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Catalytic Converter

by Larry Carley copyright 2019

Catalytic converters are one of the greatest emission add-ons ever to be installed on vehicles. By cleaning up the pollutants left over from combustion, they reduce tailpipe emissions of hydrocarbons (HC) and carbon monoxide (CO) to extremely low levels, when everything is operating normally, that is. But sometimes things do not operate normally, and when that happens engine performance may suffer or the vehicle may fail an emissions test.

Driveability symptoms such as a drop in fuel economy, lack of high speed power, rough idle or stalling are classic symptoms of excessive backpressure due to a plugged converter. Checking exhaust backpressure and/or intake vacuum will tell you if there's a blockage (more on this subject in a minute).

Elevated HC and CO tailpipe emissions, on the other hand, are often symptoms of a fouled converter or a faulty air supply (bad or leaky air pump, diverter valve or pulse air system). A fouled converter may not cause any increase in backpressure, so other methods of checking the converter are required for this type of problem (which we'll also get to shortly).

The important point to remember here is that converters don't just plug up or die for no good reason. There is usually an underlying cause which must also be diagnosed and corrected before the problem can be eliminated. Diagnosing a plugged or fouled catalytic converter is only half the fix. Replacing a bad catalytic converter will only temporarily restore things to normal because unless the underlying problem that caused the original converter to fail is identified and fixed, the replacement converter will likely suffer the same fate.


Under normal operating conditions, the converter should not have to work very hard to accomplish its job. If an engine has good compression, is not sucking oil down the valve guides, and the fuel, ignition and engine management system are all working properly, there should be relatively little HC and CO in the exhaust for the converter to burn (a few tenths of a percent CO and less than 150 ppm of HC when the engine is warm). In many late-model engines with multipoint fuel injection, combustion is so clean that the converter has little to do and the difference between the inlet and outlet temperature may only be 30 degrees F at 2,500 rpm - which is a lot less than the old rule of thumb that says a good converter should show at least a 100 degree F difference fore and aft at cruise. At idle, the converter in many late-model vehicles may cool off so much that there's almost no measurable difference in fore and aft temperatures. So checking exhaust temperatures fore and aft of the converter at idle and 2,500 rpm is NOT an accurate way to determine if the converter is working properly or not.

One thing temperature measurements will tell you, however, is if the converter is working too hard. An infrared noncontact pyrometer or a temperature probe will tell you if the converter is running unusually or dangerously hot. If the converter outlet temperature is 200 or more degrees higher then the inlet temperature, it means the engine is running rich and there's a lot of CO in the exhaust that needs to be burned. A rich fuel mixture will often produce a "rotten egg" odor in the exhaust (the smell is hydrogen sulfide). Underlying problems may include an engine management system that is not going into closed loop (check the coolant and oxygen sensors, or for a thermostat stick in the open position), plugged PCV valve, or excessive fuel pressure (bad fuel regulator). High CO levels in the exhaust can also be caused by an inoperative air pump system.

If the outlet temperature is a lot hotter (more than 500 degrees F) than the inlet temperature, it indicates unburned fuel in the exhaust. The most likely cause would be ignition misfire (fouled spark plug, shorted or open plug wire, cracked distributor cap, arcing rotor or weak coil), or a compression leak (burned exhaust valve). But other causes may include lean misfire (check for vacuum leaks, leaky EGR valve, low fuel pressure or dirty injectors). A single misfiring spark plug can cause an increase in HC emissions of 2,500 or more parts per million, which can push the converter's operating temperature well above its normal range.

A common external clue of overheating to look for is a badly discolored or warped converter shell.


Prolonged overheating or short term severe overheating are the leading causes of catalytic converter plugging. The underlying cause here is often fouled or misfiring spark plugs, or a burned exhaust valve that leaks compression and allows unburned fuel to pass through the combustion chamber into the exhaust.

The average light off temperature at which the catalytic converter begins to function ranges from 400 to 600 degrees F. The normal operating temperature can range up to 1,200 to 1,600 degrees F. But as the amount of pollutants in the exhaust go up, so does the converter's operating temperature. If the temperature gets up around 2,000 degrees F or higher, several things happen. The aluminum oxide honeycomb begins to degrade and weaken. The platinum and palladium coating on the honeycomb also starts to melt and sink into the ceramic substrate reducing its effect on the exhaust. This accelerates the aging process and causes the converter to lose efficiency.

If the overheating condition persists for more than a few minutes, or if the temperature soars high enough, the honeycomb itself may melt forming a partial or complete obstruction, causing a sharp rise in backpressure. A complete blockage will cause the engine to stall shortly after starting, and will not allow exhaust to exit the engine.

Some degree of restriction inside the converter honeycomb can also be caused by accumulated deposits: phosphorus from oil burning and/or carbon from oil burning, a rich fuel mixture or frequent short trip driving where the converter rarely reaches light-off temperature). Physical damage to the honeycomb as a result of road hazards or severe jolts may cause the relatively brittle ceramic honeycomb to break or crumble inside the converter shell. A rattling noise when you shake or thump the converter would tell you there's loose debris inside. A undamaged monolith converter should make no noise.


To diagnose a plugged catalytic converter, you can check intake vacuum or exhaust backpressure. To check intake vacuum, connect a vacuum gauge to a vacuum port on the intake manifold. Start the engine and note the vacuum reading at idle. Then increase engine speed to about 2,500 rpm and hold steady. Normal vacuum at idle for most engines should be 18 to 22 inches Hg. When the engine speed is increased there should be a momentary drop in vacuum before it returns to within a couple of inches of the idle reading. If the vacuum reading is 10 percent lower than normal and/or continues to drop as the engine runs, it probably indicates a buildup of backpressure in the exhaust. Remember, though, that intake vacuum can also be affected by retarded ignition timing and valve timing. What's more, some engines are much more sensitive to small changes in intake vacuum than others, so checking backpressure rather than intake vacuum may give you a better indication of what's going on.

Checking backpressure requires connecting a pressure gauge to the exhaust system. Use a gauge that reads up to 8 to 10 psi and is calibrated in 1/2 inch increments. Or, use a metric pressure gauge calibrated in kilo-Pascals (kPa). One psi equals 6.895 kPa.

converter backpressure checks

A backpressure gauge can be connected to the exhaust system one of several ways: by removing the oxygen sensor and connecting the gauge to the hole in the exhaust manifold; by removing the air check valve in the air pump or pulse air system and connecting the gauge here; or by drilling a small hole into the head pipe just ahead of the converter to attach the gauge (never drill a hole into the converter itself!). One drawback of drilling a hole is that the hole will have to be plugged by a self-tapping screw, plug or welded shut after you've taken your measurements. Also, drilling is not recommended if the head pipe has a double-wall construction.

Once you've made your connection, start the engine and note the backpressure reading. Depending on the application, the amount of backpressure that's considered "normal" will vary. On some vehicles, backpressure should read near zero at idle, and should not exceed 1.25 psi at 2,500 rpm. Others can handle 0.5 to 1.25 psi at idle, but should have more than 4 psi during a snap acceleration test.

If you find a relatively high backpressure reading (say 8 to 10 or more psi), there's obviously an exhaust restriction that will require further diagnosis. Don't jump to conclusions and assume the converter is plugged because it might be a collapsed pipe or muffler.

One way to rule out the pipes and muffler is to visually inspect the exhaust system for damaged components. Another way is to drill a small hole in the pipe aft of the converter and check backpressure here. If the reading is lower (or is less than about 1 psi), the rest of the system is OK and the converter is what is causing the restriction. Or, disconnect the exhaust pipe aft of the converter. No change in backpressure would indicate a blockage at or ahead of the converter. If backpressure drops back to normal, the problem is not the converter but a collapsed pipe or muffler.

If you suspect the converter is plugged, you can disconnect and remove it. Then hold a shop light by one end of the converter and look in the other end. If you can't see the light shining through the honeycomb, the converter is plugged and needs to be replaced.

You can also recheck backpressure readings with the converter removed. If readings are at or near zero, you've found the problem. But if backpressure is still high, there's an obstruction in the head pipe or manifold. Sometimes a collapsed inner tube inside a double-wall head pipe will create an obstruction that acts just like a plugged converter. Another cause can be a heat riser valve on an older V6 or V8 exhaust manifold stuck in the closed position.


There are several ways to detect a restricted, plugged or worn out converter using a scan tool. Here's what to look for

* A significant difference in Short Term Fuel Trim (STFT) and Long Term Fuel Trim (LTFT) values between the right and left cylinder banks on a V6, V8 or V10 engine. If you see such a difference and the vehicle has separate converters for each cylinder bank, one of the converters may be plugged.

* A lower than normal barometric pressure (BARO) value. If your engine has a Mass Airflow (MAF) sensor, and the engine computer uses the signal from the MAF sensor to calculate a barometric pressure (BARO) value, the calculated value may be lower than normal is the exhaust is restricted.

* A lower than normal Manifold Absolute Pressure (MAP) sensor value. A restricted converter will cause an increase in backpressure that reduces intake vacuum.

* A lower than normal Calculated Load value. The Calculated Load value (percentage or grams/second) displayed on a scan tool is a measure of the engine's volumetric efficiency. A low value means the engine isn't breathing normally because of an exhaust restriction.

* A P0420 to P0423 code. The converter may not be restricted, but it is not operating at normal efficiency. The OBD II system is really good at detecting a failing or bad converter, so if everything else if working okay and there are no exhaust leaks or O2 sensor problems, and you get a P0420 code, chances are your vehicle needs a new converter.


To clean the exhaust, the catalyst inside the converter must be exposed to the hot exhaust gases. Lead, phosphorous and silicone can contaminate the catalyst and prevent it from working its magic. Lead used to be the most common contaminant, but is no more since it was eliminated from gasoline. Phosphorus is still a threat, and comes from motor oil. So if an engine is burning oil because of worn valve guides or rings, phosphorus will shorten the life of the converter. Blue smoke in the exhaust and an emissions failure are pretty good clues that the converter has been fouled with phosphorus.

SJ, SM and SN rated motor oils contain less phosphorus (ZDDP) than earlier SH and earlier rated oils. ZDDP is an anti-wear additive, but less is needed in today's engines with roller lifters and cam followers. Lowering ZDDP levels reduces the risk of converter contamination over time and helps extend the life of the converter to 150,000 or more miles.

Silicates can find their way into the exhaust if the engine develops an internal coolant leak through a crack in a combustion chamber or a head gasket. Silicate corrosion additives will ruin the oxygen sensor as well as the catalytic converter, so chances are if the converter has been fouled the O2 sensor will also need to be replaced. White smoke in the exhaust is a clue that there's an internal coolant leak.


If a converter is not plugged and passes exhaust normally, and there are no other engine performance problems (fuel, ignition and compression all OK, and the computer going into closed loop), but HC and CO levels in the exhaust are higher than they should be, the converter may be fouled. Most original equipment converters are designed for a service life of well beyond 100,000 to 150,000 miles, so if the converter has failed at low mileage contamination may be the culprit.

One way to measure converter efficiency is to read the composition of the exhaust gases with a 4- or 5-gas exhaust analyzer. A number of companies sell small portable exhaust analyzers that are relatively affordable ($2,500 to $5,500) and can be used for a variety of diagnostic purposes. Plugging the HC Co and NOx readings before and after the converter into a conversion efficiency formula will reveal the condition of the catalyst. But this technique does NOT tell you if the converter will actually pass the OBD II catalyst monitor self-test. Only the OBD II system can do that. A high conversion efficiency reading in the 90 to 95 percent range for HC, CO and NOx should pass in many cases, but may not depending on how sensitive the OBD II catalyst monitor is calibrated.

Checking emission readings at the tailpipe will tell you whether or not they are within normal ranges and help you diagnose the cause if emissions are high. Doing a "cold start" emissions check when the engine is first started will tell you if there are any engine problems that need attention. A cold start, in this situation, is when the converter has cooled down for at least 20 minutes. It will take a couple of minutes for the converter to warm up to light off temperature, so during this time you have a relatively clear window of what's coming out of the engine. When the converter reaches operating temperature, there should be a measurable drop in HC and CO readings (the amount will depend on how dirty the baseline readings were). No change in readings would indicate a dead converter.

Another test is to create a momentary rich condition or a misfire (as described earlier) to see if the converter can clean it up. As the converter starts to react to the excess pollutants, it's operating temperature should go up as the tailpipe emission readings come down.


On 1996 and newer vehicles with OBD II onboard diagnostics, the OBD II system has a catalyst monitor to keep an eye on converter operating efficiency. The catalyst monitor may run when the vehicle is cruising at a steady highway speed of 40 to 60 mph for at least 10 minutes, or at idle depending on the vehicle application. (NOTE: The catalyst monitor will NOT run if there are any oxygen sensor fault codes present, or the oxygen sensor monitors have not completed.)

The OBD II system compares O2 sensor readings upstream and downstream of the converter, and the converter's reaction time to a sudden change in the air/fuel mixture. If the converter is slow to respond, or the downstream O2 sensor readings don't flatten out and level off at 0.45 volts, it indicates a drop off in operating efficiency and sets a P0420 catalyst efficiency code. Other converter faults may set codes ranging from P0420 to P0439.


P0420....Catalyst System Efficiency Below Threshold Bank 1
P0421....Warm Up Catalyst Efficiency Below Threshold Bank 1
P0422....Main Catalyst Efficiency Below Threshold Bank 1
P0423....Heated Catalyst Efficiency Below Threshold Bank 1
P0424....Heated Catalyst Temperature Below Threshold Bank 1
P0425....Catalyst Temperature Sensor Bank 1
P0426....Catalyst Temperature Sensor Range/Performance Bank 1
P0427....Catalyst Temperature Sensor Low Input Bank 1
P0428....Catalyst Temperature Sensor High Input Bank 1
P0429....Catalyst Heater Control Circuit Bank 1
P0430....Catalyst System Efficiency Below Threshold Bank 2
P0431....Warm Up Catalyst Efficiency Below Threshold Bank 2
P0432....Main Catalyst Efficiency Below Threshold Bank 2
P0433....Heated Catalyst Efficiency Below Threshold Bank 2
P0434....Heated Catalyst Temperature Below Threshold Bank 2
P0435....Catalyst Temperature Sensor Bank 2
P0436....Catalyst Temperature Sensor Range/Performance Bank 2
P0437....Catalyst Temperature Sensor Low Input Bank 2
P0438....Catalyst Temperature Sensor High Input Bank 2
P0439....Catalyst Heater Control Circuit Bank 2

catalytic converter
Converters do a fantastic job of cleaning up exhaust emissions.
If yours has failed, it is illegal to replace it with a straight pipe.
Replace it with a new aftermarket or OEM converter.


If your converter needs to be replaced, it may be covered under warranty. You can take you vehicle to a new car dealer and have the converter replaced for free. However, you may be charged for other components such as oxygen sensors, pipes, clamps, etc.

The federal emissions warranty that applies to ALL cars sold in the U.S. covers the catalytic converter and PCM for 8 years or 80,000 miles (which ever comes first). Some states have their own emissions warranty requirements for vehicles that are sold and registered within that state, including California, Connecticut, Maine, Massachusetts, New Jersey, New York, Rhode Island and Vermont. In these states, the converter is covered for 7 years or 70,000 miles (which ever comes first). On PZEV certified hybrid vehicles, the warranty is even better: 15 years or 150,000 miles.

NOTE: Warranty coverage is from the vehicle build date, not its sale date or model year. The build date can be found on a decal or plate usually mounted on the center door pillar.

Aftermarket converters have a shorter warranty of 2 years or 24,000 miles, and typically contain less catalyst and/or a shorter catalyst bed inside the converter shell. On some applications, they may not perform as well as the original and may cause the P0420 code to reset.


If the converter is plugged, contaminated, damaged or rusted out, it must be replaced. Likewise, if the OBD II system is showing low catalyst efficiency, the converter must be replaced. Replacing the catalytic converter will restore proper emissions performance. But a new converter will suffer the same fate as the old one if the underlying condition that caused the converter to fail has not been diagnosed and repaired. Look for fouled spark plugs or wires, low or no cylinder compression in one or more cylinders, or a computerized feedback system that stays in open loop all the time (bad coolant sensor, bad oxygen sensors, etc.).

On a dual-cat system, the side with the bad converter code will tell you which cylinder bank to check. If the converter on the right is bad, for example, check the O2 sensor, spark plugs and compression on the right cylinder bank.

Always replace the oxygen sensor. Converters needs an air/fuel mixture that is constantly flip-flopping from rich to lean. If the oxygen sensor is sluggish or dead, the fuel feedback loop will flip-flop too slowly or remain rich all the time.

Also check the air pump (if equipped) and related plumbing as these components provide fresh air for the converter to reburn the pollutants in the exhaust. If the air pump is not working right, it can reduce the operating efficiency of the catalytic converter significantly.

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