P0303: Cylinder Number 3 Misfire Event

What is P0303?

The P0303 code is an OBD II Diagnostic Trouble Code (DTC) defined as “Cylinder 3 Misfire Detected.”

It is a generic trouble code that triggers your vehicle’s check engine light. It gets registered in your Powertrain Control Module when the misfire monitor detects several misfire events on Cylinder 3.  

This Cylinder 3 misfire code may often get registered along with other misfire and lean/rich condition codes. 

The P0303 code means that your Powertrain Control Module (PCM) or Engine Control Module (ECM) has detected multiple misfires on cylinder number 3. The misfire monitor tracks crankshaft speed variations through the crankshaft position sensor (crankshaft sensor). Any relative loss in the crank speed during the cylinder’s power stroke counts as a misfire.

When that happens, the PCM creates a freeze frame data for the event, logs the P0303 code, and triggers your vehicle’s check engine light. 

Common symptoms

Look for these common symptoms if you suspect a misfire and the consequent P0303 code: 

• The check engine light comes on or is flashing 
• Vehicle jerks or hesitates during acceleration
• Rough idling or stalling
• Lack of power from the engine
• Reduced fuel economy
• Fuel smell from the exhaust

Can I still drive?

No, as the P0303 cylinder misfire code is highly critical. If your vehicle has the engine light turned on and the PCM registers this OBD-II trouble code, you should stop driving immediately. 

Since your vehicle’s engine relies on the proper functioning of its cylinders, a cylinder misfire could lead to internal engine damage, which may be costly to repair. 

Moreover, if the P0303 code isn’t diagnosed and resolved at the earliest, it could also lead to ignition failure, catalytic converter damage, and dangerous driving conditions.

P0303 causes

Several reasons could trigger a cylinder 3 misfire. Common causes include ignition system troubles, fuel delivery issues, or leaking valves.

However, a faulty or worn-out spark plug or ignition plug coil pack is the most common cause of the misfire. This is especially true if your car tune-up has been delayed for a while. 

Here are some of the possible causes that could lead to a misfire and turn on the check engine light: 

A.Troubles With Ignition System Components 

• A worn or faulty spark plug
• A failed ignition coil pack or faulty coil on plug 
• A bad spark plug wire

B. Fuel Delivery Troubles 

• A faulty fuel injector or fuel pump 
• Failing oxygen sensors
• Wiring issues in the fuel injector circuit 
• A problem with the PCM, such as a damaged injector driver

C. Engine Mechanical Troubles 

• Vacuum leak
• A worn piston ring, a damaged piston, or damaged cylinder walls
• A worn camshaft lobe or damaged lifter
• A leaking intake manifold gasket
• Poor compression
• Bad exhaust valve 
• Timing chain or belt with a skipped tooth

Diagnosis

Whenever your PCM registers the P0303 code and activates the check engine light, the first step is to diagnose what’s causing the cylinder number 3 misfire. 

It’s not an easy diagnosis, so it’s best to have a professional look into the issue. 

However, if you still want to get an idea, here’s how a mechanic will diagnose the P0303 trouble code: 

Step 1: Check If Any Other Codes Are Present 

Your mechanic will use an OBD II scan tool to check for any other current or pending code. If your PCM has registered any other check engine code, they’ll try to resolve it first. 

Step 2: Check The Freeze Frame Data 

If no other pending code is found, they’ll analyze the freeze frame data recorded for the trouble code. They’ll see at what RPM range the misfire occurred, which could help diagnose the problem better. They’ll then duplicate the conditions that set the code through a test drive. 

Step 3: Check For Loose Connections 

They’ll check for loose connectors at the ignition coil and look for any damaged wiring connected to cylinder 3. Your mechanic will check for any loose engine ground wire too. 

Step 4: Inspect The Spark Plugs and Coil Pack

Your mechanic may swap the engine cylinder 3 spark plug with that of cylinder 2. If the misfire moves to cylinder 2, the spark plug is at fault. 

They’ll also inspect the spark plug for any black soot, white deposits from overheating, or greenish deposits due to the coolant. 

Likewise, if your vehicle has individual coil packs instead of spark plug wires, they’ll remove the cylinder 2 coil pack and swap it with cylinder 4 coil. If the misfire moves to cylinder 4, the fault is in the coil pack. 

Step 5: Check For Fuel System Issues

If your mechanic doesn’t spot any issue with the ignition system, they’ll move over to the fuel system to see if that’s causing the misfire code. 

They’ll start by checking the fuel pressure, as low fuel pressure can often lead to a misfire. They’ll also review the fuel pump and the fuel pressure regulator to find out the source of low fuel pressure. 

Your mechanic will also verify if there’s a faulty fuel injector or injector wiring damage.  

Step 6: Perform An Engine Compression And Leakdown Test

If the fuel system and the ignition system are working fine, the next step is to check for any mechanical damage. 

Your mechanic will perform a leak down and compression test to spot issues like a leaking head gasket, broken piston rings, a bad exhaust valve, etc. 

There should be consistent compression readings, a minimum of 120 PSI for each engine cylinder, and at least 17-inches of steady vacuum to ensure smooth combustion.

Your mechanic may suggest different repairs or replacements based on the above diagnostic steps.

If you’re going for a DIY solution to fix the error code P0303, you should first look for simpler repairs, like fixing an air intake leak or replacing each spark plug with new plugs. 

You should also look for loose electrical connectors and broken vacuum hoses, as these issues are often ignored but can lead to engine misfires.

Possible repairs for P0303 & Costs

Here’re some of the possible fixes to resolve the P0303 code:

• Spark plug replacement for all cylinders
• Replacing the distributor cap and rotor, coil pack, or spark plug wiring
• Repairing intake air leaks 
• Repairing fuel issues
• Fixing mechanical engine issues

Once your mechanic has diagnosed the trouble code correctly, they’ll suggest a replacement or repair for fixing the misfires. 

The costs for each of these replacements are: 

• New spark plugs: $66-$250
• Spark plug wires: $180-$240
• Ignition Coil: $230-$640 
• Fuel injector: $350-$850
• Fuel pump: $400-$1200
• Fuel pressure regulator: $200-$400
• Vacuum leak: $150-$1000