P0304 on Audi: How to Diagnose & Fix
Cylinder 4 Misfire Detected
17 Audi models · 21 documented issues · $120-$7,000 repair
P0304 on Audi by model: symptoms, diagnosis and fix
The RS7's 4.0T V8 is a direct-injection engine. Fuel is sprayed straight into the cylinders, so it never passes over the intake valves to wash them, and carbon deposits build up on the valve backs over time. As the deposits grow they restrict airflow and upset the air-fuel mixture. Audi's general fuel-quality bulletin (TSB 2014753, covering all 2000-2019 Audi models) lists the results of deposits on intake valves, injectors and combustion chambers: rough running after a cold start, rough idle, hesitation, reduced performance and poorer fuel economy. In more severe cases the check-engine light comes on with misfire or lean-mixture codes. The same bulletin names low-detergent or contaminated fuel as a contributing cause.
Common Symptoms
- Rough idle
- Rough running or misfires after a cold start
- Power loss at high RPM
- Hesitation under load
- Check-engine light with a misfire or lean-mixture code (e.g. P0300, P0171/P0174)
- Turbo lag increase
How to Fix
1. Check first: read the stored fault codes. Codes from Audi's bulletin TSB 2014753 that point to this problem include misfire codes (P0300 and P0301-P0308) and lean-mixture codes (P0171/P0174). Rule out simpler misfire causes such as spark plugs and coils. If deposits are still suspected, have a professional inspect the intake valves to confirm buildup before paying for a cleaning. 2. Fuel additives will not fix valve deposits on this engine. TSB 2014753 says Audi's additive (G 001770A2) cleans injectors and combustion chambers, and intake valves only on port-injected (MPI) engines. For intake-valve deposits on direct-injection (FSI/TFSI) engines it refers technicians to a separate bulletin, TSB 2019948. 3. The fix: mechanical cleaning of the intake valves, most often walnut blasting. Crushed walnut shells are blasted onto the valves to remove the carbon and restore airflow. 4. Prevention: Audi's bulletin recommends using TOP TIER detergent gasoline and avoiding contaminated fuel. One European specialist guide quotes a cleaning interval of about 30,000-50,000 miles depending on driving conditions. Use that as a guide only and clean when symptoms or an inspection show deposits. 5. Coverage: TSB 2014753 allows warranty diagnosis claims up to 48 months/50,000 miles. It says the additive is not reimbursed under warranty, and the bulletin is informational only for cars outside warranty. Ask an Audi dealer whether a cleaning is covered on your car. No recall covers this condition.
What you need to diagnose it
The repair instructions explicitly call for reading stored codes. Use a scanner that can read the named code families before choosing a repair.The recommended scanner supports these code families; confirm your exact year, model and module coverage before buying.
ANCEL AD310 Classic OBD-II Scanner
Read & clear engine (P) codes
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Reading and clearing the code is DIY work. Module replacement or reprogramming is not — most modules are VIN-programmed, so a new one does nothing until a shop codes it to your car. Check for an open recall or service bulletin before buying anything.
Call to confirm they can assess your vehicle and perform the stated work. Search results do not confirm qualification, eligibility or coverage.
Sources for this issue (2)
The S5 suffers from carbon deposits on the intake valves, a problem common to all Audi direct-injected engines. The performance-oriented tuning makes power loss from carbon more noticeable than in base models.
Common Symptoms
- Rough idle
- Power loss
- Cold start misfires
- Hesitation under load
- Check engine light
- Poor throttle response
How to Fix
Walnut blast intake valves every 40,000-60,000 miles. Install oil catch can to reduce oil vapor reaching valves.
Sources for this issue (2)
The 4.0T twin-turbo V8 in RS6, RS7, and RS Q8 suffers from severe carbon buildup on intake valves due to direct injection. The high-performance nature and twin-turbo setup means carbon accumulates faster than in lower-output engines. Over 60,000 miles, carbon restricts airflow causing rough idle, misfires, and power loss. The V8 configuration requires removing both intake manifolds, making walnut blasting more expensive ($1,200-$2,000) than 4-cylinder engines. Failure to clean can cause valve damage requiring expensive cylinder head work on the $120,000+ RS models.
Common Symptoms
- Rough idle
- Hesitation on acceleration
- Power loss
- Poor fuel economy
- Check engine light with misfire codes
- Engine runs rough when cold
How to Fix
WALNUT BLASTING: Remove both intake manifolds and blast walnut shells through all intake ports ($1,200-$2,000 for the V8). For the confirmed cold-start misfire deposit condition, Audi TSB 2075530/1 directs removing the intake manifold or compressor and walnut-blasting the inlet valves and ports with the cylinder head installed; the bulletin does not set a cleaning interval. Confirm heavy deposits by borescope before paying for cleaning. PREVENTION (owner-community practice, not an Audi instruction): some owners fit an oil catch can to reduce PCV oil vapour reaching the valves, and keep oil changes on time; results vary, and a catch can may affect warranty or emissions compliance. Fuel or oil additives cannot reach the intake valves on a direct-injection engine and are not a remedy.
Sources for this issue (2)
The direct-injection engines in the A5 (2.0 TFSI and 3.2L V6) suffer from severe carbon buildup on intake valves. In direct-injection engines, fuel sprays directly into the cylinder, bypassing the intake valves. This means intake valves are only exposed to oil vapors from the PCV system, which bake onto the valve backs as hard carbon deposits. Over 60,000-100,000 miles, carbon restricts airflow, causing rough idle, misfires, hesitation, and power loss. In severe cases, carbon deposits prevent valves from seating properly, causing compression loss and valve damage. The only effective fix is walnut blasting—blasting crushed walnut shells through the intake to remove carbon. A5OC forums recommend this as preventive maintenance every 60,000 miles.
Common Symptoms
- Rough or unstable idle (engine shakes at stoplights)
- Hesitation or stumbling on acceleration
- Reduced power and sluggish throttle response
- Poor fuel economy (2-4 MPG drop)
- Check engine light with misfire codes (P0300-P0306)
- Engine runs rough when cold, smooths out when warm
- Hard starting or extended cranking
How to Fix
Walnut blasting: remove the intake manifold and blast crushed walnut shells through the intake ports to remove carbon ($700-$1,200). Requires specialized equipment - DIY not recommended unless experienced. Dealers charge $1,200-$1,800; independent Audi shops charge less. Confirm heavy deposits by borescope before paying for cleaning; there is no manufacturer-set cleaning interval. Prevention (owner-community practice, not an Audi instruction): some owners fit an oil catch can to reduce PCV oil vapour reaching the valves, and keep oil changes on time; results vary, and a catch can may affect warranty or emissions compliance. Fuel or oil additives cannot reach the intake valves on a direct-injection engine and are not a remedy.
Sources for this issue (2)
The 2.0 TFSI engine in early B8 A5 models (2008-2015) suffers from excessive oil consumption due to defective piston ring design. Owners report burning 1 quart of oil every 500-1,200 miles, requiring constant top-ups. The issue is worst in 2010-2012 models with EA888 Gen 2 engines. Faulty oil control rings fail to scrape oil from cylinder walls, allowing it to burn in the combustion chamber. This causes blue smoke on startup, fouled spark plugs, and carbon buildup. Low oil levels starve the turbocharger and timing chain tensioners, leading to expensive failures. Audi extended warranty coverage for some models and performed piston ring replacements under warranty. A5OC forums report many owners going through 5+ quarts between oil changes. Some cases cause cylinder scoring requiring engine replacement.
Common Symptoms
- Low oil warning light frequently (need to add oil between changes)
- Burning 1+ quarts of oil every 1,000 miles
- Blue or gray smoke from exhaust on startup or hard acceleration
- Fouled spark plugs (oil-soaked)
- Check engine light with misfire codes (P0300-P0304)
- Rough idle from carbon buildup
- Turbo whining (from oil starvation)
How to Fix
For severe consumption (over 1 qt/1,000 mi): piston ring replacement or short block replacement ($4,000-$7,000). Check Audi's extended warranty program for 2009-2015 models—some repairs covered. For moderate consumption: replace PCV valve ($150), clean carbon deposits with walnut blasting ($800), and switch to thicker oil (5W-40 instead of 5W-30). Prevention: check the oil level weekly, change oil every 5,000 miles (not 10,000), and avoid 2010-2012 models when buying used. Later Gen 3 EA888 engines have updated rings and are far more reliable.
Sources for this issue (2)
Audi states that contaminated gasoline, or gasoline low in deposit-control additives, can build deposits on the intake valves, intake manifold, fuel injectors and combustion chambers. Results can include rough running after a cold start, long cranking, hesitation, rough idle, reduced performance, poor fuel economy, and a check-engine light with misfire, lean-fuel-system or air/fuel-ratio-imbalance codes. Applies to: All Audi vehicles from model years 2000–2025 (bulletin 2014753/13, December 5, 2024), which includes the 2017–2020 and 2022–2025 Audi RS 3 with the 2.5 TFSI five-cylinder. It is a diagnosis guide, not evidence that every RS 3 develops deposits.
Common Symptoms
- Rough running after a cold start
- Extended cranking
- Hesitation or rough idle
- Reduced performance or poor fuel economy
- Check-engine light with the bulletin-listed misfire, lean-system or air/fuel-ratio codes
How to Fix
Have a dealer read the fault memory and consider the fuel source; the bulletin is a fuel-quality diagnosis guide. If contaminated fuel is suspected (unusual color or odor, water, sediment, cloudiness or separation after settling), the bulletin advises changing the fuel source. If low-additive fuel is suspected, it recommends TOP TIER Detergent Gasoline. Audi gasoline additive G 001770A2, mixed into a full tank, can be used to remove deposits from injectors and combustion chambers; intake-valve deposits on FSI direct-injection engines are handled under a separate Audi bulletin, 2019948. The bulletin is informational only, is not applicable to any Audi warranty, and the additive is not reimbursed. This is not a recall or a promise of free repair.
Call to confirm they can assess your vehicle and perform the stated work. Search results do not confirm qualification, eligibility or coverage.
Sources for this issue (1)
P0304 code reference
General diagnostic-code context. The code alone does not confirm a cause for your vehicle; use the applicable issue card and its sources.
Frequently asked questions
What does P0304 mean on a Audi?
P0304 stands for "Cylinder 4 Misfire Detected." This code means the engine computer detected a misfire in cylinder number 4 — that cylinder isn't igniting its air-fuel mixture fully or at all. The system detects this by monitoring small changes in crankshaft rotation speed as each cylinder fires. You may feel a rough idle, shaking, hesitation or reduced power, and a persistent misfire can dump unburned fuel into the exhaust and overheat or damage the catalytic converter. Since it's pinpointed to one cylinder, the problem typically lies in that cylinder's spark, fuel, or compression. On Audi it is documented across 17 models: RS7, S5, RS6, A5, RS3, A4, A6, R8, A7, Q8, S6, SQ5, S7, TTS, Q5, A8, RS4.
What are the most common causes of P0304?
The most common causes of P0304 are: Worn or fouled spark plug in cylinder 4, Failed or weak ignition coil on cylinder 4, Faulty spark plug wire or boot (where applicable), Clogged, stuck or leaking fuel injector for cylinder 4, Vacuum or intake air leak near that cylinder, Low compression (worn rings, burnt/leaking valve, head gasket), Damaged wiring or connector at the coil or injector. The specific cause varies by vehicle.
How much does it cost to fix P0304 on a Audi?
Published repair estimates on Audi range from $120 to $7,000 depending on the model and root cause. If an issue mentions a recall or campaign, check your VIN with the manufacturer or NHTSA and confirm eligibility and coverage with a dealer.
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Content compiled with AI assistance using NHTSA complaints, TSBs, and owner reports. May contain errors. Always verify with your vehicle's service manual.