P0304 on Toyota: How to Diagnose & Fix

How to Fix

Clean or replace the EGR valve (Toyota 25620-37120 for 2010-2017). Clean the intake manifold ports of carbon buildup using walnut blasting or manual cleaning. Replace EGR pipe gaskets. Some owners install an EGR cooler bypass as a long-term fix. Clean every 60,000-80,000 miles as maintenance.

How to Fix

Replace head gasket with Toyota OEM MLS gasket. Machine the cylinder head for flatness. Replace thermostat, water pump, and timing chain tensioner while engine is apart. Use Toyota Pink SLLC coolant.

How to Fix

Replace water pump (Toyota 16100-09442, which supersedes 16100-39455/16100-39456, or Aisin WPT-803). Replace the thermostat at the same time (confirm the 2GR-FE thermostat part number by VIN). Use Toyota SLLC or equivalent coolant. The pump is driven by the serpentine belt, so inspect the belt and tensioner during replacement.

How to Fix

Replace the valve cover gasket with a Toyota OEM gasket kit that includes spark plug tube seals. Clean all oil residue from the exhaust manifold after replacing. Replace the oil filler cap O-ring (or the entire cap for $10). Apply a thin bead of RTV sealant only at the cam cap corners where the valve cover gasket doesn't seal perfectly.

Owner tips & cautions

• TipThis is a 30-minute DIY job — just 4 bolts hold the valve cover on. Apply a tiny bead of RTV at the two front cam cap corners for a perfect seal.
• WarningDon't ignore oil pooling around spark plugs — oil-soaked ignition coils fail prematurely and cost $60+ each

How to Fix

Diagnosis is usually confirmed by fuel-trim readings, smoke testing, or observing idle changes when carb-cleaner or propane is directed near the intake manifold sealing area. The fix is replacement of the intake manifold gasket with the updated orange-design gasket and clearing any stored codes. Technicians should also inspect for cracked vacuum hoses and clean the throttle body if idle quality remains poor.

Owner tips & cautions

• TipIf the rough idle is worst on cold mornings and nearly disappears warm, inspect the intake gasket before replacing sensors or ignition parts.
• TipUse fuel-trim data at idle versus 2500 rpm; a strong positive trim at idle that improves off-idle often points to a vacuum leak.

How to Fix

Diagnosis involves documenting oil usage over a measured interval, checking for external leaks, and ruling out PCV issues before condemning the engine. Minor cases may improve with a fresh PCV valve and shorter oil-change intervals, but lasting repair usually requires piston/ring replacement or engine replacement. Spark plugs and the catalytic converter may also need inspection if the engine has been burning oil for a long time.

Owner tips & cautions

• TipCheck the dipstick every 500-1,000 miles on high-mileage 1ZZ-FE engines; many owners prevent engine damage simply by topping off before the level gets dangerously low.
• TipReplace the PCV valve early in the diagnosis because a restricted valve can worsen oil consumption and crankcase pressure.

How to Fix

Identify the misfiring cylinder from the codes and swap that coil to a neighboring cylinder to confirm the fault follows the coil. Replace the failed coil (and inspect/replace worn spark plugs as a set on high-mileage cars). OEM 1ZZ-FE coil PN 90919-02239 (~$40-70 each); Denso 673-1303 and other quality aftermarket units cross-reference. Use OEM/Denso-grade coils, as cheap coils commonly re-fail. Clear codes after repair.

How to Fix

Diagnose with a cylinder leak-down/compression test, a combustion-gas (block) test of the coolant, and cooling-system pressure testing to confirm internal coolant loss before replacing parts. The repair is to remove the heads and replace both head gaskets with the updated/superseded gasket, resurface and inspect the heads, and refill with the correct coolant. Replace head bolts per spec if torque-to-yield. Verify no oil-in-coolant or coolant-in-oil contamination that would indicate further damage.

How to Fix

Contact an authorised Toyota dealer or workshop with the VIN to confirm whether the car is included in the valve spring recall and to have the recall repair carried out.

How to Fix

Confirm that an exact listed DTC is present and that freeze-frame coolant temperature was 14-41 degrees F. Follow T-SB-0010-24 to verify cylinder compression, spark-plug and ignition-coil operation, direct- and port-injector resistance, and fuel-system operation. If every prerequisite is within specification and the bulletin's decision path is satisfied, replace all eight exhaust-side hydraulic lash adjusters and required one-time-use seals/gaskets using the bulletin and current VIN-specific parts information. Do not substitute a generic P0300 diagnosis, assume intake carbon is the cause, or order parts before checking current VIN-specific part numbers.

Owner tips & cautions

• TipSave the exact seven-character DTC and freeze-frame coolant temperature before clearing codes; both determine whether T-SB-0010-24 applies.
• WarningThe bulletin requires compression, ignition, both injector systems, and fuel-system checks before eight lash adjusters are authorized, and it expressly says not to use superseding part numbers from its table.

How to Fix

Have the head bolts checked for torque and the rear of the head inspected for coolant seepage. If the block threads have pulled, the cylinder head must come off and the block threads repaired with thread inserts (e.g., Time-Sert type kits) before a new head gasket is installed; where the block cannot be repaired, dealers replace the short block or engine (one owner was quoted about $6,780).

How to Fix

Replace cap, rotor, plug wires and spark plugs as a tune-up set; if oil is found inside the distributor, install the distributor oil-seal kit. Clean electrical contacts and confirm the rotor button is tight. If misfire persists after tune-up, test the igniter/ICM and wiring.