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Catalytic Converters: New Diagnostic Strategies

Catalytic converter failures on today’s vehicles are rarely caused by defects in catalytic converters. Most catalytic converter failures can be traced back to problems caused by what happens in the combustion chamber. Almost every part on the engine determines how long it will last. It could be a faulty line of computer code that pulses an injector too long, or it could be a stuck piston ring that allows oil to be sucked into the combustion chamber. These little details can limit the life of a catalytic converter.

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Catalytic converter efficiency codes can be a long diagnostic road that can either lead to a happy customer or an expensive comeback. Chances are the original converter didn’t fail on it’s own, but conditions up stream hastened its demise. 
 
Catalytic converter failures on today’s vehicles are rarely caused by defects in catalytic converters. Most catalytic converter failures can be traced back to problems caused by what happens in the combustion chamber. 
 
Almost every part on the engine determines how long it will last. It could be a faulty line of computer code that pulses an injector too long, or it could be a stuck piston ring that allows oil to be sucked into the combustion chamber. These little details can limit the life of a catalytic converter. 
Basic Chemistry

Platinum, palladium, rhodium and cerium store oxygen in the converter during periods of lean operation or by an external air source. The oxygen is used to oxidize hydrocarbons and toxic gases during periods of “rich” operation.  This oxidation changes harmful carbon monoxide (CO) into carbon dioxide (CO2). It will also oxidize hydrocarbon or fuel by turning it into inert carbon products and water (H2O). This is called reduction in chemistry and breaks down molecules into smaller parts. The precious metals act as catalysts in the process and are not changed, they just store and use oxygen to breakdown combustion products.

However, they can’t breakdown or oxidize some chemicals in the exhaust stream.

If the catalyst is blocked by carbon, silica or phosphorus, the converter will fail to work.
 
The Code
For a catalyst efficiency code to be set, a number of criteria must be met. The specific enabling criteria is different for almost every vehicle. For a code to be set, the oxygen or air fuel sensor and the rear oxygen sensor must see a reduction in the efficiency of the converter. In other words, if the oxygen levels before and after the converter do not change, the converter is not working. 
 
But, this is not an automatic pass or fail. The oxygen sensors need to see this loss in efficiency over a number of drive cycle conditions. This is why it might take a few hours or a few days for the light to come back on after an efficiency code is erased and no other service is performed. 
 
On most vehicles, an efficiency code will not be set if an oxygen sensor heater code or any oxygen sensor-related code is set. The same is true for coolant and air temperature sensors. You could repair these items only to have the customer come back with the check engine light on and an efficiency code set.  
 
Even if the converter is operating below 95 percent efficiency or the oxygen sensor is bad, the chances of the light coming right back on are slim. If you clear the code, the light might stay off for a while until the system goes through two readiness cycles. This might take a couple of days or a couple of weeks. But, no good deed goes unpunished. The customer will be back and your quick fix will be forgotten.
 
One thing to keep in mind about non-continuous OBD II monitors is that they may not catch a problem until the vehicle has been driven several times and conditions are right to detect the fault. Consequently, any time you’re troubleshooting an OBD II problem, it’s very important to use a scan tool that can tell you if all the monitor readiness flags have been set. If one or more monitors are not ready, the vehicle will have to be driven under varying speeds and loads until all the monitors are set. Then, and only then, will you get an accurate diagnosis from OBD II.
What is Efficiency?
The converter has an efficiency rating that is computed by the vehicle. This number rates the amount of reduction that is occurring in the converter and its ability to store oxygen. But, efficiency of the converter is tied to the fuel trim of the engine. Most engines minutely alter the fuel trim to replenish the oxygen in the converter and to add fuel for reduction. This helps to keep the converter at the correct temperature for the most efficient operation. 
 

If an engine is running too rich, it cannot store oxygen. If it is running too lean, the reduction process might not occur due to an inability to heat up. 
If the engine is dealing with a leaking vacuum hose or a stuck injector, it can’t switch the fuel mixture properly to replenish oxygen and reduce harmful contaminates.
 
Converter efficiency can be checked with some scan tools along with the switching between rich and lean. Lab scopes can also be used to monitor the switching. The converter efficiency threshold of a vehicle is part of a vehicle’s software. Once the efficiency drops below a specified level and other criteria are met, an efficiency code will be set. The software is designed to filter out data that may be erroneous or random signals that may interfere with the oxygen sensor. 
 
Most converters start out at about 99 percent efficiency when new, and quickly taper off to about 95 percent efficiency after 4,000 miles or so of driving. As long as efficiency doesn’t drop off more than a few percentage points, the converter will do a good job of cleaning up the exhaust. But if efficiency drops much below 92 percent, it will usually turn on the MIL lamp. With vehicles that meet the tougher LEV (Low Emission Vehicle) requirements, there’s even less room for leeway. A drop in converter efficiency of only three percent can cause emissions to exceed federal limits by 150 percent. The LEV standard allows only 0.225 grams per mile of hydrocarbons, which is almost nothing.
Problem Vehicles
Some vehicles have difficulties passing the emission warranty or replacement converter warranty before an efficiency code is set. However, there are some solutions to consider if you are stuck with a problem vehicle. 
Software
Some vehicles have more sensitive catalyst efficiency monitors. This means that the tests and parameters for testing that were programmed in at the factory for the efficiency of the converter might be a little too sensitive or the drive cycle is too narrow. The programming may not take into account real-world conditions. 
 
Many OEMs will release updated engine management calibrations that alter the enabling criteria of the catalyst monitors. The new calibration can then be re-flashed onto the ECM or PCM. For a vehicle with a damaged converter, the re-flash will do nothing. For a converter that is near the threshold, it may extend the life of the converter and prevent the light from coming on for 10,000 or 80,000 miles. 
 
It is always a good idea to check if the car has the latest calibration if the converter is being replaced; this can save you a comeback down the road. 
Oil Consumption
GM, Toyota, Honda and other manufacturers have issued Technical Service Bulletins (TSBs) concerning excessive oil consumption. Most of these problems relate to cylinder deactivation and variable valve timing. 

The main culprit in these problems is vacuum generated in the cylinders sucking engine oil past the rings and into the combustion chamber. On vehicles with cylinder deactivation, the deactivated cylinder is a negative pressure and would draw oil droplets in the crank case past the ring and eventually into the converter. This has happened on some GM and Honda engines. 
 
On some vehicles with variable valve timing (typically on the exhaust and intake cams), the valve timing could produce higher than normal vacuum pressures that could suck oil past the rings. This was the case for some recent Toyota models. 
 
While the oil getting past the rings is bad enough, the oil trapped in the rings can become carbonized and cause damage to the cylinder walls. This can lead to even more damage and more oil consumption. 
 
The oil consumption problem must be solved first before the converter is replaced. The most common fix is new engine management software designed to reduce negative cylinder pressures. Some manufacturers have also released special splash shields and oil valves to alleviate the problem.
 
These problems may occur on vehicles with as little as 20,000 miles. 
Coolant Leaks
The chemical components of engine coolant can block and prevent the precious metals of the catalyst from storing oxygen and reducing toxic components of exhaust gases. It is not the coolant that can damage the catalyst, but the silicates, phosphates and other chemicals added to the coolant to prevent corrosion. Engineers have been using alternative chemicals and lower levels to prevent leaking coolant from damaging a converter. This is why it is critical to use the right coolant for a vehicle. 
 
Some vehicles are notorious for head and intake gasket leaks. Some of these leaks may weep over time and eventually damage the converter. Most modern cooling systems do not require the coolant to be topped off regularly. Often, closed cooling systems can go 20,000 miles without needing additional coolant. But if a driver has to top off the coolant monthly, they might be damaging the converter.     
 
Always pressure check the coolant system and check for exhaust gases in the coolant before replacing a converter. Even the smallest of leaks can kill a catalytic converter. 
 
Maintenance

Even a little carbon can play havoc with the flame front.

In the past two decades, the greatest leaps forward in engine technology have been in the combustion chamber. Using high-speed cameras and quartz windows to see inside a combustion chamber, engineers are about to change the shape of the combustion chamber to produce the best possible flame front that produces more power, burns the fuel more completely and at a higher compression ratio. This is called thermal efficiency.
 
But, this increase in efficiency makes it more sensitive to changes in the combustion chamber due to lack of maintenance. Carbon deposits on the pistons and valves can cause changes in the fuel spray pattern and the velocity of the air in the combustion chamber. This can cause misfires and unburned fuel to be sent to the catalytic converter. 
 
If the spark plugs are worn, a missed combustion event can cause raw fuel to be sent to the converter and burned. This can lead to premature death of the converter. If the driver continues to drive with a misfire, the driver can kill a converter in a few thousand miles.
 
Since 1986 and the introduction of GF1 oil specifications, engine oils have had the levels of zinc, phosphorous and sulfur reduced to extend the life of the catalytic converter so the manufacturer can meet the emissions warranty of at least 80,000 miles. 
 
Zinc, phosphorous and sulfur can contaminate the catalyst and reduce the life of the converter even on low-mile engines that consume very little oil. If racing, diesel or agriculture engine oil with high levels of these additives are used, the converter will be permanently damaged. 
Clogged air filters can shorten the life of the converter. Not being able to draw in enough air, the restricted air filter can cause the fuel mixture to run rich. This can shorten the life of the converter. 
Other things to consider
PCV Valves: The spring tension of a PCV check valve is critical to the life of the catalytic converter. If there is too little tension, excessive amounts of oil can enter the combustion camber. If there is too much tension, it could cause oil sludging. Never take this inexpensive emissions device for granted because it could destroy a more expensive emissions device. 
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Some newer vehicles use an electronic PCV valve to control crank case vapors. Some TSBs have been issued and re-flash engine calibrations have been tweaked to help extend the life of the converter. 
Vibration: Broken exhaust hangers and mounts can cause the internal structure of the converter to fail. Signs of this type of damage may be a restricted converter.

Sealants: Never use silicone-based or non-approved sealants on systems or components that could enter into the combustion chamber. Most sealants can contaminate the catalyst and oxygen sensor and stop them from working. 

EGR Problems: EGR systems are designed to reduce smog-causing nitrous oxides (NOx) by recirculating a portion of the exhaust gases from each cylinder of the engine back into the intake manifold. This process lowers the combustion temperatures. Restricted flow can result in high NOx emissions and detonation (engine knock or ping) under certain driving conditions. This type of misfire can damage a converter.  

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