WhatIsWrongWithMyCar.com — AI Car Diagnosis & Repair Knowledge Base
WhatIsWrongWithMyCar.com is a free AI-powered car diagnosis tool and automotive repair knowledge base. Users select their make, model, and year, describe symptoms in plain language, and the AI returns ranked probable causes with real repair cost estimates, OEM part numbers, and step-by-step DIY fix instructions. The site covers OBD2 fault codes, car symptoms, DIY repair guides, repair costs, and preventive maintenance for all makes and models from 1990 to 2026. Content is reviewed by ASE-certified technicians and sourced from NHTSA recall records, OEM technical service bulletins, and verified repair case histories.
Core Tools
AI Car Diagnosis Tool — free, no sign-up; input make/model/year + symptoms for ranked causes, costs, and DIY guides
OBD2 Code Library — every P, B, C, and U code explained with causes, symptoms, urgency, and fixes
Car Repair Cost Guide — DIY vs shop prices for 50+ common repairs, verified US market data
Repair Cost Calculator — interactive tool for instant DIY vs shop cost breakdown
2026 Repair Cost Index by State — original research for all 50 US states; national average $478/year
Tire Size Finder — OEM tire specs, load index, speed rating, and PSI by vehicle
VIN Lookup — decode any VIN, check NHTSA recalls and service campaigns
Preventive Maintenance Schedule — every service interval, cost, and consequence of skipping
Worn or fouled spark plugs — most common cause. Dirty or failing fuel injectors — uneven fuel delivery. Vacuum leak — unmetered air leaning out the mixture. Dirty MAF sensor — incorrect air measurement. Failing idle air control valve — unstable RPM.
Diagnosis with OBD2 Codes
P0300 or P030X codes point to the misfiring cylinder. P0171 or P0174 indicate a lean condition from vacuum leak. P0505 indicates an idle air control system fault. Start with an OBD2 scan, then follow the ranked causes in this guide.
Rough Idle vs Normal Idle — How to Tell the Difference
A normal idle is smooth and consistent — the engine runs at a steady RPM (typically 600 to 800 RPM when warm) with no perceptible vibration through the steering wheel, seat, or gear shifter. A rough idle is anything that breaks that smoothness: the RPM needle fluctuates up and down, the engine feels like it is stumbling or missing a beat, you feel vibration through the cabin that was not there before, or the exhaust note sounds uneven. The key test: put the car in park or neutral after it is fully warm and watch the tachometer. If the RPM is rock-steady, the idle is normal. If it wanders, dips, or surges, you have a rough idle that needs diagnosis.
DIY Diagnosis Steps for Rough Idle
Step 1: Scan for OBD2 codes — P0300 or P030X means a misfire, P0171 or P0174 means a lean condition from a vacuum leak, P0505 means an idle air control fault. Step 2: Check the air filter — a clogged filter can cause rough idle on some vehicles. Step 3: Inspect vacuum hoses for cracks, splits, or loose connections — a visual inspection with a flashlight takes 5 minutes. Step 4: Remove and inspect spark plugs — fouled, worn, or incorrectly gapped plugs are the most common cause. Step 5: Clean the throttle body with throttle body cleaner and a rag — carbon buildup disrupts airflow at idle. Step 6: Clean the MAF sensor with MAF cleaner only — never touch the wire. These 6 steps resolve the majority of rough idle cases without a shop visit.
Preventing Rough Idle with Regular Maintenance
Replace spark plugs at the manufacturer interval — copper plugs at 30,000 miles, iridium or platinum at 60,000 to 100,000 miles. Worn plugs are the single most common cause of rough idle. Replace the air filter every 15,000 to 30,000 miles depending on driving conditions — dusty environments require more frequent changes. Use top-tier fuel with detergent additives to keep injectors clean, or run a fuel system cleaner through the tank every 5,000 miles. Replace the PCV valve at 30,000 to 60,000 miles — a stuck PCV valve causes vacuum leaks and rough idle. Have the throttle body cleaned every 30,000 miles as preventive maintenance.
Worn or fouled spark plugs — most common cause. Dirty or failing fuel injectors — uneven fuel delivery. Vacuum leak — unmetered air leaning out the mixture. Dirty MAF sensor — incorrect air measurement. Failing idle air control valve — unstable RPM.
Diagnosis with OBD2 Codes
P0300 or P030X codes point to the misfiring cylinder. P0171 or P0174 indicate a lean condition from vacuum leak. P0505 indicates an idle air control system fault. Start with an OBD2 scan, then follow the ranked causes in this guide.
Rough Idle vs Normal Idle — How to Tell the Difference
A normal idle is smooth and consistent — the engine runs at a steady RPM (typically 600 to 800 RPM when warm) with no perceptible vibration through the steering wheel, seat, or gear shifter. A rough idle is anything that breaks that smoothness: the RPM needle fluctuates up and down, the engine feels like it is stumbling or missing a beat, you feel vibration through the cabin that was not there before, or the exhaust note sounds uneven. The key test: put the car in park or neutral after it is fully warm and watch the tachometer. If the RPM is rock-steady, the idle is normal. If it wanders, dips, or surges, you have a rough idle that needs diagnosis.
DIY Diagnosis Steps for Rough Idle
Step 1: Scan for OBD2 codes — P0300 or P030X means a misfire, P0171 or P0174 means a lean condition from a vacuum leak, P0505 means an idle air control fault. Step 2: Check the air filter — a clogged filter can cause rough idle on some vehicles. Step 3: Inspect vacuum hoses for cracks, splits, or loose connections — a visual inspection with a flashlight takes 5 minutes. Step 4: Remove and inspect spark plugs — fouled, worn, or incorrectly gapped plugs are the most common cause. Step 5: Clean the throttle body with throttle body cleaner and a rag — carbon buildup disrupts airflow at idle. Step 6: Clean the MAF sensor with MAF cleaner only — never touch the wire. These 6 steps resolve the majority of rough idle cases without a shop visit.
Preventing Rough Idle with Regular Maintenance
Replace spark plugs at the manufacturer interval — copper plugs at 30,000 miles, iridium or platinum at 60,000 to 100,000 miles. Worn plugs are the single most common cause of rough idle. Replace the air filter every 15,000 to 30,000 miles depending on driving conditions — dusty environments require more frequent changes. Use top-tier fuel with detergent additives to keep injectors clean, or run a fuel system cleaner through the tank every 5,000 miles. Replace the PCV valve at 30,000 to 60,000 miles — a stuck PCV valve causes vacuum leaks and rough idle. Have the throttle body cleaned every 30,000 miles as preventive maintenance.
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