Part-2 continuation post from yesterdays ‘Cat’ post.

On 1996 and newer vehicles that have On Board Diagnostics II (OBD II), there is a “catalyst monitor” that monitors the operating efficiency of the converter. A second oxygen sensor is mounted behind the converter to compare oxygen levels in the exhaust before and after the converter.

Under normal operating conditions, the downstream O2 sensor should have little switching activity. But if the rate at which the downstream O2 sensor is switching starts to increase, it tells the OBD II system converter efficiency is dropping and there’s a potential emissions problem. If the problem may cause emissions to exceed 1.5 times the federal limit, the Malfunction Indicator Lamp (MIL) will come on and the PCM will log a diagnostic trouble code for “catalyst is below threshold efficiency” (P0420, P0421, P0422, P0430, P0431 or P0432). The bottom line here is you have a bad converter — unless the problem is something else like a bad oxygen sensor or open fuel feedback control loop.

Contamination causes failures:

When other reactive substances find their way into the exhaust, they can cause problems with the catalyst inside the converter.

Prior to 1975, tetraethyl lead was used to boost the octane rating of gasoline and to lubricate the exhaust valves. When catalytic converters were added in 1975, leaded gasoline was gradually phased out. Fuel restrictors were incorporated into the fuel filler inlet pipe so motorists couldn’t fill up with the leaded gasoline — but many managed to defeat these devices because leaded fuel was cheaper than unleaded. Eventually, leaded fuel disappeared in the U.S. so this should not be a concern unless somebody fills up a car with racing fuel or is traveling south of the border.

Phosphorus is the main additive used nowadays that ruins a converter. Phosphorus is found in motor oil. So is zinc, which can also cause trouble. Normally these trace metals do not cause a problem. But in a high mileage engine with worn valve guides, rings and/or cylinders, oil burning can pump enough oil into the exhaust to foul the converter. Once this happens, there is no fix other than to replace the converter. Trouble is, the new converter will eventually suffer the same fate as the old one unless the cause of the oil burning is also repaired — which typically means a rebuilt or used replacment engine.

Sulfur is another contaminant. It is found in small amounts in gasoline. As long as the concentration is limited, it causes no problem. But too much sulfur in a tank of bad gasoline can create a rotten egg odor in the exhaust and cause the converter to ignite at a higher than normal temperature, which ncreases pollution and probably damaging the converter.

Silicone is an ingredient in traditional antifreeze. Silicone is used to provide corrosion protection for aluminum parts. As long as it stays inside the cooling system, it has no effect on the converter. But if the head gasket starts to bleed coolant into the combustion chamber, or the head develops a hairline crack that leaks coolant, silicone can get into the exhaust and ruin the converter. As with phosphorus contamination, it is essential to eliminate the source of the coolant leak before the converter is replaced, otherwise the new converter will have the same fate as the old one.

If the converter has failed because of contamination, the oxygen sensors should also be checked because they may be contaminated, too.

When a converter gets too hot:

The converter can handle quite a bit of heat. High levels of pollutants coming out of the engine cause the catalytic converter’s temperature to increase dramatically. This can damage the converter.

If the converter overheats (1800 degrees F-plus), it can actually melt the ceramic honeycomb inside the shell. The result may be a partial or complete blockage that causes a sharp increase in exhaust backpressure and a big drop in driveability and fuel economy. If the converter is completely plugged, it will cause the engine to stall or damage it. This is where a remanufactured or used engine comes into play.

Underlying causes here include things like fouled spark plugs, bad plug wires, leaky engine valves or a leaky head gasket. Any of these can allow large quantities of unburned fuel to pass into the exhaust. .

Checking for restrictions is easier and doesn’t require any special equipment. A restriction problem might be suspected if your engine has a power loss, uses a lot of gas, or stalls after it starts and won’t restart until cooldown. A good diagnosis will prove it.

Low intake vacuum readings are a classic symptom of to much backpressure which may be due to a plugged converter.

Note the reading at idle, then hold rpm at 2,500. The needle will drop when you first open the throttle, then stabilize. If the reading then starts to drop, backpressure is building up in the exhaust system.

You can also attempt to measure exhaust backpressure directly. If the car has air injection, disconnect the check valve from the distribution manifold, and insert a pressure gauge. Or, remove the oxygen sensor and take your reading at its hole in the manifold or headpipe. Refer to the backpressure specs for the application. Generally speaking, more than 1.25 PSI of backpressure at idle, or more than 3 PSI at 2,000 rpm tells you there’s a blockage.

A “bump it” test on the outside with a soft rubber mallet will tell you if the catalyst inside is loose. You should not hear any rattling inside a monolithic converter. If you do, it means the honeycomb inside is broken.

The EPA’s has strict rules for replacement: a competent repair shop cannot simply replace a converter until it is out of factory warranty and a qualified need for replacement has been established and documented. The repair shop must also obtain your authorization for repairs in writing, keep the paperwork for a period of time, as well as the old cat to prove it’s case.

The replacement converter must be the same type as the factory original and installed in the same location. .

The federal emission warranty on OEM converters is at least 8 years or 80,000 miles. If your OEM converter is still under warranty, you should be able to get a free replacement from your new car dealer. If it is out of warranty, you can take it to any on the level competent repair shop or change it yourself.

Replacement converters must be certified to meet the factory standards. We have covered catalytic converters in full now on the GotEngines.com blog……to add more info to our library for your benefit.