Knowledgebase: General
O2 Sensors & Catalytic Converter Diagnostics
Posted by Alex (Im) E. on 01 February 2013 12:53 AM


Converters typically fail one of two ways: they get too hot, melt and plug up, or the catalyst becomes contaminated and useless.

Diagnosing a plugged converter is fairly easy, but diagnosing converter efficiency requires tapping into the OBD II system with a scan tool to check for codes and to compare the upstream and downstream O2 sensor readings.



Plugging is usually caused by unburned fuel entering the exhaust system. A fouled spark plug, bad plug wire or a bad coil on a DIS system causes ignition misfire that overloads the converter with fuel. A leaky exhaust valve can do the same thing, allowing unburned fuel to pass right through the engine without being ignited.

Most converters are designed to operate at a temperature of 800 to 1600 degrees F. When the exhaust temperature hits about 500 degrees, the catalyst starts to trigger the chemical reactions that break down the pollutants.

This releases heat, causing the converter to light off. The temperature shoots up and things really start cooking. But if there's too much HC in the exhaust, the converter may get too hot. At temperatures above 1600 degrees F, the catalyst can begin to melt and even the ceramic or metallic honeycomb that supports it.

We've seen converters that have been melted into a solid glob, creating a complete blockage of the exhaust.We've seen others that still pass some gas, but create enough restriction that backpressure chokes the engine and causes a noticeable loss of power and fuel economy.

If a vehicle lacks high speed power, is struggling to breathe or stalls after starting, the converter may be plugged.

To confirm the diagnosis, you can hook up a vacuum gauge to the intake manifold and note the vacuum reading. If intake vacuum is low and continues to drop while the engine is idling, exhaust backpressure is building up and choking the engine.

You can also check exhaust backpressure ahead of the converter by connecting a pressure gauge to the EGR exhaust port, an O2 sensor port, or an exhaust manifold air line connection. More than 1.25 psi of backpressure at idle, or more than 3 psi at 2,000 rpm usually indicates a blockage.



Diagnosing converter efficiency to see if the converter is still doing its job can be done a couple of ways. If you have a five-gas infrared exhaust analyzer, you can compare pre-cat and post-cat exhaust readings of HC, CO and NOX. There should be a significant drop in the readings if the converter is working. The question is, how much of a drop?

A brand new converter theoretically operates at 100% efficiency. But in the real world, a new converter "ages" slightly during its first 5,000 miles of operation, then levels off and operates at a fairly constant level of 95% or better efficiency for tens of thousands of miles.

The design life of most converters today is 150,000 miles. But over time, a gradual buildup of contaminants reduces converter efficiency to the point where tailpipe emissions may increase beyond acceptable limits.

Contaminants include phosphorus from burning oil (worn valve guides or seals, or worn piston rings or cylinders), silicone from internal coolant leaks (bad head gasket or cracks in combustion chamber) and high levels of sulfur in gasoline. Since there's no way to clean a contaminated converter, replacement is the only repair option.

Comparing the pre-cat and post-cat exhaust readings allows you to calculate converter efficiency percentages for each of the three pollutants.

For example, if the upstream HC reading is 120 ppm and the downstream HC reading is 12 ppm, the downstream reading is 10% of the upstream reading -- which means the converter is working at 90% efficiency. That's probably good enough to keep emissions within limits.

As a rule, if converter conversion efficiency drops below 80% for any pollutant, the vehicle may experience elevated tailpipe emissions. But for late model Low Emission Vehicles (LEV) and Ultra Low Emission Vehicles (ULEV), there's even less margin for error. Vehicle emissions may be a problem if converter efficiency is less than 90 to 94%.

It's hard to predict how emissions will increase as converter efficiency drops off because it depends on how clean the engine is, the emissions level to which the vehicle has been certified, operating conditions and even the type of drivetrain (automatic or manual transmission).

The only way to know if emissions are really a problem or not is to give the vehicle an emissions test. That's why states test vehicles every couple of years to check emissions compliance.

A loaded mode emissions check on a dyno such as an I/M 240 or ASM test gives the most accurate test results because the vehicle is tested under simulated road conditions. A converter that works well enough to keep emissions within limits at idle may literally run out of air at higher speeds allowing too much pollution to exit the tailpipe.

To work efficiently, a converter needs to capture and hold oxygen when the mixture is lean so it can burn the pollutants when the mixture is rich.

A special "washcoat" of aluminum oxide on the honeycomb increases its surface area by a factor of almost 7,000 times, allowing it to trap and hold the extra oxygen.

But if the washcoat has been damaged or contaminated, the converter may not be able to trap enough oxygen causing a drop in efficiency.



The catalyst monitor is one of the most difficult monitors to set on OBD II vehicles. It's supposed to run on every "trip, which is a drive cycle that begins with a cold start and continues until the engine reaches normal operating temperature.

But the monitor monitor won't run until all the "enabling criteria" are met. This may require driving the vehicle at various speeds and/or loads until the system decides it is ready and runs the test. As a rule, the vehicle will have to be driven at highway cruising speeds for at least 15 minutes to get the catalyst monitor to run.

The catalyst monitor will not run if the OBD II system has detected a conflict with other tests (such as the EGR, fuel system or purge tests), or there are hard codes or pending codes for a misfire condition or oxygen sensor.

If the test runs and the OBD II system detects too much of a drop off in converter efficiency, it will set a pending code. If the same condition is noted on a second or third trip, the code will mature and turn on the MIL lamp. If you plug a scan tool or code reader into the vehicle and discover a generic OBD II code P0420 to P0439, you've diagnosed a converter problem.

Now the question becomes, is the converter really bad? That's a question that isn't so easily answered. An OBD II vehicle with a converter efficiency code may or may not have an emissions problem. The only way to know for sure is to give it an emissions test. But here's the catch. Even if it passes a tailpipe test, it may not be emissions legal as long as the MIL lamp remains lit. The only way to extinguish the MIL lamp may be to replace the converter.

Remember, the OBD II system is calibrated to detect potential emissions problems that may cause emissions to exceed federal limits by 1.5 times.

Some vehicle manufacturers are more cautious than others and may calibrate their OBD II systems to turn on the MIL lamp at the first indication of trouble -- even though actual emissions may still be within legal limits. Even so, if the MIL lamp is on and there's a converter code it means the converter is failing.

Sooner or later converter efficiency will drop to the point where it may cause a real emissions problem. So the best advice is believe the MIL lamp.

If you want confirmation and have a scan tool or oscilloscope, you can compare the upstream and downstream O2 sensor readings yourself to judge converter efficiency.

If there are no O2 sensor codes and both sensors are functioning normally, you should see a lot of switching activity in the upstream O2 sensor(s) and very little switching activity in the downstream O2 sensor(s) if the converter is good.

If the downstream O2 sensor activity mirrors the upstream O2 sensor, the converter is dead and needs to be replaced.

On V6 and V8 applications with dual cat exhausts, you'll have upstream O2 sensor readings for each side of the engine and downstream O2 sensor readings for each converter. Check and compare both sides.



Most scan tools can access a menu called "Mode 06." This is usually found by choosing "global" or "generic" OBD II on the scan tool main menu rather than entering the vehicle year, make & model.

The Mode 06 menu lists self-test data for all the non-continuous monitors, including the catalyst monitor.

The nice thing about the Mode 06 data is that it will show you if the catalyst or oxygen sensor heaters are operating within normal limits.

A problem may not have set a DTC yet, but if it is misbehaving you can see it in Mode 06. See the Mode 6 section for more information on how to use Mode 06 diagnostics.



On 1995 and newer vehicles, the federal emissions warranty on the converter is 8 years or 80,000 miles.

There are strict rules that apply to replacing converters that are still under warranty. Even so, OEM converters can be replaced with aftermarket converters even when vehicles are still under warranty (but at the customer's expense) for any of the following reasons (which must be documented):

  • The vehicle failed an emissions test.
  • The original converter is damaged (clogged or rusted), or missing.
  • The converter has been contaminated.

A replacement converter must be the same type as the original (two-way, three-way or three-way plus oxygen), EPA certified and certified as being OBD II compliant.



You may run into a situation where the catalyst monitor won't run because there's an O2 sensor code. The OBD II system can't evaluate the converter if the upstream and downstream O2 sensors are not functioning normally.

So any O2 sensor problems have to be dealt with before you can use the OBD II system to diagnose the converter.

O2 sensors typically fail because of old age or contamination. Older generation O2 sensors may become sluggish once they've seen 50,000 miles of service. Newer generation O2 sensors should last 100,000 miles or more, but may also become sluggish if contaminants from the engine have fouled the sensor.

Silicone from internal coolant leaks and phosphorus from burning oil are the two main contaminants that can kill an O2 sensor.

If an engine has suffered a recent head gasket failure, the O2 sensors may need to be replaced because of contamination.

A good O2 sensor should show a lot of switching activity with readings bouncing back and forth from 0.3 volts or less to 0.8 to 0.9 volts. If the sensor is sluggish or reads low, it needs to be replaced.

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