Nothing lasts forever

Catalytic converters almost never fail. In a well-maintained vehicle, the catalytic converter should last the lifetime of the vehicle. When it does fail, it is often the result of something else going wrong in the emissions system or a problem with the vehicle engine that finally kills-off the catalytic converter. Correctly diagnosing a failed or failing catalytic converter is a key step in eventually tracking down what may have killed it. It’s no use simply replacing the converter if what is causing its demise is left unrepaired.

Starting with the basics

What will tip-off a technician that there may exist a problem in a catalytic converter is when the customer comes into the shop with a ‘Check Engine’ light on or having failed a mandated emissions test. In a ‘Check Engine’ situation, a quick use of a scan tool may likely produce a P0420 code which will signal to the technician there is a possible catalytic converter problem. Still, a P0420 code is generic fault code. It simply says that there is some sort of emissions issue taking place and it is triggered when the ODB II system compares the switching between the upstream and downstream oxygen sensors when the exhaust gases produced from the engine travel across the sensors; the upstream sensor taking note of the exhaust gases as they leave the engine and the downstream sensor, the one behind the catalytic converter, as the gases leave the converter. In a working catalytic converter what a technician will see is that the upstream sensor will show a lot of switching activity as the engine’s onboard computer system adjusts the fuel mixture from rich to lean.

“When the catalytic converter is working properly, the rear oxygen sensor will be working somewhere above 450 and 500 mV, and pretty much stays steady,” says Russell Rockstad, certified smog instructor with Universal Technical Institute (UTI) in Rancho Cucamonga, Calif. “This is when the catalytic converter is working correctly and it is being fed the right air-to-fuel mixture. What tells the (onboard) computer that (the catalytic converter) has failed a ‘self-test’ is when that rear oxygen sensor starts going up and down within about 75 per cent of the front oxygen sensor. If the back oxygen sensor begins to resemble the front oxygen sensor then it means the catalytic converter has failed and the computer uses this as an indicator that the catalytic converter is no longer working to reduce emissions and it will produce a catalytic converter inefficiency code.”

In vehicles that are 1996 and newer, the ‘Check Engine’ light will signal that a problem has occurred. In older vehicles, a failed emissions test will alert the technician that there is problem, adds Rockstad.

Using a checklist to eliminate other problems

Joseph Bacarella, manager of technical assistance with Walker Exhaust Systems, a division of Tenneco Inc., and an ASE certified Master Technician with L1 certification for advanced engine performance, points out that even if a failed emissions test or trouble code indicates a problem, before assuming the catalytic converter has failed the technician needs to run through a set of preliminary tests before attention can be given to the converter.

The reason is simple: Most catalytic converters fail, or are unable to work efficiently, due to a problem occurring somewhere else in the emissions system and engine. These issues need to be eliminated as possible causes of catalytic converter efficiency first. It is not useful to replace an expensive catalytic converter only to have it damaged by an unresolved problem elsewhere in the vehicle.

The first step is to check the vehicle’s service history, looking to see if the catalytic converter has been replaced before, if the engine has been performing properly or if it has been experiencing misfires. The next is to correct any other engine trouble codes that appear and to check and correct any exhaust system leaks, and then to check the converter temperature to make sure it has ‘lit off’ correctly. This can be done using an infrared thermometer. Technicians should also check the backpressure and examine the oxygen sensors to make sure they are not damaged, followed by the cooling system and fuel system for any internal leaks or pressure problems. Finally, it is recommended that cylinder balance and volumetric efficiency be looked at and occasionally a PCM re-flash might correct the emissions problem.

Testing the catalytic converter

Once that is done and all other issues have been taken care of, there are several tests that can be performed to directly examine the catalytic converter. UTI’s Rockstad says the most popular test is a carbon dioxide cranking test which measures the performance of the catalytic converter. The test involves running the vehicle at 2,500 rpm for three minutes to ensure the catalytic converter is warmed-up and then to disable the ignition system while preventing the catalytic converter from cooling down. At this point, the engine is cranked for 10 seconds and using a five-gas analyzer check to see if the CO2 levels go above 12.5 per cent. If it is less or the hydrocarbon levels exceed 500 parts per million then the catalytic converter is weak.

Rockstad says to test for a damaged catalytic converter technicians perform an intake vacuum or exhaust backpressure test.

“You should bring the engine up again to 2,500 rpm and using a vacuum gauge the pressure should remain anywhere between 18-22 inches,” he says. “If it drops, there is an obstruction.”

Rockstad also recommends a more intrusive test using a gas reading and a five-gas analyzer. The technician drills a hole in the exhaust just before the catalytic converter to get a baseline reading. “Then we measure our gas readings after the converter and what we are looking for is a 35 per cent reduction in HC emissions.”

Walker’s Bacarella says many ODBII systems have moved to a more advanced converter test, Oxygen Storage Capacity. One of the reason for the switch is the move by car makers from oxygen sensors to AFR sensors for fuel control.

“Once the engine is warmed up and after the vehicle has gone through a specific drive cycle -which varies by manufacturer,” Bacarella says. “With the vehicle idling and after the computer knows the converter is ‘lit-off,’ it will switch the fuel system open loop and produce a rich condition. This will empty the converter of any stored oxygen. The onboard computer will then ask for a lean condition and measure the oxygen storage capacity of the converter in seconds.”

What a technician is looking for is for how long the rear oxygen sensor takes to register the lean condition. It may take anywhere between two seconds to as long as 20 seconds.

“Normally, the cut-off point is two seconds, so if you have two seconds of oxygen storage capacity it typically means the catalytic converter has enough capacity to do its job properly, Once you know the converter is capable of doing what it is designed to do, you can move to other engine systems to determine the cause of an emissions efficiency issue.”

SSGM

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REFERENCE LIST

Universal Technical Institute www.uti.edu

Walker Exhaust Systems www.walkerexhaust.com