C atalytic converters have been used on vehicles since 1975 to reduce

exhaust pollutants. First generation "two-way" converters functioned like an afterburner to convert carbon monoxide (CO) and hydrocarbons (HC) in the exhaust into carbon dioxide (CO2) and water vapor. The result was a huge decrease in tailpipe emissions and a significant improvement in air quality.

Second generation "three-way" converters appeared in the early 1980s that added the capability to reduce oxides of nitrogen (NOx) emissions in the exhaust with a second catalyst bed. More modern versions of the three-way converter use a single catalyst bed that both stores and releases oxygen in response to cyclic changes in the engine's air/fuel ratio.

Another older variation was the "three-way-plus-oxygen" converter that used an air pump or PULSE air valve to inject extra air between the two catalyst sections inside the converter. This type of converter has a small air tube on the side.

OBD II ConvertersIn 1996, the arrival of second generation Onboard Diagnostics (OBD II) mandated yet another change in converter technology. OBD II converters contain more catalyst to improve efficiency and durability, and are required on all 1996 and newer vehicles. So if a 1996 or newer vehicle needs a replacement converter, it must be OBD II certified. Converters for pre-OBD II vehicles cannot be installed on late model vehicles.

Some NAPA replacement converters use catalysts that contain a special blend of catalysts or an extra dose of catalyst so the converter will continue to function efficiently in older, high mileage vehicles. This can help keep the Check Engine light off so the vehicle will meet emissions for a longer period of time.

New original equipment converters are covered by an 8-year or 80,000-mile

federal emissions warranty. Aftermarket replacement converters carry a 5-year or 50,000-mile durability warranty on the shell and pipe connections, and a 2-year or 24,000 mile performance warranty.

Special California RulesCalifornia emissions certified vehicles require a special "CA-approved" OBD II converter. CA-approved converters contain even more catalyst for increased efficiency and extended durability. CA-approved converters have a CARB (California Air Resources Board) five digit "EO" (Executive Order) number stamped on the shell that starts with the letter "D". Other markings include the words "OBD II" and additional coded information to identify the date and origin of manufacture.

CA approved converters are also required in other states that have adopted California emission standards (such as New York since June 2013).

If a vehicle registered in the state of California needs a new converter, it must be a CA-approved converter. But if a California vehicle is registered in another state (except those with CA emission standards), the original converter can be replaced with a CA-approved converter or a less expensive 49-state converter.

©2013 National Automotive Parts Association

Product Knowledge for NAPA AUTO PARTS Store Employees

Why Catalytic Converters Have to be Replaced ISSUE 12.2013

}  OBD II Converters

}  Special California Rules

}  Why Good Converters Go Bad

}  Cooked Converter

}  Plugged Converter

}  Converter Contamination

}  Clogged Converter

}  Crunched or Cracked Converter

}  No Converter

}  Converter Replacement Rules

}  Related Sales

continued inside

The converter cleans up left over pollutants in the exhaust, reducing tailpipe emissions dramatically.

Is this OBD II converter legal for use on California vehicles? Check the markings (yes, it is).

California warranty requirements for new original equipment converters can vary from 7-years or 70,000-miles up to 15-years or 150,000-miles depending on the year and emissions certification of the vehicle. For CA-approved aftermarket converters, the rules require a 5-year or 50,000-mile warranty against defects or performance.

Used converters have been banned in California since July 2008.

Why Good Converters Go Bad Original equipment converters on vehicles that are in good mechanical condition (no oil burning, misfiring or coolant leaks inside the engine) should last well beyond 100,000 miles. Many go more than twice this distance. But others don't for a variety of reasons.

The first sign of trouble is often a Check Engine light, accompanied by a P0420 or P0430 Diagnostic Trouble Code (DTC). A P0420 code is a catalyst efficiency code. If this code is present, it means the converter is not functioning up to par. Depending on the year, make, model and emissions certification of the vehicle, a P0420 code may be set if converter efficiency drops below 90 to 93 percent. The exact

threshold where this occurs will vary depending on how the OBD II catalyst monitor is calibrated, but sooner or later it will detect a failing converter.

The OBD II catalyst monitor compares the switching activity of the upstream and downstream oxygen sensors to estimate converter operating efficiency. The downstream O2 sensor (located in or behind the converter) should usually flat line and produce a steady voltage signal when the converter is operating at normal efficiency. But if the switching rate of the downstream O2 sensor increases and rises above a certain rate, it signals a drop in converter efficiency. A P0420 code will be set and the Check Engine light will go on. The vehicle may continue to run just fine with no noticeable change in performance or fuel economy. But the emissions coming out of the tailpipe will show a measurable increase.

A vehicle with an illuminated Check Engine light (for ANY reason) will NOT pass an OBD-II plug-in emissions test. It will not pass if the converter is operating at low efficiency, and it will not pass if any of the oxygen sensors are malfunctioning. The vehicle also will not be accepted for testing if the catalyst monitor or any of the other required OBD II monitors have not

run and successfully completed.The OBD II catalyst monitor may

or may not run every time the vehicle is driven. The catalyst monitor requires a certain mix of operating conditions and speeds before it will do its thing. If these conditions are not met, the catalyst monitor won't run and the converter won't be tested. So driving for 10 to 15 minutes, including a short time at highway speeds often won't run the catalyst monitor. It may not run for days or weeks depending on operating conditions. And if the Check Engine light is already on and there are any oxygen sensor, other major sensor or fuel trim codes present, these will usually prevent the catalyst monitor from running.

Cooked ConverterConverters can also fail as a result of ignition misfire or compression leaks. If large amounts of unburned fuel pass through the engine and enter the exhaust, it can cause the converter's operating temperature to soar. If the converter gets hot enough, it can literally melt the substrate inside the converter that supports the catalyst, forming a melted glob that may partially or completely block the flow of exhaust.

©2013 National Automotive Parts Association

The OBD II catalyst monitor uses the downstream O2 sensor to monitor the operating efficiency of the converter.

A P0420 code will set if catalyst efficiency drops below a certain percentage. Further diagnosis is usually necessary to confirm the converter needs to be replaced.

Why Catalytic Converters Have to be Replaced

Plugged ConverterA plugged converter will increase exhaust backpressure, causing a noticeable drop in both performance and fuel economy. If the blockage is complete, it will cause the engine to stall. A restriction inside the converter will also back heat up into the engine, causing the engine to run hot and possibly overheat.

Replacing a plugged converter will solve the exhaust restriction problem — but only temporarily unless the underlying cause of the overheating is diagnosed and repaired. The spark plugs, ignition wires and/or ignition coil(s) all need to be inspected for problems that could be a source of ignition misfire. Also, compression needs to be checked to see if a leaky or burned exhaust valve is responsible for overloading the converter with unburned fuel.

Other possible sources of overheating include worn valve guides and valve guide seals, and worn or broken piston rings that are allowing oil into the cylinders. An overly rich fuel mixture caused by a dead or defective oxygen sensor, leaky fuel injector(s) or excessive fuel pressure may also be another cause.

Converter ContaminationBack in the 1970s when converters were first introduced, leaded gasoline was still available and soon became a major source of catalyst contamination. Leaded pump gasoline is ancient history today, but leaded racing fuels and octane-boosting fuel additives are still available and may be a source of contamination if used frequently.

Phosphorus in motor oil (as a component in ZDDP anti-wear additive) can also foul the converter (and oxygen sensors) over time if an engine is burning oil. In recent years, the amount of ZDDP allowed in motor oils for gasoline engines has been reduced to extend the service life of the converter and O2 sensors. The amount in diesel oil has also been reduced, but not as much as in oils for gasoline engines. If someone is using diesel oil, "racing oil" or a crankcase supplement that contains high levels of ZDDP in their engine, and their engine is using oil, they risk shortening the life of their converter and O2 sensors.

Silicates from internal engine coolant leaks (leaky head gasket or cracks in the head or block) can enter the exhaust stream and foul the converter and O2 sensors. So too can freshly applied RTV sealer that is not converter and O2 sensor safe.

If a converter has failed because of a contamination problem, the underlying source of the contamination must be identified and eliminated otherwise the replacement converter (and O2 sensors) will suffer the same fate and fail prematurely.

Clogged ConverterIn some cases, a converter may become clogged with carbon or sulfur deposits. This may be the result of infrequent short trip low speed driving or prolonged

idling where the converter never gets hot enough or runs long enough to burn off the contaminants. Some highway driving combined with several hard accelerations may heat the converter up enough to burn off the carbon and sulfur deposits, but it won't do a thing for contamination caused by phosphorus, silicates or lead. A contaminated converter that is not operating at normal efficiency is a bad converter that needs to be replaced.

High sulfur concentrations in some cheap gasolines can also contribute to converter clogging as well as a noticeable "rotten egg" odor when the vehicle is first started. Switching fuel brands may be necessary to avoid such problems.

Crunched or Cracked ConverterMost original equipment converter shells are made of stainless steel so they will last a long time before they rust out. However, they are still vulnerable to road damage if the vehicle runs over something that hits the converter.

Vibration and motion in the exhaust system may lead to metal fatigue and cracking after many miles. Cracks may form where the pipe connections are

©2013 National Automotive Parts Association

continued on back

This dual bed converter has an oxidation catalyst for HC and CO, and a reduction catalyst for NOx. Overheating can damage the catalyst as can various contaminants.

Converters can die for a variety of reasons, so it's important to make sure the underlying cause has been diagnosed and eliminated before a new converter is installed.

©2013 National Automotive Parts Association

Product Knowledge for NAPA AUTO PARTS Store Employees

welded to the converter shell, or the shell itself may crack and leak. Either way, the converter will have to be replaced.

No ConverterThe platinum, palladium and rhodium catalysts that are used in most converters are expensive metals with a potential recycle value ranging from $25 to as much as $100 or more. This makes converters a potential target for thieves, especially on trucks and SUVs that have a lot of ground clearance underneath. If a thief uses a portable power saw to cut out the converter, he usually destroys the pipes on either side leaving an expensive repair for the vehicle owner.

Some people might be tempted to replace a missing converter with a straight pipe, but EPA rules prohibit "emissions tampering." Besides, a vehicle won't pass an emissions test if the converter has been

removed or gutted. And there is little to be gained in either performance or fuel economy by removing a converter from the exhaust.

Converter Replacement RulesOriginal equipment converters that are still under warranty cannot be replaced unless they are defective — and only then by an new car dealer (for free, by the way).

A converter is deemed defective if:n The vehicle has failed an emissions test

(tailpipe or OBD II plug-in test) due to a converter related issue (such as low efficiency).

n The original converter is plugged, damaged or missing.

n The converter is contaminated.A replacement converter must be the

same type as the original, and installed in the same location as the original.

Aftermarket replacement converters are either the "universal" variety (which provide broader coverage with fewer SKUs) or "direct fit" (easier to install and don't require any pipe adapters). Both work equally well from an emissions standpoint. If you're selling a customer a universal replacement converter, make sure you include the required pipe adapters and installation hardware.

Related Salesn Exhaust pipes, clamps, hangars, etc.n Exhaust tools (saws, chisels, pipe

expander, etc.)n Air pump & pump plumbing (diverter

valve, check valve, etc.)n Oxygen sensors

Q. My Check Engine light is on and I have a P0420 code. Does that mean I need a new converter?

A. A P0420 code indicates low converter efficiency. It usually indicates the converter is failing.

Q. My mechanic says my converter is plugged. Is there any way to clean it out?

A. No. Clogging is the result of a partial meltdown from severe overheating. Your converter is damaged and needs to be replaced.

Q. I installed a new converter six months ago, and I'm getting a code P0420 again. What gives?

A. The new converter is probably being contaminated or damaged because of an internal engine problem that was not repaired when you replaced your converter. The underlying problem needs to be identified before you replace the converter again.

Review Questions December 2013

Answers: 1. a, 2. c, 3. c

1. A "three-way" converter reduces all of the following pollutants except:

a. Carbon dioxide (CO2)b. Carbon monoxide (CO)c. Hydrocarbons (HC)d. Oxides of nitrogen (NOx)

2. A converter can overheat and be damaged if:

a. There's too much oxygen in the exhaust

b. There's not enough oxygen in the exhaust

c. There's too much unburned fuel in the exhaust

d. There's not enough unburned fuel in the exhaust

3. Converter damage or contamination can be caused by any of the following except:

a. Ignition misfireb. An internal engine coolant leakc. Driving too fastd. Compression leak

Questions a Customer Might Ask About Catalytic Converters: