Automotive Diagnostic OBD2 Scanner

Catalytic Converters for OBD II engines

The Role of Catalytic Converters and O₂ Sensors in OBD II Engines
Catalytic converters and oxygen (O₂₎ sensors are factory installed on every OBD II engine for one simple reason: – Strictly Controlled Exhaust Emissions. These factory OEM devices can easily last for about 150,000 miles, even more if the engine is always running in perfect tune. With that said, there are a number of factors which will determine the catalytic’s life span, and if it fails to operate effectively the vehicle will NOT pass the emissions test. According to most States, the E.U. and most developed countries, it is a big offense to drive a vehicle without one, by simply bypassing the catalytic converter with a straight pipe and threaded plugs for the O₂ sensors.

History and Types of Catalytic Converters

Two-Way Converters:
The first production catalytics, which started appearing on new vehicles around 1975, were introduced to meet the ever-tightening rules of the Environmental Protection Agency (EPA). These were known as the “Two-Way” or as the “Oxidizing” converters and only reacted with hydrocarbons (HC) and carbon monoxide (CO) found in the exhaust emissions. They did not reduce nitrogen oxide (NO_x) produced from the exhaust. This type of catalytic converter is used on diesel engines to reduce hydrocarbon and carbon monoxide emissions. They were also used on gasoline engines, mostly in the USA’s market till 1981, when the two-way converter’s inability to control NO_x led to its substitute, the “Three-Way Converters” (TWC).

Three-Way Converters:
These converters started appearing around the end of 1980 in USA and many other countries. They differed mainly from the Two-Way converters because they had two catalysts inside, one to oxidize HC and CO, and a second to reduce NO_x. The oldest TWC have have an air pipe connected to an air pump or aspirator valve to supply air between the oxidation and reduction catalysts. Latest TWC converters don’t need this air pipe and rely on oxygen in the exhaust to burn the pollutants.

These Three-Way catalytic converters are engineered with three important specific functions, and so have been designed to:

1. Reduce the nitrogen oxides into nitrogen and oxygen
2. Reduce oxidation of carbon monoxide into carbon dioxide
3. Reduce oxidation of unburnt hydrocarbons (HC) to carbon dioxide and water

Generally, OBD2 engines fitted with 3-way catalytic converters are equipped with a computerized closed-loop feedback fuel injection system employing one or more oxygen sensors. While a TWC catalyst can be used in an open-loop system, the NO_x reduction efficiency is low. Within a narrow fuel/air ratio band surrounding stoichiometry, conversion of all three pollutants is nearly complete. However, outside of that band, conversion efficiency falls off very rapidly. When there is more oxygen than required, then the system is said to be running lean, and the system is in oxidizing condition. In that case, the converter’s two oxidizing reactions (oxidation of CO and hydrocarbons) are favored, at the expense of the reducing reaction. When there is excessive fuel, then the engine is running rich. The reduction of NO_x is favoured, at the expense of CO and HC oxidation.

The Powertrain Control Module (PCM) constantly adjusts the air/fuel mixture when the engine is warm by monitoring the rich/lean signal from the oxygen sensor in the exhaust. When the O₂ sensor reads lean, the PCM makes the fuel mixture go rich. When the O₂ sensor sends back a rich signal, the PCM shortens the pulse of the fuel injectors and leans the fuel mixture. The O₂ sensor then sends back a lean signal, and the PCM increases the pulse of the injectors to make the fuel mixture rich again. By rapidly changing the air/fuel mixture back and forth, the overall mixture averages out and keeps emissions at a minimum.

On some newer vehicles, a new type of Wideband Oxygen Sensor is used, also known as (Air/Fuel Ratio Sensor). Instead of producing a high or low signal voltage, the signal changes in direct proportion to the amount of oxygen in the exhaust. This provides a more precise measurement for better fuel control and tells the PCM the exact air/fuel ratio. On most applications, the air/fuel ratio or lambda value can be read through certain sophisticated diagnostic OBD2 scanners.

What Causes Catalytic Failures:
Catalytic converters are quite durable when the engine is working without any ‘Check Engine’ or any other problems; even though they work in extreme operating temperatures of around 600°F to 1000°F. The main converter destroyers are:

• Ignition misfire — through fouled spark plug and, or shorted plug wire
• Compression misfire — through leaky valves or head gasket
• Internal coolant leaks — through cracked or porous head and, or leaky head gasket
• Burning of oil — through worn valve guides, seals, rings, cylinder-bores
• Fuel contamination — mainly through lead-based fuels
• Phosphorus fouling — from worn engine components
• Rust or any physical damage to the converter body

Bad engine sensors can also contribute to destroying a catalytic converter if the faults are left unattended to. These can be:

• Faulty O₂ sensors
• Faulty ECT sensors (engine coolant temperature sensors)

Catalytic Contaminants:
When other reactive substances find their way into the exhaust, they can cause problems with the catalyst inside the converter. These include phosphorus, silicone and lead.

Before 1975 tetra-ethyl 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. This gasoline change eventually started passing on through other European countries too, with lead being nearly completely phased out. It is still possible to buy “modified” leaded fuels, but these are becoming difficult to find and are much more expensive.

Phosphorus is the main concern today for fouling a converter. Phosphorus and zinc are used as an anti-wear agents in modern oils. These trace metals are added for longevity to the engine’s life-cycle and work well as intended but have a bad impact on older, worn out engines.These high mileage engines with worn valve guides, valves, rings and possibly even worn out cylinders (need of re-bore) will have burnt oil finding its way through the exhaust, and this will eventually destroy the converter. Once this happens, no repairs can be done — replacement is the only option. Unless the main cause of the catalytic failure is repaired, the new converter will eventually suffer the same fate as the old one. In the case of burnt oil passing through rings and bore, this means overhauling or replacing the engine.

Sulfur is another contaminant, and is found in small traces in gasoline. As long as the concentration is limited, this should not cause any problem, especially if it is not permanent (one-time bad batches of fuel). Too much sulfur in a batch of bad fuel is another story though, this can severely increase the risk of destroying the catalytic converter. The exhaust odor with high amounts of sulfur present resembles that of a rotten egg odor in the exhaust and cause the converter to light off at a higher than normal temperature, increasing pollution and possibly damaging the converter.

Silicone is usually found in the antifreeze mix of the coolant system. Silicone is used mostly to provide protection against corrosion for aluminum castings. As long as it stays inside the cooling system, it has no effect on the converter. But if the head gasket fails and coolant starts to seep through, finding its way in 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 suffer the same fate.

These substances mentioned: Silicone, Phosphorus and Lead will also eventually contaminate oxygen sensors too. If the converter has failed because of contamination, the oxygen sensors should also be tested because they may have been contaminated too.

The Catalyst Monitor:
On 1996 and newer vehicles that have On-Board Diagnostics II (OBD II), there is a “Catalyst Monitor” that keeps track on the operating efficiency of the converter. A second oxygen sensor is mounted “downstream” or 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’s activity starts to increase, it tells the OBD II system that the converter efficiency is decreasing — meaning a potential emissions problem might exist. If the problem will end up causing emissions to exceed 1.5 times the federal limit, the Malfunction Indicator Lamp (MIL), also known as ‘Check Engine’ will come On.
The PCM will log a diagnostic trouble code for “catalyst is below threshold efficiency” [P0420, P0421, P0422, P0430, P0431 or P0432].

A faulty converter can be confirmed by comparing levels of CO and HC in the exhaust before and after the converter. If you see little or no reduction in HC and CO levels, the converter has reached the end of its life-cycle and needs to be replaced.

Catalytic Converter Replacement Regulations:
The EPA’s regulations for replacement are very stringent and state: “A repair facility cannot replace a converter until it is out of warranty and a legitimate need for replacement has been established and documented (such as a blockage, failure of an emissions test, or to replace a converter that someone removed). The repair facility must also obtain your authorization for repairs in writing, keep the paperwork for six months and the old converter for 15 days. The replacement converter must be the same type as the original and installed in the same location.” These rules do NOT apply to the vehicle owner, so you may replace the converter yourself if the converter is defective.

The Federal emission warranty on OEM converters is 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 repair facility or change it yourself.

Replacement converters must be the same type as the original, and OBD II vehicles require an OBD II certified converter. The new converter must also be installed in the same location as the original.