Misfire
Ford EcoBoost P0301 Misfire Under Boost: Coil Swap Test Before Buying Injectors
This diagnostic guide explains why a P0301 misfire only occurs under boost, how to perform a safe ignition coil swap test, and when professional injector...
Repair & Diagnostics
OBD-II Codes
Search common OBD-II trouble codes, check severity, symptoms, likely causes, and first inspection steps before replacing parts.
Repair & Diagnostics
Latest in OBD-II Codes
Browse the newest obd-ii codes resources and keep your diagnosis path focused on the right checks first.
Braking
Audi P0299 Underboost on Highway: Diverter Valve, Wastegate, or Boost Leak?
A P0299 code on your Audi, especially when it only appears during highway merging or hill climbs, points to a calculated air charge not meeting the target. In...
Emissions
P0128 Code in Cold Weather: Thermostat Stuck Open or Bad Coolant Sensor?
In brief: P0128 means the engine is not reaching the expected coolant temperature quickly enough or is operating below the expected temperature range under...
Misfire
P0101 Code After Replacing the Air Filter: MAF Sensor or Intake Leak?
Triggering a P0101 code immediately after an air filter change is common and almost always due to forgetting to plug the MAF sensor back in, leaving a hose...
EVAP
Can I Drive With a P0455 Code? EVAP Leak Safety, Inspection, and Fix Cost
A P0455 code means your vehicle's computer has detected a gross leak in the EVAP system. It is generally safe for daily driving, but it should be addressed soon...
Misfire
P0171 System Too Lean but No Vacuum Leak Found: What to Test Next
In brief: If P0171 remains after no vacuum leak is found, the next tests should usually focus on fuel trim data, MAF or MAP plausibility, fuel pressure and...
EVAP
P0455 EVAP Large Leak but Gas Cap Is Tight: What to Check Next
A tight gas cap doesn't always fix a P0455 code. Here are the most common secondary causes, including the purge valve, vent solenoid, and cracked EVAP lines...
Misfire
P0171 and Rough Idle: MAF Sensor, Vacuum Leak, or Low Fuel Pressure?
Editorial illustration for Vacuum Leak Smoke Test.Editorial illustration for MAF Sensor Cleaning Spray.
Misfire
P0300 Random Misfire Only Under Load: Plugs, Coils, Vacuum Leak, or Fuel Issue?
In brief: A P0300 under load means the engine computer has detected random or multiple cylinder misfires, often during acceleration or higher engine load....
Misfire
P0420 Keeps Coming Back After Clearing: 9 Causes That Are Not Always the Cat
A P0420 code returning after being cleared means the engine computer still detects lower-than-expected efficiency in the exhaust system. While it often points...
Misfire
P0420 Code but the Car Runs Fine: O2 Sensor, Exhaust Leak, or Bad Catalytic Converter?
A P0420 code indicates your catalytic converter is operating below its efficiency threshold. If the car runs perfectly fine, the issue is often a faulty...
Engine
Volvo S40 B5244S4 Engine Most Common Error Codes and DTC Guide
B5244S4-related codes should be checked by code family first: misfire, fuel trim, EVAP, oxygen sensor, catalyst efficiency, and intake-air leaks.