Diagnostics
OBD-II diagnostic trouble codes
A code tells you which circuit or system reported a fault — not which part to replace. Each guide covers what the code actually means, the checks that come before parts, and realistic repair costs.
Codes in the library
Generic codes use the same definition on every manufacturer.
- P0420ModerategenericCatalytic Converter Efficiency Low (Bank 1)Catalyst System Efficiency Below Threshold (Bank 1). The engine control module compared the oxygen sensor signals before and after the bank 1 catalytic converter and determined the converter is no longer storing and releasing oxygen well enough to meet the emissions standard the vehicle was certified to.Typical repair $150–$2,800
- P0300HighgenericRandom or Multiple Cylinder MisfireRandom/Multiple Cylinder Misfire Detected. The engine control module has counted combustion failures across more than one cylinder, or misfires it cannot attribute to any single cylinder, based on momentary variations in crankshaft acceleration between firing events.Typical repair $150–$1,200
- P0301HighgenericCylinder 1 MisfireCylinder 1 Misfire Detected. The engine control module has identified repeated combustion failures in cylinder number 1 specifically, measured as a momentary slowdown in crankshaft acceleration at the point in the firing order where that cylinder should have delivered power.Typical repair $80–$4,000
- P0303HighgenericCylinder 3 MisfireCylinder 3 Misfire Detected. The engine control module has counted repeated combustion failures in cylinder number 3, identified by a momentary loss of crankshaft acceleration at that cylinder's position in the firing order.Typical repair $90–$4,200
- P0171ModerategenericEngine Running Lean (Bank 1)System Too Lean (Bank 1). Long-term fuel trim on bank 1 has reached the maximum positive correction the engine control module is permitted to apply — typically around +25% — meaning the computer has run out of authority to add enough fuel to reach the target air-fuel ratio.Typical repair $20–$1,000
- P0455LowgenericLarge Fuel Vapor (EVAP) System LeakEvaporative Emission Control System Leak Detected (Large Leak). During its leak-detection test the engine control module could not develop or hold the required pressure or vacuum in the sealed fuel vapor system, indicating an opening substantially larger than 0.040 inch — a gross leak.Typical repair $15–$900
- P0442LowgenericSmall Fuel Vapor (EVAP) System LeakEvaporative Emission Control System Leak Detected (Small Leak). The engine control module ran its leak-detection test and measured a pressure or vacuum decay rate consistent with an opening of approximately 0.040 inch — too small to see or smell, large enough to exceed the federal standard.Typical repair $15–$900
How to read a trouble code
Every code is one letter and four digits. The letter tells you which family of systems reported the fault; the first digit tells you whether the definition is shared across manufacturers or specific to yours.
- PP — PowertrainEngine, transmission, fuel and emissions.
- BB — BodyAirbags, lighting, climate control and comfort systems.
- CC — ChassisABS, traction control, steering and suspension.
- UU — NetworkCommunication faults between control modules.
Before you replace anything
- 1Write the code down before clearing it. Clearing wipes the freeze-frame data that shows the conditions when the fault occurred.
- 2Check for more than one code. A single root cause often sets several codes, and the cheapest fix is usually the one that explains all of them.
- 3Read live data, not just the code. Fuel trims, sensor voltages and misfire counters separate a failing sensor from a genuine mechanical fault.
- 4Fix the cause, then verify. The code should stay away through a full drive cycle before you call the job done.
Not sure a code is even stored? Start with the check engine light.