Automotive Catalytic Converters – a Primer

Automobiles have come equipped with catalytic converters since the mid-1970s. If you’ve been working on cars old or modern, catalysts or “cats” are quite familiar. These devices, installed in the exhaust stream of internal combustion engines, are designed to convert harmful vehicle tailpipe emissions to safer substances.

Depending on engine configuration, a manufacturer may install one or more catalysts in the exhaust system. Typically, a four-cylinder engine is equipped with one catalyst. Six- and eight-cylinder models nearly always are equipped with 2 or more catalysts.

The bulk of gasoline engine tailpipe emissions and their effects are as follows:

Carbon monoxide or CO, an odorless gas which is toxic to air-breathers.

Hydrocarbons or HC, a mixture of unburned organic molecules causing odors and toxicity.

Nitrous oxides or N_xO , odorless gases which, combined with hydrocarbons in the presence of sunlight, produce “smog”.

Catalysts installed in the 1970s and early 1980s were called two-way or oxidation catalytic converters. The function of this kind of device was to interrupt the flow of some of the exhaust gases and convert them. CO was oxidized to CO₂ (carbon dioxide), and HC was oxidized primarily to water vapor and CO₂. [Bear in mind that CO₂ is a significant factor in global climate change, though it is not toxic per se.]

In the mid-1980s, emission control regulations required that automobile manufacturers equip vehicles with three-way or oxidation-reduction catalysts (also referred to as TWC). These perform the same older functions of oxidization of carbon monoxide and hydrocarbons. But they have an additional function: Reduction of nitrous oxides to nitrogen (N₂) and oxygen (O₂).

The working core of a TWC is a fine honeycomb ceramic structure coated with a thin layer of the precious metals platinum and rhodium. When nitrous oxide gases pass over these metal coatings, the catalysts remove the nitrogen atoms and release free oxygen. This is the reduction stage. The free oxygen is in “storage”; this oxygen as well as unused free oxygen in the exhaust stream, oxidizes CO and unburned hydrocarbons (HC) as needed.

In order for a TWC to function efficiently, a precise ratio (by weight) of air and fuel, approximately 15 : 1, referred to as the  stoichiometric ratio, is required. Software in the engine control module (ECM) controls the amount of fuel injected into the intake to maintain stoichiometry throughout the phases of engine operation. Among the signals that the ECM receives from various engine sensors, the primary input indicating the correct air-fuel ratio comes from oxygen sensor(s).

At times referred to as the lambda-sensor [the Greek letter lambda (λ) is the symbol for oxygen] or the O2 sensor, the oxygen sensor is a device installed in the exhaust stream. There are several designs in use by automobile manufacturers, an early version being a zirconia sensor. This type of sensor, being immersed in the exhaust stream, produces a small voltage that varies between 100 and 900 millivolts (mV). Higher voltage indicates a richer mixture, equivalent to lower oxygen content.

A more up-to-date version of the oxygen sensor, called the wide-band sensor, is currently in use in newer vehicle models. The signal from this sensor is different from the zirconia version and more accurate. (We’ll dive deeper into oxygen sensors in a later article.)

Closed loop mode is controlled by pre-catalyst oxygen sensor(s) installed in the exhaust manifold(s) close to the exhaust valves. This mode assures that the catalyst(s) receive exhaust from a closely monitored stoichiometric exhaust stream.

Once the stoichiometric exhaust gases enter the catalyst, it begins to perform its function of reducing N_xO and oxidizing CO and HC using stored oxygen. However, as catalytic converters age, performance declines. Contamination as well as physical or thermal damage can greatly reduce efficiency of the ceramic matrix or the catalyzing metals.

To meet the goals stipulated by clean-air regulations, second generation on-board diagnostics (OBD II) standards requires that catalytic converter oxidation and reduction be monitored. The primary technique for this is for the ECM to monitor oxygen storage capacity by comparing output(s) from secondary (post-catalyst) oxygen sensor(s), which are installed after the catalysts, with that from the primary (pre-catalyst) sensor(s).

To sum up what we know so far:  The signal from pre-catalyst oxygen sensor(s) in closed loop mode cause the ECM to vary the fuel mixture slightly from rich to lean and back again to stay very close to the stoichiometric ideal. The reduction portion of the catalyst(s) then strip the N_xO to elemental N₂ and O₂. The released oxygen plus unused oxygen in the exhaust is then used to oxidize the CO and HC components of the exhaust stream; the final tailpipe output is nitrogen, oxygen, CO₂ and water vapor.

The operation of the catalyst(s) is intended to smooth out the lean-rich swing of closed-loop operation. With fresh, clean catalyst(s) in service, the result is a steady output of these gases. The fluctuation in signal amplitude (and oxygen content) by the pre-catalyst sensors is smoothed out so that the secondary (post-catalyst) sensors show low fluctuation in oxygen content. But with age, damage or other malfunctions, catalyst efficiency suffers and the output of the secondary sensors begins to approach that of the primary sensors.

A catalyst monitoring subroutine in the ECM compares the output of the two sets of oxygen sensors and calculates oxygen storage capacity in the catalyst. If the secondary sensor output begins to approach the fluctuations of the primary, oxygen storage capacity is calculated to be below a certain programmed limit and a fault is set. This usually indicates that the catalyst is worn out or broken inside and is due to be replaced.

In a vehicle equipped with primary and secondary catalysts, catalyst monitoring is done a bit differently. The primary catalyst has a greater capacity for oxygen storage than the secondary catalyst. Because of this the ECM intervenes during fuel control to test catalyst efficiency. Fault diagnosis is done by measuring oxygen storage capacity during a rich to lean changeover during engine idle as well as part-load operation.

In the first step of this process, the oxygen accumulator is completely emptied with the engine running rich. The secondary oxygen sensor signal voltage will be at 650 mv. In the second step of the process, with the engine running lean, the amount of oxygen absorbed reaches (storage) overflow calculated with help from the mass flow (MAF) sensor (air mass) signal and the primary oxygen sensor. The point of overflow is indicated by the secondary oxygen sensor voltage reaching 200 mV. Both steps are referenced by the ECM against a stored figure to determine if a fault is present or not. Monitoring the catalyst this way, during a rich to lean switch, allows for greater accuracy as well as less dependency on temperature and sulfurization (conversion of sulfur from exhaust gases).

When diagnosing catalyst efficiency manually on wideband equipped vehicles, you will not be able to use the graphing scan tool function to do so. You will need to to use a scope to display and calculate the conversions. 

When finalizing your diagnosis, always check for a completely sealed exhaust as well as intake system. Most times, catalysts fail from age or contamination (physical / thermal damage). Do not overlook items that can be a contributing factor to the failure. Check for gasket leaks, excessive oil in the positive crankcase ventilation (PCV) system (intake side) or even leaking turbocharger seals. Once you rule out all the possible contributing causes, replace the catalyst or catalysts as needed.