Understanding Three-Way Catalytic Converters (TWC), Heated Oxygen Sensors, And MIL

Ever wonder why your aftermarket header or s-pipe (sometimes) will cause the dreaded MIL (Maintenance Illumination Light). All aftermarket headers, save one which we will discuss later, are designed without the primary Three-Way Catalytic Converters (TWC). This alters the readings taken by the Heated Oxygen Sensor (HOS). A couple of s-pipes in the past were built with the O2 sensor bung in the wrong location which caused an error in reading as well. The O2 sensor can also have skewed readings if there is any airleaks in the exhaust stream from cracks or failed gaskets. There are many reasons but what are the causes of these errors and why does the MIL come on?

To understand this, we must first have a grasp on the functionality, as well as, the relationship between the HOS and the TWC. Let's start off with the TWC and how it works. What better way to start off than with this How Stuff Works Article that I further quote:

As we can extrapolate from the equations, Nitrogen is captured from the NOx emissions with the resultant leftover being Oxygen. This O2 is then utilized to combust unspent hydrocarbons and convert toxic carbon monoxide to carbon dioxide. It is important to note that only a percentage of NOx is broken down and not all O2 is converted. This percentage is variable based on many factors.

Now that we comprehend the basics of catalytic reaction in the converter, we can correlate how this affects HOS detection data. O2 sensors work by making constant comparisons between oxygen levels in the exhaust stream and the external ambient O2. This comparison is converter to an electric signal that interpreted by the ECM. The 2AZ-FE, like most modern engines, utilize two O2 sensors to monitor oxygen levels in the exhaust. The primary sensor is an A/FR wideband that is in the runner collector of the header just before the TWC. The secondary sensor is the HOS narrowband located in the s-pipe between the primary and secondary catalytic converters. In Closed Loop mode, the ECM utilizes the A/FR sensor (Bank1Sensor0 - wideband) data along with all other engine sensors to set fuel/ignition tables for Stoichiometric. A/FR sensor data is ignored in Open Loop operation because after a cold start, the the A/FR and HOS are not up to temperature for accurate readings. This is usually between 400~600°F.

This article by Rick Kirchoff discusses more in detail on it's operation.

"OK, thanks for the chemistry lesson, but what does this all mean?". Simply put, leaks allow ambient air to be siphoned into the exhaust stream during low pressure pulses thru a means called eduction. For this reason, gaskets must make a positive seal.

Understanding Your TWC AND Bank1Sensor2 Heated Oxygen Sensor (Narrowband)

DESCRIPTION
A three-way catalytic converter (TWC) is used in order to convert the carbon monoxide (CO), hydrocarbon (HC), and nitrogen oxide (NOx) into less harmful substances. To allow the TWC to function effectively, it is necessary to keep the air-fuel of the engine near the stoichiometric air-fuel ratio. For the purpose of helping the ECM to deliver accurate air-fuel ratio control, the Heated Oxygen (HO2) sensor is used. The HO2 sensor is located behind the TWC, and detects the oxygen concentration in the exhaust gas. Since the sensor is integrated with the heater that heats the sensing portion, it is possible to detect the oxygen concentration even when the intake air volume is low (the exhaust gas temperature is low). When the air-fuel ratio becomes lean, the oxygen concentration in the exhaust gas is rich. The HO2 sensor informs the ECM that the post-TWC air-fuel ratio is lean (low voltage, i.e. less than 0.45 V). Conversely, when the air-fuel ratio is richer than the stoichiometric air-fuel level, the oxygen concentration in the exhaust gas becomes lean. The HO2 sensor informs the ECM that the post-TWC air-fuel ratio is rich (high voltage, i.e. more than 0.45 V). The HO2 sensor has the property of changing its output voltage drastically when the air-fuel ratio is close to the stoichiometric level.



The ECM uses the supplementary information from the HO2 sensor to determine whether the air-fuel ratio after the TWC is rich or lean, and adjusts the fuel injection time accordingly. Thus, if the HO2 sensor is working improperly due to internal malfunctions, the ECM is unable to compensate for deviations in the primary air-fuel ratio control. The sensing portion of the Heated Oxygen (HO2) sensor has a zirconia element which is used to detect the oxygen concentration in the exhaust gas. If the zirconia element is at the appropriate temperature, and the difference between the oxygen concentrations surrounding the inside and outside surfaces of the sensor is large, the zirconia element generates voltage signals. In order to increase the oxygen concentration detecting capacity of the zirconia element, the ECM supplements the heat from the exhaust with heat from a heating element inside the sensor. When the current in the sensor heater is outside the standard operating range, the ECM interprets this as a malfunction in the sensor heater and sets a DTC.



Example:
The ECM sets DTC P0038 when the current in the HO2 sensor heater is more than 2 A. Conversely, when the heater current is less than 0.3 A, DTC P0037 is set.

HINT:
Sensor 2 refers to the sensor mounted behind the Three-Way Catalytic Converter (TWC) and located far from the engine assembly.

MONITOR DESCRIPTION
The ECM monitors the rear Heated Oxygen (HO2) sensor to check for the following malfunctions. If any of the malfunctions are detected, the ECM illuminates the MIL and sets a DTC:
• The HO2 sensor output voltage remains above 0.45 V (rich) or below 0.45 V (lean) while the vehicle is accelerated and decelerated for 8 minutes.
• The HO2 sensor output voltage remains below 0.05 V for a long period of time while the vehicle is driven.
• The HO2 sensor output voltage does not decrease below 0.2 V (extremely lean condition) within 7 seconds after fuel-cut is performed while the vehicle is decelerated. The ECM interprets this as the sensor response having deteriorated.

Understanding The Bank1Sensor1 A/FR Oxygen Sensor (Wideband)

DESCRIPTION
The ECM uses information from the Air-Fuel Ratio (A/F) sensor to regulate the air-fuel ratio and keep it close to the stoichiometric level. This maximizes the ability of the Three-Way Catalytic Converter (TWC) to purify the exhaust gases. The A/F sensor detects oxygen levels in the exhaust gas and transmits the information to the ECM. The inner surface of the sensor element is exposed to the outside air. The outer surface of the sensor element is exposed to the exhaust gas. The sensor element is made of platinum coated zirconia and includes an integrated heating element.


The zirconia element generates a small voltage when there is a large difference in the oxygen concentrations between the exhaust gas and outside air. The platinum coating amplifies this voltage generation. The A/F sensor is more efficient when heated. When the exhaust gas temperature is low, the sensor cannot generate useful voltage signals without supplementary heating. The ECM regulates the supplementary heating using a duty-cycle approach to adjust the average current in the sensor heater element. If the heater current is outside the normal range, the signal transmitted by the A/F sensor will be
inaccurate, as a result, the ECM will be unable to regulate air-fuel ratio properly.
When the current in the A/F sensor heater is outside the normal operating range, the ECM interprets this as a malfunction in the sensor heater and sets a DTC.



Example:
The ECM sets DTC P0032 when the current in the A/F sensor heater is more than 10 A. Conversely, when the heater current is less than 0.8 A, DTC P0031 is set.

HINT:
• Although the DTC titles say oxygen sensor, these DTCs relate to the Air-Fuel Ratio (A/F) sensor.
• Sensor 1 refers to the sensor mounted in front of the Three-Way Catalytic Converter (TWC) and located near the engine assembly.
• When any of these DTCs are set, the ECM enters fail-safe mode. The ECM turns off the A/F sensor heater in fail-safe mode. Fail-safe mode continues until the ignition switch is turned off.
• The ECM provides a pulse width modulated control circuit to adjust the current through the heater. The A/F sensor heater circuit uses a relay on the +B side of the circuit.