Fix it right the first time.
Practical diagnosis and maintenance guides for heavy-duty trucks and trailers — written for the people who turn the wrenches. New guide every day.
Using PartsNow
Step-by-step help for the website and the mobile app.
Using PartsNowHow to Find the Right Truck Part on PartsNow.ai
Search 50,000+ heavy-duty truck and trailer parts by part number, VIN, photo — or just describe what you need in plain words. Here's how.
Using PartsNowHow to Order on PartsNow: Shipping or Free Knoxville Pickup
From cart to checkout on partsnow.ai: nationwide shipping with real-time UPS/USPS/FedEx rates, or free pickup in Knoxville, TN — ready in 1-2 business days.
Using PartsNowHow to Use the PartsNow AI Assistant (Chat, Photo & VIN)
PartsNow's AI assistant finds parts, checks compatibility and crosses part numbers — by chat, photo or VIN. Here's how to get the most out of it.
Using PartsNowGetting Started With the PartsNow App (iOS & Android)
Download the PartsNow app for live video help from Mike, photo part scanning, catalog search and checkout — truck parts from the driver's seat.
Using PartsNowMike Live: Show Your Truck Problem on Video, Get the Part
In the PartsNow app, Mike Live opens a video session — point your camera at the part or problem and Mike identifies it and finds what you need.
Using PartsNowHow to Identify a Truck Part From a Photo (Web & App)
Got the old part in your hand but no number on it? Upload a photo on partsnow.ai or scan it in the app — the AI identifies it and finds matches.
Repair & maintenance
A new guide every day, grounded in real field manuals.
Engine & CoolingDetroit DD15 Common Parts and Maintenance Intervals
DD15 maintenance intervals and the parts that fail most: filters, injectors, EGR coolers, and cooling system components owner-operators replace.
Filtration & MaintenanceFreightliner Cascadia Filter Guide: Oil, Fuel and Air
Complete Freightliner Cascadia filter guide covering oil, fuel and air filters — what fits, change intervals, and warning signs.
Filtration & MaintenanceCummins X15 Oil Filter and Filtration Guide
Complete Cummins X15 filtration guide covering oil, fuel, and coolant filters, part selection, and service intervals for owner-operators.
Filtration & MaintenanceClass 8 Oil Change Kit: Filters, Intervals & What You Need
Complete guide to a Class 8 oil change kit — oil filter, fuel filter, drain plug, spec oil — and how to set the right service interval.
Filtration & MaintenanceHeavy-Duty Air Filter: How to Choose & When to Replace It
Heavy-duty air filter guide: how to pick the right one, read restriction gauges, and set change intervals for dusty vs highway service.
Brakes & Wheel EndBrake Drum Sizing: How to Read 16.5x7 and Pick the Right Drum
Understand brake drum sizing — diameter, width, bolt pattern and load rating — so you order the correct replacement drum on the first try.
Engine & CoolingEngine Mount Failure: Symptoms, Sag & When to Replace
Spot engine mount failure early — vibration, clunks, sag and alignment drift. A shop foreman's guide to inspection and replacement, with Peterbilt 337 notes.
Air SystemAir Dryer Failure: Diagnosing Moisture & Freeze-Up Issues
Diagnose air dryer moisture buildup, freeze-ups, and desiccant failures. Step-by-step checks for fleet mechanics — repair vs. replace guidance.
Brakes & Wheel EndBrake Drum Cracks & Wear: DOT Inspection Guide
Learn how to inspect brake drums for cracks, heat checking and out-of-round wear before your next DOT inspection flags them out of service.
Suspension & SteeringAir Bag Suspension Failures: Sagging, Leaks & Ride Height Fixes
Diagnose air spring leaks, sagging ride height, and bracket wear before frame damage hits. Practical air bag suspension guide for owner-operators.
Engine & CoolingClutch Slipping or Chattering? Diagnosis Guide for Manual Trucks
Diagnose clutch slipping, chattering, or hard engagement on manual trucks like the Western Star 4900 with this step-by-step troubleshooting guide.
Aftertreatment & EmissionsEGR Valve Problems: Symptoms, Codes & Diagnosis Guide
Spot EGR valve problems early—rough idle, power loss, soot, and fault codes—before they trigger a derate. Diagnosis tips for fleet mechanics.
Aftertreatment & EmissionsEGR Valve & Cooler Failure: Diagnosing Codes & Power Loss
Diagnose EGR valve failure and EGR cooler cracks causing power loss, coolant loss, or rough idle on diesel engines like the International ProStar.
Electrical & LightingTrailer Wiring Shorts: Diagnosing Flickering or Dead Lights
Fix flickering trailer lights and blown fuses fast. A step-by-step guide to diagnosing trailer wiring shorts, bad grounds, and 7-way connector faults.
Engine & CoolingFan Clutch Failure: Diagnosing Overheating & Fan Noise
Fan clutch failure causes overheating and loud roar. Learn roadside checks, viscous vs electric testing, and replace-or-rebuild calls for fleet trucks.
Suspension & SteeringPower Steering Pump Failure: Symptoms & Diagnosis Guide
Catch power steering pump failure early. Symptoms, fluid checks, and diagnostic steps for older conventional trucks like the Pete 379.
Suspension & SteeringLeveling Valve Problems: Diagnosing Ride Height Issues
Fix a tractor or trailer that sits wrong or bounces. Step-by-step leveling valve diagnosis, linkage adjustment, and exhaust port checks.
Suspension & SteeringDriveline Vibration: U-Joints, Yokes & Balance Diagnosis
Diagnose driveline vibration in Class 8 trucks—worn U-joints, bent yokes, imbalance—before it wrecks trans and axle seals.
Filtration & MaintenanceFuel Filter & Water Separator: When to Change and Why
Learn fuel filter change intervals, water separator warning signs, and OEM vs aftermarket tips to protect your engine and injectors.
Electrical & LightingNo-Start Diagnosis: Batteries, Alternator & Starter Issues
Step-by-step no-start diagnosis for trucks: test batteries, alternator output, starter draw, and grounds to find the real problem fast.
Engine & CoolingCascadia Coolant Leaks: Finding the Source Before Overheat
Step-by-step guide to tracing coolant leaks in Freightliner Cascadia engines using pressure testing and dye checks before overheating causes damage.
Air SystemAir Compressor Failure: Symptoms, Diagnosis & Fixes
Spot the warning signs of air compressor failure early—slow build-up, oil in tanks, noise—before it strands your truck. Full diagnostic guide.
Brakes & Wheel EndWheel Bearing Noise & Failure: Diagnosis Guide for Trucks
Spot wheel bearing noise and failure early—humming, grinding, wobble—before a hub separation. Diagnosis and repair steps for fleet trucks.
Brakes & Wheel EndSlack Adjuster & Brake Chamber Problems: Diagnosis Guide
Diagnose brake drag or pull from a worn slack adjuster, weak chamber, or bad stroke before your next DOT inspection.
Engine & CoolingTurbocharger Failure Symptoms: Boost Leaks, Whine & Diagnosis
Learn turbocharger failure symptoms—whine, smoke, low power, oil in intake—plus boost leak testing and rebuild vs replace advice.
Suspension & SteeringSteer Tire Wear Patterns: What They're Telling You About Alignment
Learn to read steer tire wear patterns—cupping, feathering, edge wear—to catch alignment and suspension issues early.
Aftertreatment & EmissionsDPF Clogged? Diesel Particulate Filter Symptoms & Fixes
Spot DPF clogging symptoms early and fix them right — power loss, excess regens, derate warnings, and step-by-step repair options.
Suspension & SteeringKingpin & Bushing Wear: Steer Axle Play Diagnosis Guide
Learn the dial indicator method to check kingpin and bushing wear on your steer axle before DOT pulls you out of service.
Electrical & LightingTrailer ABS Light Won't Go Off? Sensor & Wiring Guide
Trailer ABS light staying on? Step-by-step diagnosis for wheel speed sensors, harness corrosion, and ECU faults before you swap parts.
Aftertreatment & EmissionsDEF Lines Freezing? Winter Prevention & Fixes for Fleets
Stop DEF freezing from parking your trucks. Learn heated-line checks, tank insulation, and SCR fault codes before derates hit.
Trailer & CouplingFifth Wheel Not Locking? Diagnosis & Fix Before It Fails
Fifth wheel not locking or slipping under load? Step-by-step diagnosis, lube checks, jaw inspection, and rebuild-vs-replace guidance.
Brakes & Wheel EndGrease vs. Oil-Bath Hubs: Picking the Right Wheel End Lube
Grease vs oil-bath wheel end lube compared: intervals, seal compatibility, and failure signs to help you pick right for your fleet.
Air SystemAir Dryer Not Purging? Diagnose Wet Tanks Fast
Air dryer not purging? Diagnose wet tanks fast with checks for governor, purge valve, heater, and cartridge on trucks like Kenworth T800.
