DTC P0303 Cylinder 3 Misfire Detected. Geo Prizm 1997

For 1990-2009 cars only

Makes 🢒 Geo 🢒 1997 🢒 Prizm 🢒 DTC 🢒 DTC P0303 Cylinder 3 Misfire Detected

Schematic Reference: Engine Controls Schematics

Circuit Description

The Powertrain Control Module (PCM) uses the crankshaft position sensor and camshaft position 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 MIL will light. If the misfire rate is high enough and the driving conditions will cause catalyst overheating, the MIL blinks when misfiring occurs.

Conditions for Setting the DTC

•	Engine speed is between 200 and 1000 RPM

•	Battery voltage is between 9 volts and 17 volts.

•	Engine Coolant Temperature (ECT) is between -7° C (20° F) and 123°C (254°F).

Action Taken When the DTC Sets

Malfunction Indicator Lamp (MIL) illuminates after two consecutive driving cycles with the fault active, which causes a deterioration in emission. The MIL will begin flashing if the misfire is severe enough to cause catalyst damage.

Conditions for Clearing the MIL/DTC

•	The PCM turns OFF the MIL on the third consecutive trip cycle during which the diagnostic has been run and the fault condition is no longer present.

•	A DTC will clear after 40 consecutive warm-up cycles have occurred without a fault.

•	A DTC can be cleared by using the scan tool Clear Information function or by disconnecting the PCM battery feed.

Diagnostic Aids

•

An intermittent malfunction may be caused by a poor connection, rubbed through wire insulation, or a wire broken inside the insulation. Inspect harness connectors for backed out terminals, improper mating, broken locks, improperly formed or damaged terminals and poor terminal-to-wire connections before component replacement.

•	Review "Misfire RPM" and "Misfire Load" located in the scan tool "Engine Data" list. This information can help determine the vehicle conditions when the misfire occurred.

•	The information included in the Freeze Frame data can be useful in determining the vehicle operating conditions when the DTC first set.

•	If DTC P0303 is intermittent, refer to Symptoms.

•	An intermittent can also be the result of a defective reluctor wheel. Remove crankshaft sensor and inspect reluctor wheel through sensor hole. Check for the condition of the wheel for porosity, nicks, dents etc. and repair as necessary.

•	Insure the power and ground to the PCM have a good, clean connections.

•	Check that EGR valve is operating properly, insure that the EGR is closing properly and not hanging open causing excessive EGR flow.

•	Check MAP sensor and ECT sensor typical scan for proper operation.

Test Description

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

The 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.

Verifies whether the misfire is present. Misfire can sometimes be cause by a rough road condition.

A vacuum leak can cause a lean misfire condition.

This step is to verify if the misfire is cause by the ignition system.

Contaminants in the fuel, such as alcohol or water, can create a misfire condition.

Step

Action

Value(s)

Yes

Was the "Powertrain On Board Diagnostic (OBD) System Check performed?

Go to Step 2

Go to Powertrain Control Module Connector End Views

Clear the DTC.
Using the scan tool, place the vehicle into Check Mode.
Operate the vehicle within the "Freeze Frame" conditions as noted.

Does DTC P0303 set?

Go to Step 3

Go to Diagnostic Aids

Visually and physically inspect the following for vacuum leaks:

•	The EGR valve

•	The PCV valve

•	The Intake manifold gasket

•	The Vacuum hoses for splits, kinks, and improper routing

If a problem is found, repair vacuum leaks as necessary

Did your inspection reveal a vacuum leak?

Go to Step 23

Go to Step 4

Install a spark tester at the end of the #3 ignition wire, one at a time.
Crank the engine while observing the spark tester
A crisp, blue spark should be observed.

Is adequate spark present?

Go to Step 9

Go to Step 5

Remove and visually and physically inspect the #3 ignition wire for any of the following problems.

•	Carbon tracking

•	Insulation damage

Make any repairs that are necessary.

Was a repair necessary?

Go to Step 23

Go to Step 6

Using a DMM, measure the resistance of the #3 ignition wire.
Replace the #3 ignition wire that measured greater than the specified value.

Did the #3 ignition wire need replacement?

25 000 ohms (25 K ohms)

Go to Step 23

Go to Step 7

Perform a visual/physical inspection on the distributor cap and ignition rotor, look for the following problem during inspection.

•

Cracks

•

Moisture

•	Poor terminal connection

•	Carbon tracking

Repair as necessary.

Was a repair necessary?

Go to Step 23

Go to Step 8

Perform a visually/physical inspection of the signal rotor in the distributor.

•	signal rotor air gap

•	damage signal rotor

Repair as necessary, refer to Ignition System.

Was a repair necessary?

Go to Step 23

Go to Step 22

Remove and visually/physically inspect the #3 spark plug, during inspection check for the following:

•	The terminal screw - bent, loose (rotate and pull)

•	The ceramic Insulator - cracks, tracking, deposits (red or black dust)

•	The tip insulator - cracks, tracking, deposits (oil, carbon, glazing, fuel additive), loose (rattles)

•	The center electrode - loose (wiggles side to side).

Replace as necessary.

Did the #3 spark plug need replacement?

Go to Step 23

Go to Step 10

Turn the ignition "OFF."
Install a fuel pressure gauge.
Note fuel pressure with the engine running.

Is the fuel pressure within the specified value?

206-255 kPa (31-37 psi)

Go to Step 11

Go to Fuel System - Circuit Opening Relay Diagnosis

Inspect for contaminated fuel. Refer to Engine Fuel.

Is the fuel OK? (uncontaminated)

Go to Step 13

Go to Step 12

Replace contaminated fuel.

Is action complete?

Go to Step 23

Turn the engine "OFF."
Disconnect the #3 injector connector from the injector.
Install an injector test light on the injector harness connector.
Crank engine and note the test light.

Does the injector test light blink at the #3 injector harness connector?

Go to Step 14

Go to Step 15

Perform the Injector Coil Test/Balance Test Procedure, refer to Fuel Injector Coil Test/Fuel Injector Balance Test Procedure.

Are injectors OK?

Go to Step 22

Go to Step 17

Disconnect the injector test light.
With a test light connected to ground, probe the power feed terminal of the #3 injector harness connector that did not blink.
Crank the engine.

Does the test light illuminate?

Go to Step 16

Go to Step 20

With a test light connected to battery voltage, probe the injector driver side of the injector #3 harness connector.
Crank the engine.

Does the test light flash?

Go to Step 22

Go to Step 18

Replace any faulty injectors.

Is action complete?

Go to Step 23

Check the #3 injector driver circuit for any of the following problems:

•	An open circuit

•	A short to ground

•	A short to voltage

Was a problem found?

Go to Step 19

Go to Step 21

Repair the open or shorted injector driver circuit.

Is the action complete?

Go to Step 23

Repair the open or short in the power feed wire to the #3 fuel injector.

Is the action complete?

Go to Step 23

Replace the PCM.

Is the action complete?

Go to Step 23

Check for an engine mechanical problem. Refer to Engine Mechanical Diagnosis to diagnose the following conditions:

•	A faulty or incorrect camshaft

•	A leaky or sticky valves or rings

•	An excessive valve deposits

•	A weak valve springs

•	An incorrect valve timing

•	A leaking head gasket

•	An EGR operation

If a problem is found, repair as necessary.

Was a basic engine mechanical problem found and repaired?

Go to Step 23

Go to Diagnostic Aids

After repairs, use the scan tool clear information function and road test vehicle within "Freeze Frame" conditions that set the DTC. Check for DTCs. Review scan tool data and if no DTCs are stored repair is complete.

Are any DTCs displayed on scan tool?

Go to the applicable DTC table

System OK