OXYGEN SENSOR HEATER MONITOR (NGC)
DESCRIPTION—If the Oxygen sensor (O2S) DTC as well as a O2S heater DTC is present, the O2S Heater DTC
MUST be repaired first. After the O2S Heater is repaired, verify that the sensor circuit is operating correctly.
The voltage reading taken from the O2S are very temperature sensitive. The readings taken from the O2S are not
accurate below 300° C (572°F). Heating the O2S is done to allow the engine controller to shift to closed loop control
as soon as possible. The heating element used to heat the O2S must be tested to ensure that it is heating the
sensor properly. Starting with the introduction on the NGC module the strategy for checking the heater circuit has
changed. The heater resistance is checked by the NGC almost immediately after the engine is started. The same
O2S heater return pin used to read the heater resistance is capable of detecting an open circuit, a shorted high or
shorted low condition.
CATALYST MONITOR
To comply with clean air regulations, vehicles are equipped with catalytic converters. These converters reduce the
emission of hydrocarbons, oxides of nitrogen and carbon monoxide.
Normal vehicle miles or engine misfire can cause a catalyst to decay. A meltdown of the ceramic core can cause a
reduction of the exhaust passage. This can increase vehicle emissions and deteriorate engine performance, drive-
ability and fuel economy.
The catalyst monitor uses dual oxygen sensors (O2S’s) to monitor the efficiency of the converter. The dual O2S
strategy is based on the fact that as a catalyst deteriorates, its oxygen storage capacity and its efficiency are both
reduced. By monitoring the oxygen storage capacity of a catalyst, its efficiency can be indirectly calculated. The
upstream O2S is used to detect the amount of oxygen in the exhaust gas before the gas enters the catalytic con-
verter. The PCM calculates the A/F mixture from the output of the O2S. A low voltage indicates high oxygen content
(lean mixture). A high voltage indicates a low content of oxygen (rich mixture).
When the upstream O2S detects a high oxygen condition, there is an abundance of oxygen in the exhaust gas. A
functioning converter would store this oxygen so it can use it for the oxidation of HC and CO. As the converter
absorbs the oxygen, there will be a lack of oxygen downstream of the converter. The output of the downstream O2S
will indicate limited activity in this condition.
As the converter loses the ability to store oxygen, the condition can be detected from the behavior of the down-
stream O2S. When the efficiency drops, no chemical reaction takes place. This means the concentration of oxygen
will be the same downstream as upstream. The output voltage of the downstream O2S copies the voltage of the
upstream sensor. The only difference is a time lag (seen by the PCM) between the switching of the O2S’s.
To monitor the system, the number of lean-to-rich switches of upstream and downstream O2S’s is counted. The
ratio of downstream switches to upstream switches is used to determine whether the catalyst is operating properly.
An effective catalyst will have fewer downstream switches than it has upstream switches i.e., a ratio closer to zero.
For a totally ineffective catalyst, this ratio will be one-to-one, indicating that no oxidation occurs in the device.
The system must be monitored so that when catalyst efficiency deteriorates and exhaust emissions increase to over
the legal limit, the MIL (check engine lamp) will be illuminated.
Monitor Operation—To monitor catalyst efficiency, the PCM expands the rich and lean switch points of the heated
oxygen sensor. With extended switch points, the air/fuel mixture runs richer and leaner to overburden the catalytic
converter. Once the test is started, the air/fuel mixture runs rich and lean and the O2 switches are counted. A switch
is counted when an oxygen sensor signal goes from below the lean threshold to above the rich threshold. The
number of Rear O2 sensor switches is divided by the number of Front O2 sensor switches to determine the switch-
ing ratio.
The test runs for 20 seconds. As catalyst efficiency deteriorated over the life of the vehicle, the switch rate at the
downstream sensor approaches that of the upstream sensor. If at any point during the test period the switch ratio
reaches a predetermined value, a counter is incremented by one. The monitor is enabled to run another test during
that trip. When the test fails three times, the counter increments to three, a malfunction is entered, and a Freeze
Frame is stored. When the counter increments to three during the next trip, the code is matured and the MIL is
illuminated. If the test passes the first, no further testing is conducted during that trip.
The MIL is extinguished after three consecutive good trips. The good trip criteria for the catalyst monitor is more
stringent than the failure criteria. In order to pass the test and increment one good trip, the downstream sensor
switch rate must be less than 80% of the upstream rate (60% for manual transmissions). The failure percentages
are 90% and 70% respectively.
PM
EMISSIONS CONTROL
25 - 5