P0306 on Audi: How to Diagnose & Fix

How to Fix

Walnut blast all eight intake runners. Dual-injection models (2019+) are less affected.

How to Fix

Confirm carbon buildup by scanning for misfire faults (commonly random/multiple or cylinder-specific misfires), reviewing fuel trims, and inspecting the intake valves with a borescope after removing the intake manifold or accessing the ports. The proper repair is an intake valve carbon cleaning, typically walnut shell blasting each intake port with the valves closed, then replacing intake manifold gaskets, injector seals/O-rings or any disturbed PCV-related seals as needed and clearing/adapting the ECU afterward. If buildup is severe or returns quickly, inspect the PCV/oil separator system and crankcase ventilation hoses for excessive oil carryover.

How to Fix

Walnut blast every 30,000-40,000 miles. Install dual catch can setup for both turbo sides.

How to Fix

Walnut blast intake valves every 30,000-40,000 miles. Install catch can immediately on new vehicles.

How to Fix

WALNUT BLASTING: Remove the intake manifold and blast walnut shells through the intake ports ($800-$1,500). Where confirmed cold-start misfire deposits match Audi TSB 2075530/1, the bulletin directs walnut blasting of the inlet valves and ports with the cylinder head installed. 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.

How to Fix

Walnut blast intake valves every 40,000-60,000 miles. Install oil catch can to reduce oil vapor reaching valves.

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, oil interval, additive or driving prescription. 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.

How to Fix

WALNUT BLASTING: Remove intake manifolds and blast all intake ports with walnut shells. V8 (4.2L): $800-$1,200. V10 (5.2L): $1,200-$2,000 due to 10 cylinders. Perform a leak-down test BEFORE walnut blasting to ensure intake valves seat properly. Repeat every 50,000-70,000 miles. CHEMICAL CLEANING: Less effective than walnut blasting but some owners use intake valve cleaners (BG Products, CRC) as interim maintenance. DRIVING HABITS: Regular high-RPM driving (the R8's natural habitat) helps slow carbon accumulation. PREVENTION: Use quality fuel (91+ octane) and change oil every 5,000 miles.

How to Fix

Diagnose the misfiring cylinder(s) via the stored misfire counters, then replace the failed coil(s) along with spark plugs; most specialists replace the full set of 8 (V8) or 10 (V10) plus plugs to avoid repeat visits. The canonical 4.2 V8 coil is the revision-'T' red coil, Audi/VW PN 077905115T (superseded family, aftermarket cross-reference 06E905115E). The Gen-2 5.2 V10 uses Audi PN 07L905115B. Address any valve-cover or plug-tube oil leak at the same time so new coils are not contaminated.

How to Fix

Have the cold-start event and stored faults diagnosed before authorizing intake cleaning. The B7 RS4 bulletin first checks whether its fuel-additive and follow-up procedure resolves injector-deposit misfires. Current TSB 2014753/13 warns that gasoline additive can clean FSI injectors and combustion chambers but not FSI intake valves. If the misfire remains, the shop should continue guided diagnosis and confirm inlet-valve deposits before using the applicable mechanical-cleaning procedure. Do not buy a scanner, oil bundle or cleaning service solely from this card.

How to Fix

Confirm the failing cylinder, then swap the suspect coil to another cylinder to verify the fault follows the coil. Replace the failed coil; best practice on the 4.0T is to replace all eight coils and the spark plugs as a set since the access labor is shared and mixed-age coils tend to fail in sequence. Use OE or quality OE-equivalent coils (the 4.0T uses the 079905110-series coil). Inspect spark plugs for fouling/oil that could indicate a secondary issue.

How to Fix

Confirm fuel quality, symptoms and stored DTCs before replacing or cleaning components. If contaminated gasoline is suspected, advise changing the fuel source; if low detergent quality is suspected, Audi recommends Top Tier detergent gasoline. Continue workshop diagnosis if symptoms persist. On 2018-2025 EA839 SQ5 models, do not recommend unapproved cleaners or additives; follow the owner manual and use only Audi-approved petrol additives. Do not prescribe routine walnut blasting, a catch can or chemical cleaning from this record.

How to Fix

Confirm the exact gasoline-powered vehicle, fuel quality, symptoms and stored DTCs before cleaning or replacing components. If contaminated gasoline is suspected, change fuel source; if detergent quality is suspect, Audi recommends Top Tier detergent gasoline. Continue guided diagnosis if the condition persists and identify the actual deposit location before selecting any cleaning procedure. Do not prescribe routine walnut blasting, a catch can, chemical cleaner or high-RPM driving from this record.

How to Fix

Record every stored DTC and freeze-frame before clearing faults. Confirm that the vehicle is a 2019-2021 A6 with the 3.0L engine, then have an Audi dealer or qualified VAG specialist compare the ECM J623 part number and software version with Audi's bulletin and perform the specified Software Version Management update when applicable. For MY2019 the bulletin lists 4K0907559A version 0007 updating to 0008 with SVM action 01A255; MY2020 lists 4K0907559D version 0002 updating to 0003; MY2021 lists 4K0907559K versions 0001-0002 updating to 0004. Recheck for misfires after the update. If faults persist, diagnose ignition, plugs, injectors, fuel quality, air leaks, compression, and only then inspect the intake valves before authorizing cleaning. Do not buy a catch can, cleaner, ignition part, or walnut-blasting service solely from P0300-P0306.

Owner tips & cautions

• TipSave the complete scan and freeze-frame before an update or repair. The cylinder pattern and operating conditions are useful if the misfire remains.
• WarningP0300-P0306 do not identify carbon buildup by themselves. Verify the Audi software level and diagnose the remaining possible causes before paying for walnut blasting or ordering parts.
• WarningThe published fix is software, so ShowMeTheParts cannot validate it and no retail product link belongs on this record.

How to Fix

Scan for cylinder-specific misfire faults, inspect spark plugs, and swap suspect coils to confirm the fault follows the coil. The standard repair is replacement of the failed coil packs; many owners replace the full set and install updated revision coils. Spark plugs should be checked or replaced at the same time if worn or fuel-fouled.

Owner tips & cautions

• TipLabel the coils before swapping them and rescan after the swap. A misfire that stays on the original cylinder points away from the coil.
• WarningDo not use recall 04V-133 as coil evidence; it is the A6 headlight-switch wiring and fire-risk recall.

How to Fix

WALNUT BLASTING: Remove intake manifold and blast crushed walnut shells through intake ports ($1,000-$1,800 for A8 due to packaging complexity). Requires specialized equipment. Repeat every 60,000 miles as PREVENTIVE maintenance. PREVENTION: Install catch can ($400-$700) to filter PCV vapors—extends cleaning interval to 100k+ miles. Add Liqui Moly Intake Valve Cleaner to every oil change. Change oil every 5,000 miles to reduce vapor contamination. Budget for this as routine maintenance.

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.

How to Fix

WALNUT BLASTING: Remove the intake manifold (supercharger on the 3.0T) and blast walnut shells through the intake ports to remove carbon ($800-$1,500 for A7; $1,200-$2,000 for S7/RS7 due to complexity). Requires specialized equipment - not DIY-friendly. 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.

How to Fix

WALNUT BLASTING: Remove the intake manifold and blast walnut shells through the intake ports ($900-$1,500). Requires specialized equipment. 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.