Is your BMW’s check engine light on, with the scanner showing the dreaded P0420 or P0430 catalytic converter efficiency below threshold fault codes? If so, you may have already been shocked and horrified by a $3,000+ repair estimate and you’re wondering what to do next. This article has some answers, and an explanation of what the codes mean . . .
A catalyst efficiency fault means the converter is no longer able to do its job. The only cure is a new converter. However, before replacing parts we need to know which converter has failed.
We see quite a few of these faults at Robison Service, especially on 2002-2006 vintage cars. We begin by connecting the scan tool and reading all the faults. In this photo you can see a typical set of readings, these from a 2002 330 convertible.
We’ve got oxygen sensor, fuel trim, and catalyst faults – a common combination. Sometimes the converters just wear out, and you get catalyst faults all by themselves. However, that is rare in my experience. Most times, we see fuelling faults and efficiency faults together, and my sense is the fueling errors cause stress on old catalysts, which helps them fail.
They were probably weak already, but the other fuel management issues are the thing that puts them over the edge. So it’s vital that we fix all the problems in a situation like this.
The next step is looking at the live data stream to see if the converters have really gone bad. Modern cars operate by switching between rich and lean states, sometimes as often as several times a second. The cycling is seen in a constant oscillation of the oxygen sensor signals before the catalytic converter.
When things are normal we see that signal swinging back and forth between 0.05 volt and 0.75 volt or so. You can see an example in the live data view below.
The catalyst should be smoothing the exhaust flow, so that the rear oxygen sensor should show a hear constant signal, somewhere in the middle of the range. In this car, we see the post-catalyst oxygen sensors swinging in tandem with the front sensors – a sure sign that the cat has failed. It isn’t doing anything at all anymore.
A look at the live data allows us to see which converters are working and which are not. We can also observe the oxygen sensor performance, as the sensors are threaded into the catalysts. If the sensors are old we recommend changing them with the cats because they are probably nearing the end of their useful life and the stress of removal of refit may lead them to fail tomorrow anyway.
Another thing the tester allows us to do is conform which side is which. Some fault codes refer to Bank 1 or 2, while others refer to Bank A or B. I’ve seen quite a few capable home mechanics defeated when they read a fault for a Bank A converter, and changed it, only to discover their identification of Bank A was wrong.
Here’s what I suggest: If you are only changing one cat in the system, unplug its oxygen sensor while watching the live data. You should see the reading fall to zero, and you know for absolute certain which side is which. If you unplug what you thought was the right side, and you turn out to be wrong, you’ll thank me for that tip!
Next we looked at the fuel trim faults. In this car, those were quickly traced to a split intake boot, but there are certainly many other potential fueling issues on these cars. Be sure you tackle all the faults, otherwise you risk shortening the life of the expensive new converters you are about to install.
The final thing to think about is parts – should you buy original or aftermarket? Genuine BMW catalysts tend to run near $1,000 each, and our cars have several. Given that price many people turn to the Internet and the chain stores, where cats can be found for as little as $200.
In my experience, the only cats that work in these cars are the ones from BMW. You may be able to put a generic converter in a Buick, but BMW seems to ask more of its parts and a generic converter that works when installed will often be giving that same efficiency fault a month or two later.
Frankly, I don’t see how some of these parts are sold, as they are failing before any reasonable warranty period is up.
After a run of bad experiences, all we use are genuine converters. I have no problem buying the oxygen sensors from Bosch, Denso, or other reputable sources. I wish there was such a place for converters, but if there is, I have not found it.
This article was written around BMW, but the fault is common on many cars and my advice is applicable to most any modern vehicle.
John Elder Robison
Founder, J E Robison Service