Primary System - Based Diagnostics
Primary System-Based DiagnosticsThere are primary system-based diagnostics which evaluate system operation and its effect on vehicle emissions. The primary system-based diagnostics are listed below with a brief description of the diagnostic function:
Heated Oxygen Sensor Diagnosis
The fuel control Heated Oxygen Sensor (HO2S) is diagnosed for the following conditions:
^ Slow response
^ Response time (time to switch R/L or L/R)
^ Inactive signal (output steady at bias voltage approx. 450 mV)
^ Signal fixed high
^ Signal fixed low
The catalyst monitor Heated Oxygen Sensor (HO2S2. is diagnosed for the following conditions:
^ Heater performance (time to activity on cold start)
^ Signal fixed low during steady state conditions or power enrichment (hard acceleration when a rich mixture should be indicated)
^ Signal fixed high during steady state conditions or deceleration mode (deceleration when a lean mixture should be indicated)
^ Inactive sensor (output steady at approximately 438 mV)
If the oxygen sensor pigtail wiring, connector or terminal are damaged, the entire heated oxygen sensor assembly must be replaced. Do not attempt to repair the wiring, connector or terminals. In order for the sensor to function properly, it must have clean reference air provided to it. This clean air reference is obtained by way of the heated oxygen sensor wire(s). Any attempt to repair the wires, connector or terminals could result in the obstruction of the reference air and degrade heated oxygen sensor performance.
Misfire Monitor Diagnostic Operation
The misfire monitor diagnostic is based on crankshaft rotational velocity (reference period) variations. The Engine Control Module (ECM) determines crankshaft rotational velocity using the Crankshaft Position (CKP) sensor and the Camshaft Position (CMP) sensor. When a cylinder misfires, the crankshaft slows down momentarily. By monitoring the CKP and CMP sensor signals, the ECM can calculate when a misfire occurs.
For a non-catalyst damaging misfire, the diagnostic will be required to monitor a misfire present for between 1,000 - 3,200 engine revolutions. For catalyst-damaging misfire, the diagnostic will respond to misfire within 200 engine revolutions.
Rough roads may cause false misfire detection. A rough road will cause torque to be applied to the drive wheels and drive train. This torque can intermittently decrease the crankshaft rotational velocity. This may be falsely detected as a misfire.
A rough road sensor or G sensor, works together with the misfire detection system. The G sensor produces a voltage that varies along with the intensity of road vibrations. When the ECM detects a rough road, the misfire detection system is temporarily disabled.
Misfire Counters
Whenever a cylinder misfires, the misfire diagnostic counts the misfire and notes the crankshaft position at the time the misfire occurred. These "misfire counters" are basically a file on each engine cylinder. A current and a history misfire counter are maintained for each cylinder. The misfire current counters (Misfire Cur #1 - 4) indicate the number of firing events out of the last 200 cylinder firing events which were misfires. The misfire current counter will display real time data without a misfire Diagnostic Trouble Code (DTC) stored. The misfire history counters (Misfire Hist #1 - 4)indicate the total number of cylinder firing events which were misfires. The misfire history counters will display 0 until the misfire diagnostic has failed and a DTC P0300 is set. Once the misfire DTC P0300 is set, the misfire history counters will be updated every 200 cylinder firing events. A misfire counter is maintained for each cylinder.
If the misfire diagnostic reports a failure, the diagnostic executive reviews all of the misfire counters before reporting a DTC. This way, the diagnostic executive reports the most current information.
When crankshaft rotation is erratic, a misfire condition will be detected. Because of this erratic condition, the data that is collected by the diagnostic can sometimes incorrectly identify which cylinder is misfiring.
Use diagnostic equipment to monitor misfire counter data on On-Board Diagnostic (OBD II) compliant vehicles. Knowing which specific cylinder(s) misfired can lead to the root cause, even when dealing with a multiple cylinder misfire. Using the information in the misfire counters, identify which cylinders are misfiring. If the counters indicate cylinders numbers 1 and 4 misfired, look for a circuit or component common to both cylinders number 1 and 4.
The misfire diagnostic may indicate a fault due to a temporary fault not necessarily caused by a vehicle emission system malfunction. Examples include the following items:
^ Contaminated fuel
^ Low fuel
^ Fuel-fouled spark plugs
^ Basic engine fault
Fuel Trim System Monitor Diagnostic Operation
This system monitors the averages of short-term and long-term fuel trim values. If these fuel trim values stay at their limits for a calibrated period of time, a malfunction is indicated. The fuel trim diagnostic compares the averages of short-term fuel trim values and long-term fuel trim values to rich and lean thresholds. If either value is within the thresholds, a pass is recorded. If both values are outside their thresholds, a rich or lean DTC will be recorded.
The fuel trim system diagnostic also conducts an intrusive test. This test determines if a rich condition is being caused by excessive fuel vapor from the Evaporative (EVAP) emission canister. In order to meet OBD II requirements, the control module uses weighted fuel trim cells to determine the need to set a fuel trim DTC. A fuel trim DTC can only be set if fuel trim counts in the weighted fuel trim cells exceed specifications. This means that the vehicle could have a fuel trim problem which is causing a problem under certain conditions (i.e., engine idle high due to a small vacuum leak or rough idle due to a large vacuum leak) while it operates fine at other times.No fuel trim DTC would set (although an engine idle speed DTC or HO2S2 DTC may set). Use a scan tool to observe fuel trim counts while the problem is occurring.A fuel trim DTC may be triggered by a number of vehicle faults. Make use of all information available (other DTCs stored, rich or lean condition, etc.) when diagnosing a fuel trim fault.
Fuel Trim Cell Diagnostic Weights
No fuel trim DTC will set regardless of the fuel trim counts in cell 0 unless the fuel trim counts in the weighted cells are also outside specifications. This means that the vehicle could have a fuel trim problem which is causing a problem under certain conditions (i.e. engine idle high due to a small vacuum leak or rough due to a large vacuum leak) while it operates fine at other times. No fuel trim DTC would set (although an engine idle speed DTC or HO2S2 DTC may set). Use a scan tool to observe fuel trim counts while the problem is occurring.