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Chasing down cat codes

The number one code techs deal with is also one of the most misdiagnosed.
Wednesday, May 29, 2013 - 06:08
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The No. 1 codes in the country are the P0420 and P0430 (Catalyst Efficiency Below Threshhold) diagnostic trouble codes. They cover the same problem, with the only difference being to which side of the engine they are referring. But there is more to be considered than the catalytic converters alone. Other key components involved are the O2 sensor heaters, the O2 sensors themselves, and the Powertrain Control Module (PCM) software that performs the catalytic efficiency testing.

What is catalyst efficiency? The catalytic converter is the last step in cleaning up any dirty gasses left over after combustion. Simply stated, its efficiency is a measure of just how well it can do its job.

A borescope can be used to inspect the condition of the substrate. Damaged substrate (broken, clogged, melted) means something killed the cat before its time.

Most vehicles (those certified to OBDII) have both a pre-catalytic (upstream) oxygen sensor (or air fuel sensor) and another oxygen sensor located after at least one of the catalytic converters (downstream). Conventional oxygen sensors respond to the amount of oxygen in the exhaust stream and provide feedback to the PCM for fuel control. But the downstream sensor should sense a relatively stable voltage signal. When this rear sensor voltage zigzags up and down at a steady RPM, it is an indication of a decrease in catalytic efficiency. The PCM programming determines how much zigzagging is acceptable. If the PCM interprets the readings to indicate a catalytic converter failure, a P0420 – P0430 pending DTC (Diagnostic Trouble Code) is first set and if the decreased efficiency is still occurring on the next consecutive test, a hard DTC is set and freeze frame data PIDs (Parameter Identifiers) are recorded and stored.

Remember, Not Just The Cat
As with any other diagnostic challenge, you must go after the root cause of these codes using a systematic approach. Even if you do determine that the converter has failed, there is often an underlying reason for that failure. Miss that, and the new converter will quickly suffer the same fate as the old one.

An often overlooked, but extremely valuable, source of information is the code’s Freeze Frame recording. Even if you own the OE scan tool, you should first make use of the Generic/Global mode of the tool. The reason? Generic/Global does not substitute engine values (PIDS). It also provides access to freeze frame data and Mode $06 test results if needed. The freeze frame data is going to be especially helpful to you in diagnosing these DTCs. Be sure you do not erase/clear DTCs since that will also erase/clear freeze frame. Freeze frame data might help you uncover a sensor value that is out of normal range only under the conditions that were present when the DTC was set.

Remember, though, that freeze frame data does not record at the exact moment the fault occurred. It can be in a 15-degree-frame window, meaning what you see in the freeze frame may have happened 15 degrees-frame before or 15 degrees-frame after the store screen. It can still be very helpful in duplicating the problem by providing some idea of the conditions the vehicle was under when the problem was discovered. Diagnosing and repairing a vehicle without freeze frame would be difficult, to say the least.

The next step in diagnosing any problem is in understanding how and why the PCM detected the fault. In most cases, the PCM is comparing the input signal from the upstream oxygen sensor to the input of the downstream sensor. As discussed earlier, the downstream sensor should remain relatively stable. And while an aged cat can cause the rear sensor to swing in synch with the upstream one, it is not the only reason why. And, as mentioned earlier but absolutely worth repeating, if the cat is indeed failed it is imperative to know why to avoid sealing the fate of the replacement.

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