OBD-II reference
Don't buy the expensive part first.
OBD-II trouble codes, diagnosed in the right order
Look up a stored code to understand the monitored condition, possible causes and diagnostic checks. Each guide includes an illustrative graph, repair budgets and videos where available. Match the guidance to your vehicle's service procedure before carrying out a test.
A code reports a failed test, not a broken part. The test results help you choose which repair is justified.
All codes 30
P0011
"A" Camshaft Position - Timing Over-Advanced or System Performance (Bank 1)
A starting point: Low, dirty or incorrect-viscosity engine oil
P0012
"A" Camshaft Position - Timing Over-Retarded (Bank 1)
A starting point: Incorrect oil level, specification or service condition
P0016
Crankshaft/Camshaft Position Correlation (Bank 1, Sensor A)
A starting point: Oil condition or recent timing work
P0101
Mass or Volume Air Flow "A" Circuit Range/Performance Problem
A starting point: Contaminated mass-airflow sensing element
P0113
Intake Air Temperature Sensor 1 Circuit High Input
A starting point: Loose connector, pushed-out pin or spread terminal
P0128
Coolant Thermostat Below Regulating Temperature
A starting point: Thermostat stuck open
P0135
Oxygen Sensor Heater Circuit Malfunction (Bank 1, Sensor 1)
A starting point: Open or failed heater inside Bank 1 Sensor 1
P0141
Oxygen Sensor Heater Circuit Malfunction (Bank 1, Sensor 2)
A starting point: Open or failed heater inside Bank 1 Sensor 2
P0171
System Too Lean (Bank 1)
A starting point: Unmetered air leak — vacuum hose, intake boot or gasket
P0172
System Too Rich (Bank 1)
A starting point: Leaking or over-delivering fuel injector
P0174
System Too Lean (Bank 2)
A starting point: Intake or vacuum leak affecting bank two
P0234
Turbocharger/Supercharger A Overboost Condition
A starting point: Sticking wastegate, bypass valve or variable turbo vanes
P0299
Turbocharger/Supercharger A Underboost Condition
A starting point: Split, loose or swollen charge-air hose or intercooler leak
P0300
Random / Multiple Cylinder Misfire Detected
A starting point: Worn spark plugs, coils or leads
P0301
Cylinder 1 Misfire Detected
A starting point: Worn or fouled spark plug on cylinder one
P0325
Knock Sensor 1 Circuit (Bank 1 or Single Sensor)
A starting point: Chafed, open or shorted wiring and loose connector terminals
P0335
Crankshaft Position Sensor A Circuit Malfunction
A starting point: Failed crankshaft position sensor
P0340
Camshaft Position Sensor A Circuit (Bank 1 or Single Sensor)
A starting point: Failed camshaft position sensor
P0401
Exhaust Gas Recirculation Flow Insufficient
A starting point: Carbon-blocked EGR passages
P0420
Catalyst System Efficiency Below Threshold (Bank 1)
A starting point: Catalytic converter genuinely worn out
P0430
Catalyst System Efficiency Below Threshold (Bank 2)
A starting point: Bank 2 catalytic converter genuinely degraded
P0441
Evaporative Emission System Incorrect Purge Flow
A starting point: Loose, damaged, restricted or misrouted purge hoses
P0442
Evaporative Emission System Leak Detected (Small Leak)
A starting point: Fuel-cap seal hardened, cracked or contaminated
P0446
Evaporative Emission System Vent Control Circuit
A starting point: Damaged vent hoses, filters or connectors
P0449
Evaporative Emission System Vent Valve/Solenoid Circuit
A starting point: Damaged vent wiring or connector
P0455
Evaporative Emission System Leak Detected (Large Leak)
A starting point: Fuel cap, capless inlet or filler-neck sealing fault
P0456
Evaporative Emission System Leak Detected (Very Small Leak)
A starting point: Loose cap, damaged cap seal or capless filler seal
P0496
Evaporative Emission System High Purge Flow
A starting point: Purge valve leaking in its closed state
P0507
Idle Control System RPM Higher Than Expected
A starting point: Vacuum or intake leak bypassing the throttle
P0700
Transmission Control System Malfunction (MIL Request)
A starting point: A specific sensor, solenoid or pressure DTC stored in the TCM