P0304 on 2001-2009 Volvo S60: Cylinder 4 Misfire Causes and Fixes

What's Unique About the 2001-2009 Volvo S60

The first-generation Volvo S60 (P2 platform) is well-known for ignition coil failures. These coil-on-plug units are subjected to significant heat and vibration, and with age, they are a very common failure point leading to single-cylinder misfire codes like P0304. Owner and technician experiences overwhelmingly point to the ignition coil as the first and most probable part to check. Using cheap, non-OEM coils is often a mistake, as they can have poor fitment and a short service life. Many owners on forums like SwedeSpeed and VolvoForums report success with OEM Volvo or OE-supplier Bosch coils.

Symptoms You May Notice

• Rough or vibrating idle.
• Engine hesitation or stumbling during acceleration.
• Noticeable loss of power.
• Solid or flashing Check Engine Light.
• Reduced fuel economy.
• Difficulty starting the engine.
• Smell of unburnt fuel from the exhaust.

Most Likely Causes

1. Failed Ignition Coil 🔴 High Probability → Shop Ignition Coil The coil-on-plug ignition system on these Volvo 5-cylinder engines is a known weak point due to heat and age. It is the most frequent cause of single-cylinder misfires. Forum threads are filled with owners resolving P030x codes by replacing a coil.
   How to confirm: Swap the ignition coil from cylinder 4 with another cylinder (e.g., cylinder 3). Clear the codes and drive. If the code changes to P0303, the coil is bad. This is a definitive, no-cost diagnostic step. 🎬 See how to diagnose and fix a misfire for free.
   Typical fix: Replace the faulty ignition coil. 🎬 Watch: Step-by-step guide to replacing your S60 ignition coils. It is often recommended to use OEM or OE-supplier parts like Bosch for longevity. Many owners replace all five coils at once if they are original and high-mileage to prevent future failures.
   Est. part cost: $50-$90
2. Worn or Fouled Spark Plug 🟡 Medium Probability → Shop Spark Plug Spark plugs are a regular maintenance item with a service life of around 60,000 miles. An old, worn, or oil-fouled plug can't produce a strong enough spark to ignite the fuel mixture consistently.
   How to confirm: Remove the spark plug from cylinder 4 and inspect it. Look for heavy carbon deposits, oil, a wet fuel-soaked appearance, or a worn/damaged electrode. The correct gap is critical; for turbo models, it's typically around 0.028 - 0.030 inches (0.7mm). Compare it to a plug from a healthy cylinder.
   Typical fix: Replace the spark plug. If all plugs are old, it's best to replace them as a set with pre-gapped 🎬 Watch this easy DIY walkthrough for spark plug and coil replacement. OEM Volvo, Bosch, or NGK Iridium plugs.
   Est. part cost: $10-$25
3. Faulty Fuel Injector ⚪ Low Probability → Shop Fuel Injector Injectors can become clogged or fail electrically over time, but this is far less common than ignition system faults on this platform.
   How to confirm: After ruling out the coil and plug, swap the fuel injector from cylinder 4 to another cylinder. If the misfire code follows the injector (e.g., moves to P0303 after swapping with cylinder 3), the injector is faulty. This is more labor-intensive than swapping a coil. A professional can perform an injector balance test to measure flow rates.
   Typical fix: Replace the faulty fuel injector. Running a bottle of quality fuel system cleaner like Techron through the tank is a worthwhile first step before undertaking mechanical work.
   Est. part cost: $70-$150
4. Vacuum Leak ⚪ Low Probability Aging rubber hoses and plastic intake components can crack and create vacuum leaks. However, this often results in random misfire codes (P0300) or lean codes (P0171) rather than a persistent single-cylinder misfire, unless the leak is isolated to the intake runner for cylinder 4.
   How to confirm: With the engine running, spray brake cleaner or starter fluid around vacuum lines and the intake manifold gasket near cylinder 4. If the engine RPM changes, you've found a leak. A professional smoke test is more effective.
   Typical fix: Replace the cracked or leaking vacuum hose or gasket.
   Est. part cost: $5-$50

Rare But Worth Checking

• Low Engine Compression: This indicates a serious mechanical problem like a burnt exhaust valve, bad valve lifter, worn piston rings, or a head gasket leak. A compression test is the next step if ignition and fuel checks don't solve the P0304. Healthy compression for these engines is typically in the 150-185 PSI range, with less than 10% variation between cylinders. Zero compression in a cylinder often points to a burnt valve.
• Damaged Coil/Injector Wiring: The plastic wiring harness connectors can become brittle from heat and age, and the locking tabs often break. Wires can also fray, causing an intermittent connection to the coil or injector. Visually inspect the harness for cylinder 4 for any cracks, exposed wires, or loose-fitting connectors.

Diagnosis Steps

1. Read the trouble codes with an OBD-II scanner. Confirm P0304 is present and note any other codes.
2. If the Check Engine Light is flashing, avoid driving the vehicle to prevent catalytic converter damage.
3. Locate cylinder 4. On the transverse 5-cylinder engine, it is the fourth cylinder from the timing belt (passenger) side, or the second from the driver's side.
4. Swap the ignition coil from cylinder 4 with the coil from an adjacent cylinder (e.g., cylinder 3 or 5). Use a Torx bit (often T30) to remove the plastic engine cover and a 10mm socket for the coil bolt.
5. Clear the codes with the scanner, start the engine, and let it run or drive it briefly until the Check Engine Light returns.
6. Re-scan for codes. If the code is now P0303 (or P0305), the ignition coil is the confirmed problem and needs to be replaced.
7. If the code remains P0304, the coil is likely good. Remove the spark plug from cylinder 4. Inspect for wear, fouling, or incorrect gap. If it looks bad, replace it. It's good practice to replace all 5 if their age is unknown.
8. If the code still remains P0304, the problem is likely related to the fuel injector, wiring, or a mechanical issue. Inspect the wiring connector for the coil and injector for damage.
9. To test the fuel injector, swap it with another cylinder and see if the code follows. This is more involved than swapping a coil.
10. If all else fails, perform a compression test to check for mechanical engine health. A healthy range is 150-185 PSI, with minimal variation between cylinders. Zero compression is a strong indicator of a burnt valve.

Parts You'll Likely Need

• Ignition Coil (OEM #30713416) — This is the most common failure part for a P0304 code on this specific vehicle.
  Trusted brands: Bosch (OEM supplier, P/N 0221604008), Genuine Volvo, NGK
  OEM price range: $70-$100
  Aftermarket price range: $45-$75
• Spark Plug (OEM #8692071) — A worn spark plug is the second most likely cause and should be inspected or replaced when servicing the ignition coil.
  Trusted brands: Genuine Volvo (re-branded Bosch/NGK), Bosch (FR7DPP+, 7955), NGK (IFR6B, Iridium IX)
  OEM price range: $15-$25
  Aftermarket price range: $8-$15

Related Codes That Often Appear With This One

• P0420 — A persistent misfire from P0304 dumps unburnt fuel into the exhaust. This can overheat and destroy the catalytic converter, triggering a P0420 (Catalyst System Efficiency Below Threshold) code. This indicates expensive secondary damage.
• P0300 — If the root cause is a large vacuum leak, a failing fuel pump, or a clogged PCV system affecting the whole engine, you might see a P0300 (Random/Multiple Cylinder Misfire) code alongside or instead of the P0304.

Platform-Specific Known Issues

• Brittle Coil and Injector Connectors: The plastic connectors for the ignition coils and fuel injectors become very brittle over time due to engine heat. The locking tabs frequently snap off during removal, which can lead to a poor connection and intermittent misfires if the connector isn't properly seated or secured. Care must be taken during diagnosis and repair.

Mechanic-Grade Diagnostic Values

• Fuel Pressure (at fuel rail) — expected: Approximately 3.8 bar (55 PSI) at idle. The P2 platform uses a returnless fuel system where pressure is controlled by a fuel pressure sensor and pulse-width modulation of the pump, not a traditional regulator.. Failure: Significant deviation from 55 PSI or erratic readings can indicate a failing fuel pump, clogged filter, or faulty fuel pressure sensor.
• Ignition Coil Primary/Secondary Resistance — expected: While specific OEM resistance values are not commonly published for modern coil-on-plug units, a comparative test is effective. A good coil should have similar primary and secondary resistance readings to other known-good coils on the engine.. Failure: A reading of OL (Open Loop/infinite resistance) or a value that is drastically different from the other coils on the engine indicates a failed coil.
• Engine Compression — expected: 150-185 PSI, with less than 10% variation between cylinders.. Failure: A reading of 0 PSI on a cylinder is a strong indication of a burnt valve. Readings significantly lower than other cylinders point to issues like worn piston rings or a head gasket leak.

Hidden / Shadow Codes Worth Checking

• Misfire Counters per Cylinder: Volvo's proprietary diagnostic tool, VIDA (Vehicle Information & Diagnostics for Aftersales), can display live misfire counts for each individual cylinder. This is far more detailed than a standard OBD-II scanner and can help identify intermittent or low-frequency misfires that haven't yet triggered a CEL. (see via Requires connection to a laptop running Volvo VIDA software with a DICE interface tool.)

Scan Tool Commands That Help

• Volvo VIDA/DICE: Injector Cut-out Test — After ruling out ignition components, this function allows a technician to disable one injector at a time while the engine is running. When the injector for cylinder 4 is disabled, if there is no change in engine RPM or smoothness, it confirms that the injector was not contributing, pointing to a failure in that injector or cylinder. A drop in RPM on other cylinders confirms they are working.
• Volvo VIDA/DICE: Read ECM Status / Misfire Diagnosis — This function provides a detailed summary from the Engine Control Module (ECM) about the conditions under which the misfire was detected (engine speed, load, temperature), which can help diagnose intermittent issues.

Wiring & Ground Locations

• Engine Ground Strap (Main) — A braided strap typically running from the top of the engine (near the timing cover on the passenger side) to the chassis at the passenger-side strut tower.. A corroded or loose main engine ground can cause a host of intermittent electrical issues, including weak spark and erratic sensor readings, which can lead to misfires. Ensuring this connection is clean and tight is a critical, often overlooked step.
• 31/4 (Engine Block Ground) — A primary ground point directly on the engine block.. This is a crucial ground reference for many engine sensors. A poor connection here can affect the signals the ECM receives, potentially leading to misdiagnosis of other components.
• Ignition Coil Connector Pins — The 4-pin connector on top of the ignition coil for cylinder 4.. Technicians can back-probe this connector to verify power, ground, and the trigger signal from the ECM. Pin 4 is typically 12V+ power, Pin 3 is the signal from the ECM, and Pin 2 is the ground. Verifying these signals can distinguish between a bad coil and a wiring or ECM driver issue.

Real Owner Repair Stories

• VolvoForums user thread (2002 Volvo S60 AWD, 98,000 miles) — Rough idle after warm-up, check engine light on, little power.
  ❌ Tried (didn't work) Replaced all spark plugs., Replaced the ignition coils on cylinders #1 and #4.
  ✅ What actually fixed it The problem was only resolved after replacing all five of the original ignition coils with new ones. This suggests that even though only one cylinder was consistently logging a code, other original coils were likely weak and contributing to instability.

OEM Part Supersession History

• 9125601 → 30713416 — Updated design for improved reliability and performance. The 30713416 is the current Genuine Volvo and OEM Bosch replacement part number.
  Heads up: The newer 30713416 coils are fully backward compatible with vehicles that originally used 9125601.

Model Year Variations Within This Range

• 2005-2009: A facelift in 2005 brought cosmetic updates and a mechanical change to the T5 engine, which was bored out from 2.3L to 2.4L, increasing power to 257 hp. The core ignition and fuel systems relevant to a P0304 code remained largely the same across the 2001-2009 generation, however.
• 2001-2004 vs 2005+: Early (pre-2005) high-pressure turbo models (like the T5) used a 2.3L engine, while later models used a 2.4L. Additionally, some very early P2 models had different timing belt tensioner designs, which is irrelevant for a P0304 but important for general maintenance. The ignition coils and injectors remained functionally identical for this diagnosis.

Diagnostic Flowchart

Other Known Issues on This Vehicle

Issues unrelated to this code that are worth knowing about as an owner of this generation:

• Clogged PCV/Breather System 🔴 High — Very common, especially on vehicles over 80k miles or with infrequent oil changes. Can occur sooner in harsh conditions.
• Driver Information Module (DIM) Failure 🟠 Medium — Common failure on 2001-2004 models. Less frequent on later years.
• Electronic Throttle Module (ETM) Failure 🔴 High — Primarily affects 1999-2002 models with the Magneti Marelli ETM. A contactless replacement was introduced later. (Ref: There was a warranty extension and recall program, but it has long since expired.)
• AWD Angle Gear Collar Sleeve Failure 🟠 Medium — A known weak point on all P2 platform AWD models, especially pre-2006.
• Automatic Transmission Valve Body Failure 🔴 High — Common on higher-mileage (80k-120k miles) vehicles, especially with the Aisin-Warner AW55-50/51SN transmission.

Used vs. New Parts: Buying Guide for This Vehicle

When a used part is the smart pick: For this repair, a used ignition coil or fuel injector from a reputable salvage yard can be a cost-effective option, especially for a DIY diagnosis to confirm a fault before buying a new part. Given the high failure rate of coils, a used one is best for testing, not as a permanent fix unless its history is known.

Donor-vehicle mileage cap: roughly under 80000 miles for the part to have meaningful remaining life.

What to inspect on the donor part:

• For ignition coils, check the plastic housing for cracks and the rubber boot for tears or swelling.
• For fuel injectors, look for clean nozzles and intact O-rings. Avoid parts with heavy corrosion or physical damage.
• Ask for the donor vehicle's mileage; lower is always better.
• Check for a return policy in case the part is dead on arrival.

OEM-only on this vehicle (don't cheap out):

• Engine Control Module (ECM) - A used ECM cannot be simply swapped; it must be 'cloned' by a specialist service to copy the immobilizer data from the original unit. A dealer will not program a used ECM.

Aftermarket brands forum-validated for this vehicle:

• Ignition Coils: Bosch (OEM supplier), NGK
• Spark Plugs: Bosch, NGK
• Fuel Injectors: Bosch (OEM supplier)

Brands owners have reported issues with on this vehicle:

• Unnamed, no-brand 'white box' ignition coils from online marketplaces. Forum users frequently report premature failure, poor fitment, and intermittent misfires with these cheap alternatives.

Real Owner Stories

Aggregated from forums and TSBs cited above. Mileages and costs reflect what owners reported in those sources.

Related OBD-II Codes

• P0300 — Random or multiple cylinder misfire code; often appears alongside P0304 if vacuum leaks or ignition issues affect more than one cylinder.
• P0303 — Misfire detected in cylinder 3; often used during diagnosis when swapping coils from cylinder 4 to confirm a faulty part.
• P0305 — Misfire detected in cylinder 5; another companion code frequently seen on these 5-cylinder engines when multiple coils begin to fail due to age.
• P0171 — System Too Lean; may appear with misfire codes if the root cause is a vacuum leak in the intake components or aging rubber hoses.

Frequently Asked Questions