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Diagnosing OBD I, OBD II catalytic converter concerns

Friday, September 23, 2016 - 07:00
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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.

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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
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