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Diagnosing OBD I, OBD II catalytic converter concerns

Friday, September 23, 2016 - 07:00
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Given this dilemma, OBD II catalyst codes require a different approach. Diagnosing a catalyst code can be done successfully on almost every OBD II-compliant vehicle if the following six steps are performed. Let us address each one individually.

Step 1: Read

It is very important to take a few moments and read the operation of the catalyst monitor for the vehicle in question. Enabling criteria and testing conditions, as well as what actually sets the code, are important to know. In addition, Technical Service Bulletins (TSBs) can provide insight into potential pattern failures or updated module calibrations. Many vehicles have calibration updates that may or may not require converter replacement. These TSBs may resolve these issues. It is always better to know this information early in the diagnostic process in order to avoid potentially unnecessary testing and wasted time.

Step 2: Are there any codes that interfere?

This step involves taking the information gathered in Step 1 and determining if other diagnostic trouble codes could be interfering with the catalyst monitor or causing false fails. For example, a P0171 Lean Bank 1 code needs to be resolved before we can even think about addressing a catalyst code. Conversely, a code stored for an oil pressure switch circuit should have no effect on the catalyst monitor.

Step 3: Verify exhaust integrity

Exhaust leaks do some weird things. A crack in an exhaust manifold can actually draw ambient oxygen into the exhaust. This additional oxygen can skew oxygen sensors and effect catalyst, and catalyst monitor, operation. A quick check of the exhaust system needs to be performed before we can proceed.

The three most common methods of checking the exhaust are: visual inspection, plugging the exhaust and listening/feeling for leaks or introducing smoke into the exhaust with a smoke machine. My personal preference is to use a smoke machine because event the smallest leaks will be easier to find.

Step 4: Verify oxygen sensor operation

In order for the catalyst monitor to run, the vehicle has to have functioning oxygen sensors since they are the inputs the PCM uses to evaluate the catalyst. In addition, the exhaust integrity addressed in Step 3 is also required for accurate oxygen sensor input. A scope or a scan tool can be used to evaluate oxygen sensor performance. Use whichever technique you deem appropriate, but do not skip this step.

Step 5: Verify the engine is running correctly

A proper running engine is required for the catalyst monitor to run correctly. A misfiring engine will definitely need to be diagnosed and repaired before a catalyst code can be addressed. In addition, fuel trim numbers should be consulted to confirm that the engine is running without any significant fuel corrections. To illustrate my point, take a look at this 2002 Pontiac Bonneville with a P0420. A shop has already replaced the catalyst and the vehicle returned a week later with a P0420 set again. At 2500 RPMs, the total fuel trim correction is -4 percent, which is definitely acceptable. However, when the vehicle is returned to idle, the total fuel trim correction is more than -40 percent.

Could these fuel trim numbers false fail a catalyst monitor that runs at idle?

On this particular vehicle, if we back up to Step 1, we would find that this catalyst monitor runs at idle. Is it feasible that a -42 percent fuel trim number at idle might be interfering with proper catalyst monitor operation? I cannot tell you why this vehicle did not set a P0172 for the rich running condition. However, it did false fail the catalyst monitor. A ruptured fuel pressure regulator diaphragm was the cause and a new regulator resolved the P0420 issue.

Step 6: Mode $06 analysis

The final step of analyzing Mode $06 data is optional. However, the data obtained could provide an additional piece of information that points at a faulty converter. It can also be valuable on a dual bank vehicle where a catalyst code has been set for only one bank. Mode $06 data for the opposite bank may indicate that its converter may not be far behind the failing bank. This could result in a justified additional converter sale and avoid potential come backs. The following example is from a Crown Victoria that set a P0430. The Mode $06 results indicate that the bank 2 converter did fail, but we also get a glance at the condition of the bank 1 catalyst.

Do both catalysts require replacement?

Could this information be valuable while deciding if a single catalyst needs to be replaced or a pair of converters is justified? Mode $06 data can also be used as repair verification after a converter has been replaced.

The OBD II diagnostic conclusion

If the first five steps pass our inspection (Step 6 was for the sake of information) then the PCM has everything it needs to analyze converter efficiency. We now must trust the PCM’s judgment and replace the converter. If any of the previous steps did not pass our inspection, then the corresponding issue must be resolved and the catalyst monitor should be run again before we condemn the converter.

Catalytic converter diagnosis obviously differs based on the age of the vehicle. The approach we chose will depend on the faults the vehicle has and model year of said vehicle. The important things to remember are: have a repeatable diagnostic procedure that covers the bases and make sure all other systems are working correctly before pulling the trigger on a new catalyst.

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