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Diagnosing OBD I, OBD II catalytic converter concerns

Friday, September 23, 2016 - 07:00
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What is being illustrated in the previous example is the converter’s ability to store oxygen, which is very similar to how some OBD II vehicles test converters. The pro here is that we can see the converter’s ability to store oxygen and convert CO into CO2. The con to this test is that we have no clue how well hydrocarbons, or HC, are being converted. It is a decent test, but if a vehicle fails for HC only, we may be lacking in our testing.

Propane Conversion Test

The propane conversion test is another test that we have available to us. It is time consuming and often not worth the time required to perform the test. It involves running the vehicle until it obtains operating temperature and a converter that has achieved a high enough temperature to function. Ignition and fuel are then disabled before the converter cools, which is the toughest part on some vehicles, and then the engine is cranked over while metering a specific amount of propane into the intake manifold. The idea is to feed HC and O2 into the converter while cranking, which feeds oxygen and hydrocarbons into the converter, and see what gases come out of the tail pipe. A functioning converter will change HC (propane) into CO2. The results can be compared to a pass/fail chart.

The pros of the propane conversion test are that we can see how well the converter is capable of converting HC into CO2. The cons of this test are the time investment and the lack of results regarding the CO to CO2 reaction.

Pre- and post-exhaust gas comparison

This test involves intrusively measuring exhaust gas before the catalytic converter, sometimes called feed gas, and comparing it to the tailpipe readings. Tapping into the exhaust before the converter requires special adapters. This technique also requires above-average knowledge of exhaust gasses and is time consuming. Even with extensive knowledge of exhaust gas theory, this technique can be tough without a dynamometer. Different driving conditions — specifically loaded conditions — change the exhaust volume and can affect our results. Technically, the exhaust volume issue effects all of the previous tests. For example, a pre- and post-exhaust gas comparison test might pass in the shop when the vehicle is operated at 1500 RPM in park but may fail an IM240 (or similar test) while being operated on a dynamometer under load at 50 miles per hour.

Secondary air injection

An additional thing to keep in mind is that a malfunctioning secondary air injection system can also effect catalytic converter efficiency. Air injection provides additional oxygen to the catalyst under certain situations to enhance its effectiveness. Again, oxygen is required for the desired reaction and a malfunctioning secondary air system could reduce a catalyst’s efficiency. It is important to note, that while performing the previously mentioned tests, an air injection system should be disabled in order to obtain accurate results.

OBD I summary

All of the tests previously covered are valid tests to help the technician judge how well the converter may be working. Provided everything else is working correctly on the vehicle, and the converter fails one or more of these tests, it’s a pretty safe bet that the converter is not up to par.

OBD II converter testing

A failed converter — or more specifically a catalyst code — on an OBD II-compliant vehicle gives us a completely different diagnostic approach. Any one of the previous tests can be performed on an OBD II vehicle, and we can make a judgment call on how well the converter works. However, it doesn’t matter how good or bad we think the converter is. Who decides if the converter is bad? The PCM and the catalyst monitor. For example, a vehicle may have acceptable test results using any of the tests previously described. But if the PCM still sets a P0420, the vehicle is still broken. This means that on an OBD II vehicle, all of the previous tests discussed are worthless.

Think about it this way — the PCM is the component that makes the judgment call on the condition of the converter. As technicians, our job is to make sure the PCM has everything it needs to make that judgment call correctly… and then we have to trust its assessment.

A perfect example of this would be comparing the upstream and downstream oxygen sensors on a scan tool. If we observe these inputs, just as the PCM does, can we decide if a converter passes or fails? This next example is from a 2000 Honda Odyssey with a P0420 code. Just as we have been taught, the upstream oxygen sensor is switching back and forth while the downstream oxygen sensor is relatively flat.

Comparing upstream to downstream oxygen sensor switching...is this catalyst good?

Our judgment call might be that the converter is good, yet the PCM still sets a P0420. In this case, the catalyst was bad and needed replacement.

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