Diagnosing OBD I, OBD II catalytic converter concerns

Sept. 23, 2016
We will explore catalytic converter issues and testing for a wide range of vehicles. In order to do this, we will break down diagnosis into two parts: pre-OBD II vehicles and OBD II compliant vehicles.

This article was supposed to be about diagnosing P0420 and P0430 issues. However, because I travel to work with shops around the country, I realize that pre-OBDII vehicles are still emissions tested in some areas. As a result, we will explore catalytic converter issues and testing for a wide range of vehicles. In order to do this, we will break down diagnosis into two parts: pre-OBD II vehicles and OBD II compliant vehicles. Please note that if your demographic mainly consists of OBD II vehicles, the first half of this article will be valuable to you as it pertains to the understanding of how a catalytic converter works. With the disclaimer out of the way, we shall move forward.

Regardless of which type of vehicle you are working on, OBD I or OBD II, the vehicle needs to be running correctly before we can even consider testing the converter. For example, misfires, exhaust leaks or fuel system issues need to be fixed prior to proceeding with converter diagnosis. It is also important to know that what happens inside of a catalytic converter is a chemical reaction. Conditions have to be correct to facilitate this reaction. If the vehicle is running correctly then the conditions for appropriate catalytic converter operation should exist.

Ah ha, the chemical reaction! We could spend a lot of time describing how the oxidation reaction in a catalytic converter changes CO into CO2, HC into CO2 and water, and NOx into N2 and O2… but we will not go there. As technicians, our understanding of what happens in the catalytic converter is valuable, but not completely necessary. I do, however, encourage technicians to learn the details of the reaction that occurs. That being said, oxygen is the key.

OBD I catalytic converter testing

There are a variety of tests that are available to technicians that can be used to diagnose catalytic converter issues. There are pros and cons to each of these tests. It is important to know the value and limitations of each test. Let's attack each testing technique individually.

Delta Temperature Test

The delta temperature test is relatively simple. It involves measuring exhaust temperature just before the converter and just after the converter with an infrared thermometer or pyrometer. The engine should be at operating temperature, in closed loop, running correctly and at approximately 2,000 revolutions per minute. The converter inlet temperature is then compared to its outlet temperature. The general industry standard for a good converter is a 50-degree Fahrenheit increase at the outlet compared to the inlet. What this indicates to us is that a chemical reaction is occurring in the converter. The chemical reaction is exothermic, which means that when it occurs, it releases heat. The pros of this test are if temperature increases, we know that a chemical reaction is occurring in the converter. The cons of this test are that we do not know what reaction is occurring and how well it is happening. Are we reducing hydrocarbons, or carbon monoxide, or both? We don’t know. In addition, we do not know how well it is performing its conversion. Obviously, if there is a temperature decrease, we know the converter is doing nothing. But, if the temperature increases, we can only say it is doing “something.” We have no way to know how well the converter is doing its job.

Oxygen storage test

The oxygen storage test requires the use of an exhaust gas analyzer. Again, the vehicle is brought to operating temperature and is running correctly. The exhaust gas analyzer is in the tailpipe and the engine is brought to around 2000 RPM. The oxygen level in the exhaust should eventually be near zero percent. If the vehicle is equipped with an air injection system, it will need to be disabled at this point. The next step is to do a hard wide-open throttle snap. The throttle snap should force the engine rich and flood the converter with carbon monoxide, causing a CO increase at the tailpipe as well. Immediately after the throttle is released, the PCM will run the engine lean and oxygen levels should increase. The timing of these events is key to determining converter function. The guage of this test is oxygen content at carbon monoxide’s peak. This means that when CO peaks, O2 should not exceed a magic number. The magic number is 1.2 percent above the initial O2 reading. To illustrate my point, refer to Figure 1, which shows a baseline oxygen reading of .2 percent. The throttle is snapped and CO rises. At CO’s peak, marked by the black cursor, O2 rises. The blue trace would indicate a bad catalytic converter. Its oxygen level rose from .2 percent to 2.1 percent. That equates to a 1.9 percent increase, which is above our 1.2 percent standard and indicates a bad converter. The green trace comes in at .9 percent O2. This .7 percent increase indicates a passing converter that is capable of storing, and using, oxygen. Remember, oxygen will hit a much higher peak on deceleration. It is important to make our measurements at the moment that CO hits its highest point.

A comparison of exygen to carbon monoxide levels for good vs. bad catalysts while performing the 02 storage test with a gas analyzer

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.

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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