-board computer software that “manages” the running of the engine. The “manager” is typically called an engine control unit (ECU), although other terms can be used. Some folks just call it the computer. The ECU tracks certain parameters to keep the engine running at its best and to also keep it running as environmentally clean as possible. When the ECU senses that the engine is not running at its best it may turn on a little wrench light on the dash or maybe a message like Engine Service Required. When the engine is not running as environmentally clean as possible you typically get a light or a message Check Engine. This does not mean the engine is getting ready to expire but it does mean something is wrong with the emissions control systems. If the light is flashing, then you might be damaging an important component, like the catalytic convertor. You may want to get it checked out pronto. If it is not flashing, you have some time to look into it.

You can stop at any auto parts store and ask them to “check your codes”. Watch carefully and you will see them insert a plug up under your dash, typically under the steering wheel. This plug is mandated by US regulations and is called the on-board diagnostics (OBD) plug. Very soon the auto parts store associate will say something like “You are showing codes xyz” and he probably will also provide a short description in words. Write the codes down, go home, and Google them. But in this case, we are talking about codes P0421 or P0431. These are technically Diagnostic Codes and they will help you, your dealership, or your independent mechanic get an idea of what the problem is. P0421 is Warm-Up Catalyst Efficiency Below Threshold Bank 1. P0431 is the same, except for Bank 2.

OK, we’re still at a high level but its getting more detailed in a hurry. All modern engines have catalytic converters. They are necessary to “clean” exhaust gases from gasoline engines and thus reduce bad things like smog and acid rain. So the word Catalyst is referring to the one or more catalytic converters on your engine. As emissions regulations got tighter, automakers resorted to two catalytic converters, one before the other. The first one is the Warm-Up Catalyst. Exactly as the name implies, it warms up quickly after the engine is started and starts doing it job as soon as possible. It is probably smaller than the main catalytic converter and thus has less mass/volume to heat up. Because catalytic converters only work when they are hot, (like over 500 degF) it is important to get them heated up as quickly as possible so they can do their job. Efficiency Below Threshold basically means that the “cat” has a minimum criteria or threshold to be considered that it is doing its job. The code indicates that it is operating below that threshold. And finally, some engines with more than 4 cylinders divide the work of the cats into two halves, denoted as bank 1 and bank 2. 3 cylinders per bank on a 6 cylinder engine and 4 cylinders per bank on an 8 cylinder engine.

So how does the ECU know that the efficiency target is not being met. The cat has an upstream and a downstream, in an airflow sense. Downstream should be the cleaned-up exhaust gas. An electro-mechanical device called an O2 sensor (O2 is science speak for 2 bonded molecules of oxygen, the most common form in our air) that measures the quality of the downstream gas. If it falls below a certain pre-set value for very long, the ECU makes note of this. If it does it beyond a pre-set period of time, it illuminates the Check Engine light on your dashboard.

What should you do. Well, unfortunately, catalytic converters are expensive, which is why you literally hear in the news that they are being stolen. It will be a value decision on the part of your mechanic but usually it is worth it to replace the O2 sensor first and see if that fixes the problem. Unfortunately, cats do fail and if they do, you will have to replace them. Note that in the US, manufacturers are required to warranty the cats for 8 years on new cars.

That is the high level discussion. If you get this code, your mechanic or dealer will probably give you a shorter version of this discussion but you are basically going to have to say “yes, try replacing the O2 sensor” or “go ahead and replace the cat”. On 2 Bank cars, they may suggest swapping the O2 sensors between banks to see if the problem “follows” the swap, is which case it is the sensor, not the cat. Not a bad idea in the great scheme of things. I have also read of cats being chemically cleaned but I can’t vouch for the merits of this. They may remove the O2 sensor and visually inspect the downstream face of the cat with a small camera. If it is broken or chunks are missing, it is pretty much game over for that cat.

In my next article, Part 2, I’m going to go into much more detail. It is probably going to be of more interest to DIY guys who are up for working on their Porsches.