What this code means
P0303 means the PCM may be detecting a misfire contribution problem on cylinder 3.
What the vehicle may do
- The vehicle may idle rough or shake.
- The vehicle may hesitate or feel weak under load.
- The MIL may flash during a severe misfire.
- The misfire may be most noticeable under certain operating conditions.
Possible fault areas
- Possible ignition faults such as a coil, spark plug, or coil control circuit issue.
- Possible fuel delivery faults such as injector flow, leakage, fuel quality, or fuel supply concerns.
- Possible air handling issues such as intake vacuum leaks or restrictions.
- Possible mechanical issues such as compression, valvetrain, combustion chamber, carbon, oiling, or VCT concerns.
- Possible wiring, connector, power, ground, or PCM control concerns.
Diagnostic path
Open and frame the fault
On this Ram 1500 Classic with the 5.7 HEMI, P0303 means the PCM may be seeing a misfire on cylinder 3. The truck may run rough, shake at idle, lose power under load, or flash the MIL if the misfire is severe. The possible fault area can be ignition, fuel delivery, an intake leak, fuel quality, compression or valvetrain trouble, combustion chamber carbon, VCT or oiling issues, wiring, connectors, or in the last part of the path, PCM control. Start with the basic system checks, then work the misfire path in order. The PCM is looking at crankshaft speed variation between cylinders, and it sets the fault when that variation exceeds a calibrated value for the current engine speed and load. Treat the run conditions as monitor gates: make sure the misfire monitor was allowed to run before you trust the test result, and remember it is disabled during decel fuel shutoff.
Confirm the active cylinder 3 misfire
First, turn the ignition on and use the scan tool to record the stored codes and the captured operating conditions. Drive the truck under those same conditions and note exactly when the misfire shows up, because that helps later when you separate an idle miss from a load-related miss. Watch the downstream O2 sensor behavior while the miss is happening. Consistently low voltage points you toward a lean condition or lack of fuel, while consistently high voltage points toward too much fuel reaching the catalyst, which can happen with no spark or a leaking injector. Use the misfire monitor to confirm whether cylinder 3 is the cylinder with excessive counts right now. Also keep in mind that during a severe misfire with a flashing MIL, the PCM can disable the injector for the affected cylinder for a calibrated number of revolutions. That can make a high-speed or heavy-load miss look like it is still there when the engine comes back to idle.
Check for codes that change the path, then inspect the engine
Next, look at the other recorded codes. If there are active or pending faults in the fuel system, ignition system, air handling system, VVT system, or VVL system, handle the applicable diagnostic path for those first. If that does not explain the cylinder 3 miss, move into a mechanical and visual inspection. Look for belt wear, pulley misalignment or binding, poor sensor mounting at the crank, cam, or MAP sensor, poor connector and terminal contact at key engine components, PCM power or ground corrosion, vacuum leaks, intake or exhaust restrictions, signs of internal engine trouble, moisture on ignition parts, fuel supply or fuel quality issues, and load-related operating problems like bogging or towing overload. If a problem is found here, repair it and verify the repair. If the no-fault return point in the path is not clear in front of you, pause, recheck the earlier diagnostic path, and avoid guessing.
Rule out intake leaks and bad fuel
Now check the engine and intake manifold for vacuum leaks. A leak can affect one cylinder or several cylinders depending on where it is located, so do not assume cylinder 3 means the leak has to be directly at that runner. If an intake leak is found, repair the leak and verify the repair. If the intake checks clean, check the fuel itself: water in the fuel, the wrong fuel type, old or contaminated fuel, or low fuel can all put you on the wrong track. Check the ethanol level using the proper fuel-quality procedure. If the fuel is the problem, replace the fuel and, if needed, flush and clean the fuel system, then verify the repair.
Compare injector pressure drop
For the injector check, remember the fuel system can hold pressure even with the engine off, so release pressure before opening any fuel line, hose, or fitting. Install the Fuel Pressure Decay Tester 8978A at the fuel rail. Turn the ignition on and let fuel pressure reach maximum and stabilize; cycling the ignition off and on a couple of times may be needed. Record that stabilized value as the starting pressure. Use the scan tool to actuate the injector for cylinder 3 while watching the gauge, then record the ending pressure. Repeat the same process on several known-good cylinders, starting each actuation from the same starting pressure, and compare the ending pressures. They should be about the same. If cylinder 3 has a much smaller pressure drop, check for debris in the fuel rail or injector and repair as needed. If the pressure drop is much larger, suspect a faulty or fouled injector as the issue. After each injector actuation, start the engine to clear fuel from the cylinder, because leaving that fuel in place can damage the engine.
Check spark, the plug, compression, and carbon
If injector pressure drop is about the same as the good cylinders, move to spark. Turn the ignition off, remove the pressure tester, and reconnect the fuel lines before continuing. Remove the ignition coil from cylinder 3, reconnect the coil connector, disconnect that cylinder’s fuel injector connector, install a spark tester, and crank the engine. You are looking for a crisp blue spark. If spark is present, shut the ignition off and inspect the cylinder 3 spark plug for cracks, carbon tracking, fouling from oil, coolant, or excess fuel, incorrect gap, or a loose or broken electrode. Also look for signs of oil or coolant in the cylinder. If any of those plug conditions are present, replace the spark plug; if oil or coolant is in the cylinder, diagnose and repair the cause, then verify the repair. If the plug and cylinder inspection are clean, run a compression test on all cylinders and compare cylinder 3 to the known-good cylinders. If compression is not within specification, use a cylinder leak-down test to separate valvetrain trouble from combustion chamber trouble, repair as needed, and verify. If compression passes, swap the cylinder 3 spark plug with a known-good cylinder, or replace it if access makes swapping impractical, then start the engine and watch the misfire monitor. If the miss follows the plug, replace the plug. If replacing the plug makes the miss go away, the repair is complete after verification. If the miss does not follow the plug, de-carbon the engine with Mopar Combustion Cleaner, drive it to clear the cleaner, erase the codes, reproduce the captured conditions, and watch the misfire monitor. Carbon on valves and guides can cause a miss across many operating conditions, often more noticeable at cruise or heavy load.
Check oiling, VCT operation, and valvetrain parts
If the misfire returns after de-carboning, check the engine oil level and condition before going deeper. Look for mechanical tolerances out of specification, oil pressure out of specification, low oil level, the wrong oil filter, dirty or deteriorated oil, contaminated oil, incorrect viscosity, or aerated oil. These checks matter because skipping them can lead to a misdiagnosis. If an oiling or mechanical condition is found, repair it and verify the repair. If those checks pass, make sure the PCM C2 connector is connected, turn the ignition on, and check VCT solenoid operation by measuring the VCT Solenoid control circuit at the solenoid connector. The VCT solenoid driver is a 12.0 volt PWM circuit, and the voltage reading will vary with the actuation percentage selected. If the voltage is not present or does not match the selected percentage approximately, repair the VCT solenoid driver circuit for an open or short to ground and verify. If VCT control passes, visually inspect the camshaft lobes, rocker arms, and springs for excessive wear or broken parts. These faults may or may not show up in a compression test, and the operating condition of the miss depends on what is worn and how badly. If damage is found, make the appropriate mechanical repair and verify.
If spark was missing, test coil feed and control
If the earlier spark test did not show a crisp blue spark, go into the ignition coil circuit. Stay clear of the fan, belts, and pulleys any time the engine is operating or cranking. With the ignition off, disconnect the cylinder 3 ignition coil connector. Turn the ignition on, command the ASD relay with the scan tool, and use a 12-volt test light connected to ground to probe the ASD Relay F343 output circuit at the coil connector. The test light should illuminate brightly. If it does not, stop scan tool actuations and repair the open or short to ground in that output circuit between the PDC and the ignition coil connector, then verify. If the feed is good, connect a 12-volt test light to a 12 volt source and back probe the coil control circuit at the ignition coil connector. Crank the engine for 5 seconds. If the light blinks brightly, replace the ignition coil and verify. If it stays on constantly, check the coil control circuit for a short to ground. If it stays off constantly, check the coil control circuit for an open.
Check coil control wiring and connector integrity
For the short-to-ground check, turn the ignition off, disconnect PCM C2, and use the GPEC Diagnostic Adaptor 10436 on the engine-harness side. Do not probe the PCM harness terminals directly; that can damage the terminals and create a poor connection. Check continuity between ground and the coil control circuit at the ignition coil connector. If continuity is present, repair the short to ground and verify. If there is no short to ground, move to the final connector checks. For the open-circuit check, measure resistance in the coil control circuit between the ignition coil connector and the GPEC adaptor. Resistance must be below 3.0 Ohms. If it is not, repair the open or high resistance and verify. If it is below 3.0 Ohms, continue to connector inspection. Disconnect the PCM connectors, any related in-line connectors, and the related component connectors. Inspect for connector seating, damaged locks, corrosion, water intrusion, seal damage, bent terminals, overheating, pushed-back terminals, spread terminals, and poor terminal tension. Repair anything found, reconnect every connector fully, lock them, erase codes, and operate the truck under the monitored and set conditions. If P0303 does not return, the wiring or poor connection issue has been repaired. If it does return after these checks, replace and program the PCM, then verify the repair and confirm the code stays gone.
Close
The key with P0303 is to prove when cylinder 3 is actually misfiring, then work through fuel, spark, compression, carbon, oiling, VCT, and circuit control without skipping the simple checks. Verify the repair separately at the end and make sure the code stays gone under the conditions that originally set it. For more diagnostic training, visit stepdiagnostics.com.
Final check
P0303 diagnosis often works best when cylinder 3 misfire activity is confirmed first, then each possible system is tested in order before final repair verification.
For more guided automotive diagnostics, visit STEP Diagnostics.
