(P0420, P0430, P0421 etc...) Today's on board diagnostic system, known as OBD-II, has brought many changes to the way technicians diagnose emissions and drivability problems with the sophisticated engine management computers, sensors and other components. One of these components is the catalytic convertor, which is now tested for efficiency by the PCM (powertrain control module). Before 1996, when OBD-II was first mandated, the convertor was only replaced if it was clogged or if the vehicle failed the tailpipe emissions test. Today most states do not measure the pollutants from the exhaust on 1996 and newer vehicles, but instead rely on the vehicles own diagnostic systems to catch a problem that can lead to higher than normal harmful emissions. When a vehicle's computer spots a potential problem with a catalytic convertor it will turn on the "check engine" or "service engine soon" light and store a code that can be retrieved using a diagnostic scanner. Note that this DOES NOT necessarily mean the catalytic convertor is bad. As with any code stored, the PCM can only point to a potential problem, and many tests, expensive testing equipment, and knowledge are required to pinpoint the exact fault. Before having your convertor replaced, make sure the technician inspecting your vehicle understands the system and knows the proper testing methods that I will explain in the next section. Testing For a Bad Convertor (The following is written for the professional technician) Ask 10 automotive technicians how to check a catalytic convertor and you are likely to get 10 different answers. Even one manufacturer to the next can not agree on a common way to test a convertor. The real problem is that there is not a practical way to directly test the catalytic convertor. Unlike many components, the technician can not perform any one test that the catalyst can "fail", therefore he must eliminate all other possibilities before determining that the convertor must be at fault because every other possible cause for a catalyst efficiency code tests good. One testing method used is to measure the input and output temperature on the convertor. In theory, the convertor will be hotter on the outlet side than the inlet if it is working. The problem with using this method is that a convertor can be working hard enough to generate heat and reduce oxides of nitrogen, but may have lost much or all of its oxygen storage capacity needed to oxidize carbon monoxide and hydrocarbons. Therefore a convertor can pass the temperature test and still flag a check engine light with a catalyst efficiency code. Another popular method being used in many shops today is to examine the upstream (before the convertor) and downstream (after the convertor) oxygen sensor activity. In theory, if the catalyst is working well, the upstream sensor will be steadily sweeping above and below 450mv while the downstream sensor voltage moves much more slowly, usually maintaining between 300 and 600mv. The PCM is constantly changing the amount of fuel being delivered in an attempt to maintain a perfect air/fuel ratio. As the upstream sensor voltage rises above 450mv indicating a slightly rich mixture, the PCM corrects and lowers the amount of fuel delivered. As the upstream O2 voltage falls below the 450MV range, the PCM adds more fuel to correct for the slightly lean mixture. This results in the rhythmic pattern seen below when the oxygen sensor activity is monitored on an oscilloscope or GMM (graphing multi-meter). As the mixture swings lean, the convertor uses some of the oxygen to convert carbon monoxide and hydrocarbons (CO and HC) to carbon dioxide and water (CO2 and H20). Some of the excess oxygen is then stored for use moments later when the fuel mixture is slightly rich and there is not enough oxygen in the exhaust stream to convert all the pollutants. This causes the amount of oxygen seen by the downstream sensor to stay fairly constant. Although using this testing method is more accurate than checking output temperature, there are other tests that must also be performed if an accurate diagnosis is to be made. Below are two scangraph examples of a good and bad catalytic convertor. The first one is of a 1997 Chevy suburban with 2 bad convertors. Note that the downstream Oxygen sensor has the same amount of activity as the upstream for both the left and right bank. Below is the same truck after the proper tests were performed and the convertors were both replaced. Notice the downstream activity is greatly reduced. There will always be some movement as engine speed and load are changed Now, back to the first scangraph with the bad convertors. Even if the downstream sensor mirrors the upstream, this does NOT mean the convertor is at fault. There are other checks that MUST first be made to eliminate any other problems that can and WILL cause the same pattern activity on the downstream oxygen sensor. Check for exhaust leaks. How is engine performance? Any misfiring? Any other codes stored? Rough idle? Etc. Check for TSB's (technical service bulletins) related to that specific vehicle. Check for any computer updates or PCM re-flashes for that specific vehicle. Check for vacuum leaks. Thoroughly inspect the upstream AND downstream oxygen sensors for poor performance. If any of the above are present, they should be taken care of before the convertor is replaced. After the repairs, clear the codes and inform the customer that this is only the first step in repairing the failure and the vehicle must now be driven for several days or weeks so the PCM can monitor the convertor performance. Testing the Oxygen Sensors The first five inspections are self-explanatory and any good drivability technician should be able to handle them. Many technicians, however, do not know how to properly test an oxygen sensor for performance. While most catalyst efficiency codes are indeed flagged because of a bad convertor, the second most popular fault is a defective upstream or downstream oxygen sensor which will falsely flag a P0420 or P0430. Take a look at the scangraph below from a 1998 Mazda 626.