P0303

Diagnostic Chart:

Wiring Diagram:

Circuit Description

The Powertrain Control Module (PCM) uses the Crankshaft Position (CKP) sensor and Camshaft Position (CMP) sensor to monitor changes in crankshaft rotation for each cylinder to determine a misfire. When the misfire rate equals or exceeds the count indicating that the engine condition has deteriorated, the Malfunction Indicator Lamp (MIL) illuminates. Certain driving conditions can cause the catalytic converter to overheat if the misfire rate is high enough. The MIL will blink when conditions for catalytic converter overheating are present.

Conditions for Running the DTC

^ Engine coolant temperature greater than -10°C (14°F).

^ Intake air temperature is between -10°C (14°F) and 50°C (122°F).

^ Barometric pressure greater than 75 kPa.

^ Engine speed less than 4500 RPM.

^ Fuel tank level greater than 25%.

Conditions for Setting the DTC

^ RPM change is less than 200/50 msec.

^ Manifold Absolute Pressure (MAP) change is less than 1.3 kPa/16 firing events.

^ Conditions present for 5 seconds after engine start to 1 second from fuel shut-off.

Action Taken When the DTC Sets

^ The PCM will blink the Malfunction Indicator Lamp (MIL) the first time catalytic converter damage has occurred.

^ The PCM will illuminate the MIL the second time emission thresholds are exceeded.

^ The PCM will store vehicle operating conditions which were present when the DTC set in Freeze Frame data.

^ The PCM will enter the Fail-Safe Function and stop EGR control.

Conditions for Clearing the MIL/DTC

^ The MIL will turn OFF after three consecutively passing cycles without a fault present.

^ A History DTC will clear after 40 consecutive warm-up cycles without a fault.

^ DTCs can be cleared by using a scan tool or by disconnecting the PCM battery feed.

Diagnostic Aids

Check for any of the following conditions:

^ If any DTCs other than Misfire are present. P0300 to P0303, diagnose those DTCs first.

^ Engine overheating.

^ Vacuum leaks.

^ Improper EGR system operation. Refer to EGR System for EGR system checks.

^ A functional check of the PCV valve. Refer to Positive Crankcase Ventilation (PCV) system.

^ An ignition system malfunction (spark plugs, Ignition wires, Ignition coil, and the distributor).

^ Fuel pressure that is out of specification or a malfunctioning fuel injector.

^ An engine mechanical malfunction (engine compression out of specification, valve lash adjuster malfunction).

^ A poor connection at the PCM. Inspect the harness connectors for a backed out terminal, improper mating, broken locks, improperly formed or damaged terminals, and poor terminal to wire connection.

^ A damaged wiring harness. Inspect the wiring harness for damage and repair as necessary.

^ An intermittent misfire can also be the result of a defective crankshaft timing belt pulley. Remove the crankshaft sensor and look at the crankshaft signal rotor through the sensor hole. Check the crankshaft timing belt pulley for foreign material or damaged teeth.

^ If the DTC P0303 is intermittent, Refer to Symptoms. Testing and Inspection

Due to the potentially destructive nature that engine misfire has on the exhaust catalyst always check for proper catalytic converter operation after a misfire has been repaired. This can be accomplished by driving the vehicle at a steady state between 55 to 65 km/h (35 to 45 mph) for 5 minutes. Connect the scan tool and observe the System Status Catalyst is completed to a Y (yes) state, and no DTC set.

If a DTC P0303 cannot be duplicated, the information included in the Freeze Frame data can be useful in determining vehicle operating conditions when the DTC was first set. Also, if the fault can not be duplicated, wet the secondary ignition system and operate the vehicle under the conditions that the DTC set.

If any wiring repairs need to be made, Refer to Repair Procedures in Electrical Diagrams.

Test Description

The numbers below refer to the step numbers in the Diagnostic Table.

1. The Powertrain OBD System Check prompts the technician to complete some basic checks and store the freeze frame data on the scan tool if applicable. This creates an electronic copy of the data taken when the fault occurred. The information is then stored in the scan tool for later reference.

3. Determine if fault is present. A misfire DTC can be caused by a rough road condition.

8. Additional ignition system components (noise suppressor filter and Ignition Control Module [igniter]) may cause a DTC P0303 to set. Refer to Ignition System.

11. Low engine compression and improper lash may also cause a rough idling condition. Refer to Engine Mechanical for additional checks.