PT Cruiser, 2001 - 2005

OBD II System Monitors


Catalyst Monitor

OBD II regulations require that the functionality of the Catalyst system be monitored. If the catalyst system deteriorates to a point where vehicle emissions increase by more than 1.5 times the Federal Test Procedure (FTP) standard, the MIL must be activated.

The oxygen content in a catalyst is important for efficient conversion of exhaust gases. When a lean air-fuel (A/F) ratio is present for an extended period of time, the oxygen content in the catalyst can reach a maximum. Conversely, when a rich A/F ratio is present for too long a period, the oxygen content in the catalyst can become totally depleted.

Catalyst operation is dependent on its ability to store and release oxygen needed to complete the emissions-reducing chemical reactions. As a catalyst deteriorates, its ability to store oxygen is reduced. Since the catalyst's ability to store oxygen is somewhat related to proper operation, oxygen storage can be used as an indicator of catalyst performance.

Comprehensive Component Monitors

OBD II regulations require that all emission related circuits and components controlled by the PCM that could affect emissions are monitored for circuit continuity and out-of-range faults. The Comprehensive Component Monitor (CCM) consists of 4 different monitoring strategies: 2 for inputs and 2 for output signals.


Input Strategies

One input strategy is used to check devices with analog inputs for opens, shorts, or out-of-range values. The CCM accomplishes this task by monitoring A/D converter input voltages of various Sensors.

A second input strategy is used to check devices with digital and frequency inputs by performing rationality checks. The PCM uses other Sensor readings and calculations to determine if a Sensor or switch reading is correct under existing conditions. Some tests of the CKP, CMP and VSS run continuously. Other tests run only after actuation.

Output Strategies

An Output Device Monitor in the PCM checks for any open and shorted circuits by observing the control voltage level of a particular device.

The control voltage is low when the device is on and the voltage is high when the device is off. Monitored outputs include the AC relay, ASD relay SS1, SS2, SS3, EGR and EVAP and O2 heater control.

Crankshaft Relearn

Select Crank Relearn on the scan tool to allow the PCM to enable a fast CMP and CKP Sensor relearn procedure. The PCM relearns the crank target window spacing during Decel Fuel Shutoff mode The PCM reset step requires 3 closed throttle decelerations from 55 mph in order to relearn the Crank Target Spacing.

EGR System Monitor

Emissions of NOx increase proportionally with the temperature in the combustion chamber. The Exhaust Gas Recirculation (EGR) system is designed to circulate non-combustible exhaust gases into the manifold to dilute the A/F mixture. In this manner, the EGR system lowers the combustion temperature and reduces NOx emissions and pre-detonation (knocking).

EVAP System Monitor

OBD II regulations require that all vehicles equipped with an EVAP system be monitored for component integrity, system functionality and loss of hydrocarbons. The Enhanced EVAP system test is required to detect leaks as small as 0.040" in diameter. The test must detect leaks of 0.040-0.080" in diameter on fuel tanks larger than 25 gallons.

Front And Rear Heated Oxygen Sensors

There are 2 Oxygen (O2) Sensors used to monitor the catalyst storage capability. One Sensor is mounted in front of the catalyst (pre-catalyst O2) and another Sensor is mounted in back of the catalyst (post-catalyst O2). By utilizing an O2 Sensor in front of the catalytic converter, and a second Sensor located behind the catalyst, the oxygen storage capability of the catalyst can be determined by comparing the voltage signals from the 2 Sensors.

Fuel System Monitor

To comply with clean air regulations, DaimlerChrysler vehicles are equipped with catalytic converters that reduce the amount of HC, CO and NOx emissions released from the vehicle. The catalyst works best when the A/F ratio is at or near an optimum value of 14.7:1. The PCM is programmed to maintain this optimum A/F ratio.

This is accomplished by making Short Term fuel trim corrections in injector pulse width based upon the O2 Sensor input. The programmed memory in the PCM acts as a self-calibration tool to compensate for variations in engine specifications, sensor tolerances and engine fatigue over the life span of the engine.As injector pulse width increases, the amount of fuel delivered by the fuel injector is increased. A shorter pulse width decreases the amount of fuel delivered. The PCM monitors engine load, RPM, TP Sensor, ECT Sensor signals in order to make changes to injector pulse width.

Misfire Monitor

OBD II regulations require that the OBD II system monitor the engine for misfire conditions and identify specific cylinders that experience a misfire. The PCM should also identify and set a different code to indicate if a multiple misfire condition exists. It should also be able to identify a specific cylinder that misfires in a multiple misfire condition. This monitor detects misfires related to engine mechanical, ignition or fuel system faults under positive load conditions (idle, cruise, etc.).

Revolution Counter

The PCM contains a program called a Revolution Counter. In the logic portion of this counter, each 1000-revolution window contains five 200-revolution windows. The PCM counts the misfires for each 200-window and carries that value over to the 1000-revolution window.

Secondary Air Monitor

This monitor is a PCM diagnostic that monitors the Secondary AIR system for component integrity, system functionality and faults that could cause vehicle tailpipe output levels to exceed 1.5 times the FTP Standard. The Secondary AIR System Monitor is run once per trip once the required enable criteria are met.