Prius - P0300 Random/Multiple Misfire 🔎

P0300 means the engine detected random misfires. On your Prius (NHW20, 1NZ-FXE), the usual causes are ignition coil issues, worn spark plugs, vacuum leaks, PCV hose cracks, EGR deposit at intake ports (light on ’04–’09), and fuel/air metering issues (MAF dirty).

Quick questions before we dive in

• 🧩 Do you also have specific cylinder codes (P0301–P0304) or other codes like P0171 (lean) or P0A0F/P3190 (no start/poor power)?
• 🛠️ When were the spark plugs and ignition coils last replaced (OE spec SK16R11/IFR5A11 plugs at 120k)?

What to check first (easy wins)

• 🛡️ Safety first: Ready mode keeps the engine starting/stopping. Work with the car OFF and key away. Don’t touch orange high-voltage parts.
• 🕯️ Spark plugs: Pull and inspect. Prius needs Denso SK16R11 (iridium), gap is 0.039–0.043 in (1.0–1.1 mm) pre-set. Oil or heavy wear = replace. Hand-thread to avoid cross-threading.
• ⚡ Ignition coils (one per cylinder): Look for cracks, carbon tracking, or oil in wells. Swap a coil from a suspected cylinder to another to see if misfire follows (if P0301–P0304 appear).
• 🌬️ MAF sensor (in airbox outlet): If dirty, clean with MAF cleaner only. A fouled MAF often sets P0171 with P0300. Do not touch the sensor wire.
• 🫧 Vacuum leaks: Commonly the PCV hose (rear of valve cover to intake manifold) splits. Also check airbox lid seated and intake snorkel clamps tight.
• ⛽ Fuel quality: Old fuel or a nearly empty tank on hills can cause stumble. Fill fresh Top Tier 87, clear codes, recheck.

Deeper Prius-specific checks

• 🧪 Fuel trim data (scan tool): At hot idle, STFT and LTFT together should be roughly within ±10%. High positive trims suggest a vacuum leak/MAF issue.
• 🧯 Intake manifold/EGR deposits: These engines can build carbon at intake ports causing intermittent misfire at idle. If plugs/coils are good and trims are normal, consider cleaning the throttle body and manifold runners.
• 🌀 Throttle body: Gummy throttle can cause unstable idle. Clean the bore and plate with throttle cleaner. Key OFF, do not force plate.
• 💧 Coolant leaks into plug wells: Check for pink residue near the thermostat/water outlet area above coils—rare, but any fluid in wells will cause misfire.
• 🔌 Injector balance: Listen with a stethoscope; all four should click evenly. A clogged injector will skew fuel trims and set a cylinder-specific code eventually.

Specs and basics (handy)

• 🔩 Spark plug torque: 13 ft‑lb (18 N·m) on clean, dry threads.
• 🔩 Ignition coil bolt torque: 71 in‑lb (8 N·m).
• 🧷 Coil connectors: Make sure the gray lock tabs click fully.
• 🛠️ Tools: 10 mm socket, 5/8 in spark plug socket with rubber insert, 6 in extension, torque wrench, MAF cleaner, throttle body cleaner, small pick.

Simple diagnostic path (DIY friendly)

• 🔍 Step 1: Scan and note all codes and freeze frame (data snapshot). Helps reproduce conditions.
• 🧼 Step 2: Inspect air filter, airbox, intake duct clamps. Clean MAF. Clear codes, test drive.
• 🔧 Step 3: Pull plugs. If worn/fouled, install correct iridium plugs, torque to spec.
• 🔁 Step 4: If a cylinder code appears, swap that coil to another cylinder. If the code follows, replace that coil (use Denso).
• 🫁 Step 5: If trims are high positive, smoke-test or carefully spray around the PCV hose and manifold gasket while idling to find leaks (RPM change = leak). Use nonflammable brake cleaner lightly.
• 🫗 Step 6: If still present, clean throttle body and consider removing the intake manifold to clean carbon in runners; replace manifold gasket.

When to suspect other systems

• 🚨 P3190/P0A0F with P0300: Possible severe air/fuel issue or MAF unplugged—start with MAF and major vacuum leaks.
• 💥 Rough only on cold start: Look at coolant seepage into plug wells or intake gasket leak that seals when warm.
• 🧯 Fuel pump/filter: Rare on this model but consider if trims go lean under load and injector/ignition checks pass.

Reply with any companion codes or what you find on the plugs/coils, and I’ll map your next steps.