{"model": "gemini-free", "provider": "Gemini web app, signed-in free tier ('Gemini Auto')", "task": "auto-diag", "prompt": "Write a step-by-step diagnostic walkthrough for high school automotive students: a 2012-2018 gasoline car comes in with a check engine light and code P0300 (random/multiple cylinder misfire). Show the logical order of tests from simplest to most involved, what tools are needed, what each result means, and safety notes. Format it as a student handout.", "ran_at": "2026-10-16T06:54:20-0700", "output": "Gemini Auto\n\n# Student Handout: Diagnostic Walkthrough \u2013 Code P0300 (Random / Multiple Cylinder Misfire)\n\n**Vehicle Year Range:** 2012\u20132018 Gasoline Engines\n\n**Target Audience:** High School Automotive Technology Students\n\n**Estimated Time:** 45\u201360 Minutes\n\n---\n\n## Safety First! \u26a0\ufe0f\n\n* **Hot Surfaces:** Never touch exhaust components, catalytic converters, or the engine block immediately after running. Allow the engine to cool or wear high-temperature thermal gloves.\n* **High Voltage (Ignition Systems):** Secondary ignition circuits can carry tens of thousands of volts. Avoid touching coil-on-plug (COP) connectors or spark plug wires while the engine is running to prevent severe electrical shock.\n* **Moving Parts:** Keep loose clothing, hair, jewelry, and hands away from serpentine belts, cooling fans, and pulleys when working in the engine bay with the engine running.\n* **Ventilation:** Always perform engine diagnostics in a well-ventilated shop area or with an exhaust evacuation hose properly attached to prevent carbon monoxide poisoning.\n* **Fire Safety:** Keep a Class ABC fire extinguisher nearby whenever working with fuel systems.\n\n---\n\n## Required Tools & Equipment\n\n1. OBD-II Diagnostic Scan Tool (with live data / graphing capabilities)\n2. Digital Multimeter (DMM)\n3. Basic Hand Tools (Ratchet set, spark plug socket, torque wrench, extensions)\n4. Mechanics Stethoscope or long-handled screwdriver (for sound isolation)\n5. Fuel Pressure Gauge (if applicable)\n6. Smoke Machine (EVAP / Induction leak testing)\n\n---\n\n## Diagnostic Flowchart: From Simplest to Most Involved\n\n---\n\n### Step 1: Initial Scan, Freeze Frame Data & History Check\n\n* **What to do:**\n1. Connect your OBD-II scan tool to the Data Link Connector (DLC) under the dash.\n2. Retrieve all Diagnostic Trouble Codes (DTCs)\u2014look for pending codes or specific cylinder misfire codes (e.g., P0301, P0302) alongside the P0300.\n3. Pull the **Freeze Frame Data** to see engine conditions (RPM, coolant temperature, load, fuel trim) when the misfire occurred.\n\n\n* **What it means:**\n* If individual cylinder codes (P0301\u2013P0308) accompany P0300, the issue may be isolated to specific cylinders rather than truly random.\n* If freeze frame data shows the misfire happens strictly on a cold start, it points toward different components (e.g., valve carbon buildup, leaking fuel injector) than if it occurs only under heavy load at highway speeds (e.g., weak ignition coil, lean fuel mixture).\n\n\n\n---\n\n### Step 2: Visual & Physical Inspection (The \"Under-Hood Check\")\n\n* **What to do:**\n* With the engine off, inspect the entire engine bay. Look for disconnected, cracked, or deteriorated vacuum lines and PCV hoses.\n* Check electrical connectors at each ignition coil and fuel injector for corrosion, loose pins, or broken locking tabs.\n* Listen and look for unmetered air leaks around the intake manifold gaskets and air intake boot (between the mass airflow sensor and throttle body).\n\n\n* **What it means:**\n* Modern 2012\u20132018 vehicles are heavily dependent on sealed vacuum and intake systems. A torn intake boot or vacuum leak lets unmeasured air into the cylinders, leaning out the air-fuel mixture and causing a random misfire.\n\n\n\n---\n\n### Step 3: Check Live Data (Misfire Counters & Fuel Trims)\n\n* **What to do:**\n* Start the engine and pull up **Live Data** on your scan tool.\n* Navigate to misfire counters (if supported by the vehicle make) to see which cylinders are actively skipping.\n* Monitor **Short Term Fuel Trim (STFT)** and **Long Term Fuel Trim (LTFT)** at idle and at 2,500 RPM.\n\n\n* **What it means:**\n* **Fuel Trims over +10% to +20% (Lean):** Indicates the computer is adding extra fuel because it detects too much oxygen\u2014look for a vacuum leak, clogged fuel injectors, or low fuel pressure.\n* **Fuel Trims under -10% to -20% (Rich):** Indicates the computer is pulling fuel\u2014look for a leaking fuel injector, stuck purge valve, or heavy carbon fouling.\n\n\n\n---\n\n### Step 4: The Swap Test (Isolating Ignition or Fuel Components)\n\n* **What to do:**\n* If live data points to specific misfiring cylinders (or if you want to test suspects), swap components between a **misfiring cylinder** and a **known good cylinder**.\n* *Example:* Swap the Ignition Coil from Cylinder 1 to Cylinder 3. Clear codes, test-drive until the check engine light returns, and rescan.\n* If the misfire code follows the coil to Cylinder 3, the coil is bad. If it stays on Cylinder 1, the coil is good. (You can repeat this exact method with spark plugs or fuel injectors).\n\n\n* **What it means:**\n* Isolates whether the root cause is a failing wearable component (spark plug/coil) or an internal mechanical/fuel issue.\n\n\n\n---\n\n### Step 5: Secondary Ignition & Spark Plug Inspection\n\n* **What to do:**\n* Remove the ignition coils and pull the spark plugs from the affected cylinders using a spark plug socket and torque extension.\n* Inspect the plug tips for signs of wear: oil fouling, fuel fouling, extreme gap erosion, cracked porcelain, or carbon tracking down the side of the ceramic.\n\n\n* **What it means:**\n* **Worn/Wider Gap:** Weakens spark intensity under load.\n* **Oil on Threads/Tip:** Indicates failing valve cover spark plug tube seals.\n* **Black, Sooty Deposits:** Over-fueling, weak spark, or short trips preventing proper warm-up.\n\n\n\n---\n\n### Step 6: Fuel System & Injector Testing\n\n* **What to do:**\n* Check fuel rail pressure using a mechanical gauge or scan tool PID against manufacturer specifications.\n* Perform a power balance test or electronic injector drop test using the scan tool.\n* Listen to fuel injectors clicking using a mechanic's stethoscope.\n\n\n* **What it means:**\n* Low fuel pressure starves all cylinders, triggering a P0300.\n* A dead or clogged direct-injection (DI) or port-injection injector will cause a localized misfire that spreads across the random counter. (Note: 2012\u20132018 vehicles frequently feature Gasoline Direct Injection (GDI), which are prone to carbon fouling on the intake valves).\n\n\n\n---\n\n### Step 7: Advanced Mechanical Testing (Compression & Leak-Down)\n\n* **What to do:**\n* If ignition and fuel check out fine, perform a **Relative Compression Test** via the scan tool or a physical **Cylinder Compression Test** with a pressure gauge.\n* If low compression is found on one or more cylinders, perform a **Cylinder Leak-Down Test** to determine *where* pressure is escaping (past worn piston rings into the crankcase, out a burnt intake/exhaust valve, or through a blown head gasket).\n\n\n* **What it means:**\n* 2012\u20132018 engines (especially certain high-mileage direct-injection platforms) can suffer from carbon buildup on intake valves preventing proper seating, stretched timing chains altering valve timing, or mechanical valve train wear. Mechanical issues will never be fixed by replacing spark plugs or coils!\n\n\n\n---\n\n## Student Quick-Review Checkpoint\n\n1. Why should you check Freeze Frame data before clearing codes?\n2. If fuel trims are heavily positive (+18%), what basic engine condition is most likely present?\n3. True or False: Swapping components helps isolate whether a misfire is mechanical or electrical."}