Dodge Ram Truck 1500-2500-3500. Manual — part 2083

MISFIRE MONITOR

Excessive engine misfire results in increased catalyst temperature and causes an increase in HC emissions. Severe
misfires could cause catalyst damage. To prevent catalytic convertor damage, the PCM monitors engine misfire.

The Powertrain Control Module (PCM) monitors for misfire during most engine operating conditions (positive torque)
by looking at changes in the crankshaft speed. If a misfire occurs the speed of the crankshaft will vary more than
normal.

FUEL SYSTEM 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. The catalyst works best when the Air Fuel (A/F)
ratio is at or near the optimum of 14.7 to 1.

The PCM is programmed to maintain the optimum air/fuel ratio of 14.7 to 1. This is done by making short term
corrections in the fuel injector pulse width based on the O2S sensor output. The programmed memory acts as a self
calibration tool that the engine controller uses to compensate for variations in engine specifications, sensor toler-
ances and engine fatigue over the life span of the engine. By monitoring the actual fuel-air ratio with the O2S sen-
sor (short term) and multiplying that with the program long-term (adaptive) memory and comparing that to the limit,
it can be determined whether it will pass an emissions test. If a malfunction occurs such that the PCM cannot main-
tain the optimum A/F ratio, then the MIL will be illuminated.

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. This can increase vehicle emissions and
deteriorate engine performance, driveability and fuel economy.

The catalyst monitor uses dual oxygen sensors (O2S’s) to monitor the efficiency of the converter. The dual O2S’s
sensor 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 lean condition, there is an abundance of oxygen in the exhaust gas. A function-
ing 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 will be illuminated.

DESCRIPTION - TRIP DEFINITION

The term “Trip” has different meanings depending on what the circumstances are. If the MIL (Malfunction Indicator
Lamp) is OFF, a Trip is defined as when the Oxygen Sensor Monitor and the Catalyst Monitor have been completed
in the same drive cycle.

When any Emission DTC is set, the MIL on the dash is turned ON. When the MIL is ON, it takes 3 good trips to turn
the MIL OFF. In this case, it depends on what type of DTC is set to know what a “Trip” is.

For the Fuel Monitor or Mis-Fire Monitor (continuous monitor), the vehicle must be operated in the “Similar Condition
Window” for a specified amount of time to be considered a Good Trip.

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If a Non-Contiuous OBDII Monitor fails twice in a row and turns ON the MIL, re-running that monitor which previ-
ously failed, on the next start-up and passing the monitor, is considered to be a Good Trip. These will include the
following:

•

Oxygen Sensor

•

Catalyst Monitor

•

Purge Flow Monitor

•

Leak Detection Pump Monitor (if equipped)

•

EGR Monitor (if equipped)

•

Oxygen Sensor Heater Monitor

If any other Emission DTC is set (not an OBDII Monitor), a Good Trip is considered to be when the Oxygen Sensor
Monitor and Catalyst Monitor have been completed; or 2 Minutes of engine run time if the Oxygen Sensor Monitor
or Catalyst Monitor have been stopped from running.

It can take up to 2 Failures in a row to turn on the MIL. After the MIL is ON, it takes 3 Good Trips to turn the MIL
OFF. After the MIL is OFF, the PCM will self-erase the DTC after 40 Warm-up cycles. A Warm-up cycle is counted
when the ECT (Engine Coolant Temperature Sensor) has crossed 160°F and has risen by at least 40°F since the
engine has been started.

DESCRIPTION - COMPONENT MONITORS

There are several components that will affect vehicle emissions if they malfunction. If one of these components
malfunctions the Malfunction Indicator Lamp (MIL) will illuminate.

Some of the component monitors are checking for proper operation of the part. Electrically operated components
now have input (rationality) and output (functionality) checks. Previously, a component like the Throttle Position sen-
sor (TPS) was checked by the PCM for an open or shorted circuit. If one of these conditions occurred, a DTC was
set. Now there is a check to ensure that the component is working. This is done by watching for a TPS indication
of a greater or lesser throttle opening than MAP and engine rpm indicate. In the case of the TPS, if engine vacuum
is high and engine rpm is 1600 or greater, and the TPS indicates a large throttle opening, a DTC will be set. The
same applies to low vacuum if the TPS indicates a small throttle opening.

All open/short circuit checks, or any component that has an associated limp-in, will set a fault after 1 trip with the
malfunction present. Components without an associated limp-in will take two trips to illuminate the MIL.

OPERATION

OPERATION

The Powertrain Control Module (PCM) monitors many
different circuits in the fuel injection, ignition, emission
and engine systems. If the PCM senses a problem
with a monitored circuit often enough to indicate an
actual problem, it stores a Diagnostic Trouble Code
(DTC) in the PCM’s memory. If the problem is repaired
or ceases to exist, the PCM cancels the code after 40
warm-up cycles. Diagnostic trouble codes that affect
vehicle emissions illuminate the Malfunction Indicator
Lamp (MIL). The MIL is displayed as an engine icon
(graphic) on the instrument panel. Refer to Malfunction
Indicator Lamp in this section.

Certain criteria must be met before the PCM stores a
DTC in memory. The criteria may be a specific range
of engine RPM, engine temperature, and/or input volt-
age to the PCM.

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The PCM might not store a DTC for a monitored circuit even though a malfunction has occurred. This may happen
because one of the DTC criteria for the circuit has not been met. For example, assume the diagnostic trouble code
criteria requires the PCM to monitor the circuit only when the engine operates between 750 and 2000 RPM. Sup-
pose the sensor’s output circuit shorts to ground when engine operates above 2400 RPM (resulting in 0 volt input
to the PCM). Because the condition happens at an engine speed above the maximum threshold (2000 rpm), the
PCM will not store a DTC.

There are several operating conditions for which the PCM monitors and sets DTC’s. Refer to Monitored Systems,
Components, and Non-Monitored Circuits in this section.

Technicians must retrieve stored DTC’s by connecting the DRB scan tool (or an equivalent scan tool) to the 16–way
data link connector. The connector is located on the bottom edge of the instrument panel near the steering column.

NOTE: Various diagnostic procedures may actually cause a diagnostic monitor to set a DTC. For instance,
pulling a spark plug wire to perform a spark test may set the misfire code. When a repair is completed and
verified, connect the DRB scan tool to the 16–way data link connector (1) to erase all DTC’s and extinguish
the MIL.

OPERATION - TASK MANAGER

The Task Manager determines which tests happen when and which functions occur when. Many of the diagnostic
steps required by OBD II must be performed under specific operating conditions. The Task Manager software orga-
nizes and prioritizes the diagnostic procedures. The job of the Task Manager is to determine if conditions are appro-
priate for tests to be run, monitor the parameters for a trip for each test, and record the results of the test. Following
are the responsibilities of the Task Manager software:

•

Test Sequence

•

MIL Illumination

•

Diagnostic Trouble Codes (DTCs)

•

Trip Indicator

•

Freeze Frame Data Storage

•

Similar Conditions Window

Test Sequence

In many instances, emissions systems must fail diagnostic tests more than once before the PCM illuminates the
MIL. These tests are know as ’two trip monitors.’ Other tests that turn the MIL lamp on after a single failure are
known as ’one trip monitors.’ A trip is defined as ’start the vehicle and operate it to meet the criteria necessary to
run the given monitor.’

Many of the diagnostic tests must be performed under certain operating conditions. However, there are times when
tests cannot be run because another test is in progress (conflict), another test has failed (pending) or the Task
Manager has set a fault that may cause a failure of the test (suspend).

•

Pending
Under some situations the Task Manager will not run a monitor if the MIL is illuminated and a fault is stored
from another monitor. In these situations, the Task Manager postpones monitors pending resolution of the
original fault. The Task Manager does not run the test until the problem is remedied.
For example, when the MIL is illuminated for an Oxygen Sensor fault, the Task Manager does not run the
Catalyst Monitor until the Oxygen Sensor fault is remedied. Since the Catalyst Monitor is based on signals
from the Oxygen Sensor, running the test would produce inaccurate results.

•

Conflict
There are situations when the Task Manager does not run a test if another monitor is in progress. In these
situations, the effects of another monitor running could result in an erroneous failure. If this conflict is present,
the monitor is not run until the conflicting condition passes. Most likely the monitor will run later after the con-
flicting monitor has passed.
For example, if the Fuel System Monitor is in progress, the Task Manager does not run the EGR Monitor.
Since both tests monitor changes in air/fuel ratio and adaptive fuel compensation, the monitors will conflict with
each other.

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Suspend
Occasionally the Task Manager may not allow a two trip fault to mature. The Task Manager will suspend the
maturing of a fault if a condition exists that may induce an erroneous failure. This prevents illuminating the MIL
for the wrong fault and allows more precis diagnosis.
For example, if the PCM is storing a one trip fault for the Oxygen Sensor and the EGR monitor, the Task
Manager may still run the EGR Monitor but will suspend the results until the Oxygen Sensor Monitor either
passes or fails. At that point the Task Manager can determine if the EGR system is actually failing or if an
Oxygen Sensor is failing.

MIL Illumination

The PCM Task Manager carries out the illumination of the MIL. The Task Manager triggers MIL illumination upon
test failure, depending on monitor failure criteria.

The Task Manager Screen shows both a Requested MIL state and an Actual MIL state. When the MIL is illuminated
upon completion of a test for a third trip, the Requested MIL state changes to OFF. However, the MIL remains
illuminated until the next key cycle. (On some vehicles, the MIL will actually turn OFF during the third key cycle)
During the key cycle for the third good trip, the Requested MIL state is OFF, while the Actual MILL state is ON. After
the next key cycle, the MIL is not illuminated and both MIL states read OFF.

Diagnostic Trouble Codes (DTCs)

With OBD II, different DTC faults have different priorities according to regulations. As a result, the priorities deter-
mine MIL illumination and DTC erasure. DTCs are entered according to individual priority. DTCs with a higher pri-
ority overwrite lower priority DTCs.

Priorities

•

Priority 0 —Non-emissions related trouble codes

•

Priority 1 — One trip failure of a two trip fault for non-fuel system and non-misfire.

•

Priority 2 — One trip failure of a two trip fault for fuel system (rich/lean) or misfire.

•

Priority 3 — Two trip failure for a non-fuel system and non-misfire or matured one trip comprehensive com-
ponent fault.

•

Priority 4 — Two trip failure or matured fault for fuel system (rich/lean) and misfire or one trip catalyst dam-
aging misfire.

Non-emissions related failures have no priority. One trip failures of two trip faults have low priority. Two trip failures
or matured faults have higher priority. One and two trip failures of fuel system and misfire monitor take precedence
over non-fuel system and non-misfire failures.

DTC Self Erasure

With one trip components or systems, the MIL is illuminated upon test failure and DTCs are stored.

Two trip monitors are components requiring failure in two consecutive trips for MIL illumination. Upon failure of the
first test, the Task Manager enters a maturing code. If the component fails the test for a second time the code
matures and a DTC is set.

After three good trips the MIL is extinguished and the Task Manager automatically switches the trip counter to a
warm-up cycle counter. DTCs are automatically erased following 40 warm-up cycles if the component does not fail
again.

For misfire and fuel system monitors, the component must pass the test under a Similar Conditions Window in order
to record a good trip. A Similar Conditions Window is when engine RPM is within ±375 RPM and load is within
±10% of when the fault occurred.

NOTE: It is important to understand that a component does not have to fail under a similar window of oper-
ation to mature. It must pass the test under a Similar Conditions Window when it failed to record a Good
Trip for DTC erasure for misfire and fuel system monitors.

DTCs can be erased anytime with a DRB III. Erasing the DTC with the DRB III erases all OBD II information. The
DRB III automatically displays a warning that erasing the DTC will also erase all OBD II monitor data. This includes
all counter information for warm-up cycles, trips and Freeze Frame.

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