Onboard diagnostics (OBD) II requires the heated oxygen sensors (HO2S) to be monitored for deterioration. The voltage and response rate of the primary and secondary sensors must be monitored. Also, the HO2S heaters must be monitored for correct response. This should be a vast improvement over past oxygen (O2) sensor onboard diagnostics.

The OBD I diagnostics for the O2 sensor was pretty much limited to sensor stuck lean for 30 seconds or more and sensor stuck rich for 30 seconds or more. The computer was perfectly content to use a slow or overly fast signal to work with. Slow responding sensors are generally old and coated with carbon or oil ash. O2 spikes in the exhaust stream generally cause an overly fast response from an O2 sensor from cylinder misfire. The normal response rate for an O2 sensor is from once every three seconds to five times per second depending on fuel control and age.

Looking at the HO2S time to activity can test the HO2S heater circuit operation. The time to activity will be compared with a known good value. This test will only be run from a cold start. It appears that this code will not set if other engine codes are present - this should prevent the possibility of false codes being set from a bad sensor or problems in the fuel control. The other option is to have the computer monitor the heater circuit directly. This test measures the voltages in the circuit and compares them with calibrated values.

The response time test will be used to determine if a sensor's response has slowed due to coating. The computer will monitor the switch rate from rich (over 600 mV) to lean (under 300 mV) and from lean to rich. The switching time will be compared with known good values and a code will be set for slow rates. This value will vary from vehicle to vehicle. Everything I have read makes no reference to overly fast switching related to misfire. It seems the misfire monitor will replace this functional test.

Early Society of Automotive Engineers (SAE) papers on misfire detection indicated that if a single cylinder misfire was detected, the injector could be shut off to prevent catalyst damage. Some systems have the capability of detecting which cylinder is misfiring, while others only identify misfire without specifying which cylinder is misfiring. Real-world testing on some cars indicates that when misfire is detected, the engine control defaults to a very rich mixture. The rich mixture should deprive the catalyst of O2 and protect the catalyst, while causing the car to be a gross polluter.

The final test on the HO2S is sensor voltage. This test will determine if the voltage is stuck too high or too low for a specific period of time. I hope this will finally stop a powertrain control module (PCM) from trying to use a negative voltage to set fuel control. We have seen cracked O2 sensors or sensors with plugged reference air read as much as one volt negative and the PCM use this value without setting a code. Why the original equipment manufacturers (OEMs) wouldn't have programmed the PCM to identify this problem in the past remains a mystery.

The catalyst monitor is probably one of OBD II's coolest tests after the misfire monitor. Catalyst testing in the past was inexact at best. First, let's cover some of the commonly used catalyst tests today. The most common test is to use an infrared thermometer to test the inlet and outlet temperatures of the catalyst. Most techs have agreed that the outlet temperature should be about 100 degrees Fahrenheit hotter than the inlet. If the temperature is higher, then the catalyst is working - how efficiently you can't say, but it is working. The other problem with this test is that some newer cars run so clean, there just isn't enough unburned stuff in the exhaust gas to really light off the converter. One of my favorite tests is the cold start recording 4-gas test. This test gives an indication of how well the engine runs and burns fuel, and when the converter lights off, it shows how well the converter converts the gases that are there. On my brother's truck (1988 Chevy TBI 350 CID) at 70 degrees ambient temperature, it starts at 3 percent carbon monoxide (CO) and 100 hydrocarbons (HC). Within one minute it drops to less than 0.5 percent CO and 30 HC. At idle and with these gasses, the converter never lights up and the outlet of the converter is always 75 degrees cooler than the inlet.

General Motors (GM) created a more precise test of converter efficiency, but it is quite involved. You must warm the car up to light up the converter; turn it off and disable the fuel delivery and ignition so it won't have fuel and it won't start. Depending on engine size, you introduce a calibrated amount of propane into the intake system using a flow gauge. Then you crank the engine at wide-open throttle and read the carbon dioxide (CO2) percentage on a 4-gas analyzer. Using a chart, you can precisely determine the catalyst efficiency. An easier test is to do the same as above but, rather than propane, just let the fuel system inject fuel with the ignition disabled and check the CO2 reading on your analyzer. While not as precise as the GM test, you can see if the converter is working by the presence of CO2.

My other converter test requires extreme caution. On a questionable converter, I will cause one cylinder to misfire by disconnecting and grounding a spark plug wire. While running the engine at 2,000 rpm, a helper monitors the converter shell temperature. A good converter will heat up very quickly, usually 800 degrees Fahrenheit on the outside in less than two minutes. The 800-degree outside temperature usually indicates the core has reached 1200 degrees, and that's plenty. Too hot for too long is going to melt the converter substrate. This test has two functions - first, it confirms the converter is working, and secondly, it "cleans" the converter of contaminating coatings. The converter can become coated with sulfur and/or carbon, usually from older drivers that drive short trips at low speeds. We have some elderly customers who never achieve closed loop. Every year we have to change their oil to remove the fuel in the oil and light up their converter to pass emissions. Many techs advise the highway burn out for the converter, but we have found the misfire trial by fire a better solution. Be careful!

So, back to OBD II. The catalyst monitor using the before and after heated O2 sensors is no doubt the best catalyst efficiency test ever produced. The entire engine fuel control system is designed around getting the proper gasses to the converter, and the converter is the key to reduced emissions. It makes sense to monitor the operation of the most important component in the system. The three-way converter must do two jobs - first, it must reduce nitrogen oxides (NOx) to nitrogen and carbon dioxide, and secondly, it must oxidize CO and HC to CO2 and H2O. For the reduction reaction to work in the front of the converter, there must be a lack of O2 and CO present, so the engine needs to swing rich. For the oxidizing reaction to take place in the rear of the converter the mixture needs to swing lean to produce O2. The O2 is stored in the rear of the converter for use when necessary. The storage ability of the converter is measured by looking at the swings in the O2 sensor before the converter and the smoothness (lack of swings) after the converter. When the front O2 signal starts to look like the rear O2 signal, the converter is dead. When the converter efficiency drops below 60 percent during steady state operation at 30 to 50 mph, the code is set.

OBD II is not only an attempt to eliminate tailpipe testing nationwide, it is an attempt to bring onboard emissions testing to every car produced after 1996. This should help clean the air and reduce the unfairness of people in emission areas registering their cars in non-emission areas. The concept is good; we will have to see if the execution works out as well.

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