Walker - Catalytic Converters

The catalytic converter is designed to convert harmful emissions, produced by an internal combustion engine, to less-harmful elements: H2O (Water), CO2 (Carbon Dioxide) and N2 (Nitrogen).

To perform this conversion, the catalytic converter works with a vehicle's PCM (Powertrain Control Module) and other emissions control devices.

OBDII (On-Board Diagnostics Version 2) monitors the emissions control devices and provides feedback on their operating condition.

As the EPA (Environmental Protection Agency) updates emissions standards, the OBDII system becomes more sensitive to fluctuations in emissions performance.

Component Description:
1. Stainless Steel Body. For long life and durability. The ribbed body minimizes expansion and distortion, as well as forming channels that protect the cushioning mat from direct exposure to exhaust gases.

2. Monolithic Free-Flowing Substrate. The substrates are the backbone of the converter. This is where the proprietary mix of precious metal(s) and the washcoat formulated to store O2 allow the conversion process to take place. Converters are available in single- or multiple-substrate designs.

3. Catalyst Cushioning Mat. The mat cushions the converter substrate, holding the ceramic catalyst in proper alignment. Creates a seal between the substrate and body, making sure all exhaust goes through the catalyst. Allows for thermal expansion of the body.

4. Body & Pipe Heat Shields To Match OE. Deflects heat created by the converter away from the vehicle's undercarriage.

5. O2 (Oxygen) Sensors. Another vital part of an emissions control system. These sensors are placed before and after the catalytic converter on an OBDII vehicle. They are designed to monitor the O2 storage efficiency of the converter. This information also allows the PCM to adjust fuel controls.
Two-Way Catalytic Converter
- Allows oxidation of CO (Carbon Monoxide) to less-harmful CO2 (Carbon Dioxide)
- Allows oxidation of HC (Unburned Hydrocarbons) to CO2 (Carbon Dioxide) and H2O (Water)
  In this design, exhaust gases are directed to flow through the substrate containing precious metals platinum and palladium, which allow the chemical reaction to occur. The exhaust gases increase in temperature as the conversion process takes place.
Because of the intense heat created by this process, exhaust gases leaving the converter should be hotter than the gases entering the converter. This also explains why heat shields are required on most units.

Two-way converters operate relatively efficiently with a lean fuel mixture. Ineffectiveness in controlling NOx led to the introduction of three-way converters.
Three-Way Catalytic Converter
Three-way converters have been used in vehicle emissions control systems in North America - and many other countries - since 1981.
- Allows reduction of NOx (Nitrogen Oxides) to N2 (Nitrogen) and O2 (Oxygen)
- Allows oxidation of CO (Carbon Monoxide) to less-harmful CO2 (Carbon Dioxide)
- Allows oxidation of HC (Unburned Hydrocarbons) to CO2 (Carbon Dioxide) and H2O (Water)
The three-way without air uses advanced catalyst chemistry to store and release O2, in conjunction with an O2 monitoring and control system.

This system utilizes one or more O2 sensors to oscillate the fuel mixture between lean and rich conditions. This oscillation, combined with the O2 storage and release on the catalyst surface, allows for optimum reduction of all three emissions.

Three-way converters are used in conjunction with OBDII diagnostic systems on today's vehicles. This system alerts the driver when the converter is not working at peak efficiency.
Three-Way Plus Air Catalytic Converter
Three-way plus air converters were used in vehicle emissions systems in North America during the late '70s and early '80s.
- Allows reduction of NOx (Nitrogen Oxides) to N2 (Nitrogen) and O2 (Oxygen)
- Allows oxidation of CO (Carbon Monoxide) to less-harmful CO2 (Carbon Dioxide)
- Allows oxidation of HC (Unburned Hydrocarbons) to CO2 (Carbon Dioxide) and H2O (Water)
Inside this converter there are two substrates. The front, coated with the precious metal rhodium, is used to reduce NOx emissions into simple N2 and O2. This process is most effective when little O2 is present (rich mixture). That is why it is located upstream of the air tube.

Since a rich mixture is high in HC and CO, an air pump and tube supply additional O2 to this mixture before it enters the second substrate.

The second substrate, coated with the precious metals palladium and platinum, allows oxidation of HC and CO to less harmful emissions CO2 and H2O.

This system was not very efficient and was phased out in the early '80s, when the current three-way converter was introduced.
Overheated, Melted or Broken Converters
Any malfunction causing an unusually high level of HC or CO (in conjunction with high levels of O2) to enter the converter, will dramatically elevate its temperature.

Potential causes for high HC readings may include:
- Misfires
- AFR Cylinder imbalance
- Excessive engine or vehicle load:
  a) Fuel delivery system or fuel quality
  b) Sluggish (worn out) O2 sensors
  c) Excessive backpressure
  d) Low compression* Poor spark, or weak ignition
Coated / Oil-Fouled Substrate
Catalyst poisoning occurs when the converter is exposed to emissions containing substances that coat the working surfaces, enveloping the catalyst to the point it cannot contact - and treat - the exhaust.

Potential causes for coated or fouled substrate may include:
- Excessive carbon build-up in exhaust
- Internal coolant leaks (head / intake gasket)
- Use of non-converter-safe gasket sealants
- Excessive oil consumption (burning oil)
- Improper fuels or additives (E85, diesel)
Structural Damage
The primary cause of structural damage is road debris striking the converter. Normally, some evidence of impact is visible on the converter shield.

Other conditions that may cause structural damage:
- Corrosion
- Thermal shock
- Metal fatigue
- Stress fractures
- Stripped O2 Sensor threads
- Flex pipe failure
- Air-gap pipe failure

Component Description: