What is the code P0304 on a Lexus ES 350?
The code P0304 on a Lexus ES 350 indicates a misfire in cylinder 4, which can affect engine performance and emissions. The fix typically involves checking the ignition system, fuel delivery, and engine compression, then replacing or repairing the faulty component.
What P0304 Means for a Lexus ES 350
P0304 is a manufacturer-independent diagnostic trouble code that signals a fault in cylinder 4 of the engine. When the engine control unit notices that cylinder 4 isn’t firing as expected, it sets P0304 and usually triggers a check engine light. A misfire can cause rough idle, reduced power, and higher fuel consumption, and if left untreated, can harm the catalytic converter and other exhaust components.
Common Causes
Below is a list of frequent culprits behind a cylinder 4 misfire on a Lexus ES 350. The items are listed to help you prioritize diagnostic steps.
- Worn or fouled spark plug in cylinder 4
- Clogged or leaking fuel injector servicing cylinder 4
- Vacuum leak around the intake manifold or cylinder 4 area
- Low or unstable fuel pressure affecting cylinder 4
- Compression problems in cylinder 4 (valve, piston, or head gasket issues)
- Electrical wiring or connector problems to the spark plug coil or injector
- Faulty engine control module (ECM/PCM) or software issue
Diagnosing the exact cause requires a methodical approach that narrows these possibilities.
Symptoms to Watch For
These are common signs that may accompany P0304. If you notice any of them, consider having the vehicle inspected promptly to avoid further damage.
- Rough or unstable idle, especially when the engine is cold
- Poor acceleration or reduced power
- Check Engine Light illuminated with code P0304
- Increased fuel consumption or a noticeable drop in efficiency
- Engine vibration or misfire feel, particularly at low RPM
- Possible exhaust smell or abnormal engine noises
Persistent misfires can harm catalytic converters and spark adjacents, so timely diagnosis matters.
Diagnostic Steps
To identify the root cause of P0304, follow these practical steps in sequence. If you lack tools or experience, consult a professional technician.
- Use an OBD-II scanner to confirm P0304 and note any freeze-frame data or related codes (such as P0300 or P0301-P0308).
- Inspect the spark plug in cylinder 4 for wear, fouling, or gap issues; replace if needed.
- Test or swap the ignition coil for cylinder 4 with another cylinder to see if the misfire follows the coil.
- Check the spark plug boot and ignition wiring for cracks, corrosion, or poor connections.
- Evaluate the fuel injector for cylinder 4: listen for injector operation and consider a spray test or balance test to detect restriction or leakage.
- Check for vacuum leaks near cylinder 4 and around the intake manifold gaskets and PCV system.
- Perform a compression test on cylinder 4 to assess mechanical health (compression loss points to mechanical issues).
- Review fuel pressure and fuel trim data with a scan tool to rule out low fuel delivery or injector timing problems.
- If all basic checks fail, consider more advanced diagnostics or engine mechanical inspection by a professional.
After completing these steps, reset the codes and take a test drive to confirm whether the misfire returns.
Repair and Maintenance Options
Based on the diagnosed cause, repairs may be straightforward or more involved. The following options cover common remedies for P0304 on a Lexus ES 350.
- Replace the spark plug in cylinder 4 and ensure proper torque and gap
- Replace or service the ignition coil or coil-on-plug boot for cylinder 4
- Clean or replace a clogged fuel injector for cylinder 4; test for proper spray pattern
- Repair vacuum leaks near the intake manifold, PCV hose, or related gaskets
- Address fuel delivery issues: check and possibly replace the fuel filter, pump, or related lines
- Address compression or mechanical issues if machining or parts replacement is required (valve, piston, head gasket)
- Repair or replace damaged wiring or connectors to the injector or coil for cylinder 4
- Update or reprogram the ECM/PCM if a software fault is suspected
After repairs, clear the codes and perform a road test to verify that cylinder 4 misfire does not recur.
When to See a Mechanic
Some situations warrant professional service without delay. Consider seeking help if:
- The misfire persists after replacing spark plug and coil in cylinder 4
- There are signs of fuel or coolant leaks, or you notice abnormal engine noises
- You see the Check Engine Light return with P0304 or related codes after initial repairs
- There is damage to the catalytic converter hinting at prolonged misfire
Seasoned technicians can perform precise compression testing, advanced injector testing, and wiring diagnostics to determine the exact cause.
Safety and Driving Considerations
Continued driving with an active misfire can overstress other engine components and damage the catalytic converter. If the engine runs rough, reduces power, or emits unusual noises, limit driving and arrange for a diagnostic soon.
Summary
P0304 on a Lexus ES 350 means the engine control unit detected a misfire specifically in cylinder 4. The issue is most commonly caused by a faulty spark plug, ignition coil, or fuel injector, but it can also involve vacuum leaks, compression problems, or wiring faults. A systematic diagnostic approach—starting with basic electrical checks and progressing through mechanical tests—helps pinpoint the exact cause. Repairs typically involve replacing the faulty component and clearing codes, followed by a road test to ensure the misfire does not return. If in doubt, or if themisfire persists after initial fixes, seek professional service to avoid further damage to the engine or exhaust system.
