OBD2 Diagnostic Trouble Codes — What they mean and what to do.

Your catalytic converter isn't cleaning exhaust as well as it should. The downstream oxygen sensor is seeing almost the same reading as the upstream sensor, meaning the catalyst isn't doing its job.

Common causes

• Aging or contaminated catalytic converter
• Leaking exhaust manifold or pipe before the catalyst
• Faulty downstream oxygen sensor (Bank 1, Sensor 2)
• Engine misfire depositing unburned fuel into the catalyst
• Failed O2 sensor heater circuit

Drivability

The engine is running with too much air and not enough fuel on Bank 1. The powertrain control module has compensated as far as it can and is still seeing a lean mixture.

Common causes

• Vacuum leak (intake manifold, PCV hose, brake booster line)
• Dirty or failing mass airflow (MAF) sensor
• Weak fuel pump or clogged fuel filter
• Stuck-open PCV valve
• Leaking injector seal

Drivability

The engine control module has detected misfires happening across multiple cylinders without a clear pattern. This is the engine telling you combustion timing is off in more than one place.

Common causes

• Worn spark plugs or bad ignition coils
• Vacuum leak affecting air/fuel mixture
• Low fuel pressure from a weak pump or clogged filter
• Carbon-fouled or mechanically damaged engine
• Bad crankshaft position sensor signal

Drivability

The intake camshaft on Bank 1 is running more advanced than the ECM commanded. Variable valve timing isn't landing where it should, usually indicating a control or oil-flow problem.

Common causes

• Sludged or low engine oil blocking the VVT solenoid
• Failing VVT (variable valve timing) solenoid
• Stretched timing chain and worn guides
• Faulty camshaft position sensor
• Dirty engine oil past its service interval

Drivability

The EVAP system (which traps fuel vapors from the tank) has detected a sizeable leak, often a loose or missing gas cap. It's usually not urgent but the check engine light stays on until cleared.

Common causes

• Loose, cracked, or missing fuel cap
• Split or disconnected EVAP hose
• Failed EVAP purge or vent valve
• Damaged charcoal canister
• Leaking fuel filler neck seal

Drivability

The EVAP system has a general fault — somewhere vapor isn't flowing the way it should. It's a broad code that requires pinpointing which valve, hose, or sensor is off.

Common causes

• Faulty purge valve or vent valve
• Cracked EVAP hose or loose connection
• Bad fuel tank pressure sensor
• Failed charcoal canister
• Wiring damage to an EVAP component

Drivability

The upstream oxygen sensor on Bank 1 is responding too slowly to changes in exhaust oxygen. It's aging out or contaminated, and it controls the fuel mixture the engine runs on.

Common causes

• Aging O2 sensor nearing end of service life
• Oil or coolant contamination of the sensor element
• Exhaust leak between engine and sensor
• Corroded sensor harness connector
• Weak sensor heater circuit

Drivability

The MAF (mass airflow) sensor is reporting lower airflow than the engine is actually ingesting. The ECM falls back to a default map, which usually means a rough idle and reduced power.

Common causes

• Dirty or contaminated MAF sensor element
• Damaged MAF sensor wiring harness
• Intake leak after the MAF (false-low reading)
• Failed MAF sensor
• Aftermarket intake without proper MAF recalibration

Drivability

The engine isn't reaching its normal operating temperature fast enough or is running cooler than it should. The thermostat is stuck open, or the coolant temp sensor is misreading.

Common causes

• Stuck-open thermostat
• Faulty engine coolant temperature (ECT) sensor
• Low coolant level from a slow leak
• Air pocket trapped in the cooling system
• Wiring fault at the ECT sensor connector

Drivability

The secondary air injection system — which pumps fresh air into the exhaust on cold start to warm the catalyst — has an open circuit in its switching valve. Cold-start emissions go up.

Common causes

• Failed secondary air injection switching valve
• Open or shorted wiring to the valve
• Corroded or loose valve connector
• Failed air injection pump
• ECM driver failure (less common)

Drivability

The transmission control module has flagged an internal fault and asked the engine control module to light the check engine light. There's a stored transmission code waiting to be pulled.

Common causes

• Underlying transmission solenoid fault (often P0755, P0760)
• Low or degraded transmission fluid
• Wiring fault to the TCM
• Failed transmission range sensor
• Torque converter issue

Drivability

The engine computer has detected an internal fault in its own processor. This is unusual and usually points to a deep electrical or module-level issue.

Common causes

• Internal ECM/PCM failure
• Damaged wiring or corroded connectors at the ECM
• Voltage spikes from a failing alternator
• Weak battery causing brownouts during cranking
• Aftermarket module programming corruption

Drivability

The exhaust-side camshaft on Bank 1 is running more advanced than commanded. Same family of problems as P0011 but on the exhaust cam.

Common causes

• Exhaust VVT solenoid failure
• Sludged engine oil restricting VVT oil flow
• Stretched timing chain
• Faulty exhaust camshaft position sensor
• Oil control valve (OCV) screen clogged

Drivability

The turbocharger isn't making as much boost as the ECM commanded. Either a leak, a stuck wastegate, or a failing turbo is letting boost pressure leak away.

Common causes

• Boost leak at intercooler piping or charge pipe
• Stuck or failed wastegate actuator
• Failing turbocharger (seal or bearing)
• Faulty boost pressure sensor
• Stuck turbo bypass valve

Drivability

The camshaft position sensor on Bank 2 has a fault in its circuit. The ECM loses reliable timing reference on half the engine, causing driveability issues.

Common causes

• Failed camshaft position sensor
• Damaged sensor wiring or connector
• Reluctor ring damage on the camshaft
• Stretched timing chain (sensor out of position)
• Corroded sensor ground

Drivability

The idle air control valve isn't letting enough air bypass the throttle plate at idle, so the engine idles lower than the ECM expects. Common on older cars with a dedicated IAC valve.

Common causes

• Carbon buildup around the throttle body and IAC passage
• Failing idle air control (IAC) valve
• Vacuum leak compensating the wrong direction
• Faulty throttle position sensor
• Dirty throttle plate (drive-by-wire systems)

Drivability

The engine oil pressure sensor is reporting a reading outside its expected range — either the sensor is drifting or the actual oil pressure is off. Don't ignore this.

Common causes

• Failing oil pressure sensor
• Actual low oil pressure from a worn pump
• Oil pressure relief valve stuck open
• Worn engine bearings lowering pressure
• Wiring or connector issue at the sensor

Drivability

The intake manifold runner control flap is stuck in the open position instead of switching at the right RPM. Typically a Ford-style variable intake fault.

Common causes

• Failed IMRC actuator motor
• Broken or disconnected IMRC linkage
• Carbon buildup jamming the runner flap
• Open or shorted wiring to the IMRC solenoid
• Failed intake manifold runner control module

Drivability

The two position sensors inside the electronic throttle body disagree with each other. ECM can't trust the throttle reading and usually forces the engine into limp mode.

Common causes

• Faulty throttle position sensor pair inside the throttle body
• Damaged throttle body wiring harness
• Pedal position sensor mismatch
• Corroded throttle body connector
• ECM driver fault (less common)

Drivability

The MAF sensor is reporting airflow outside its expected range for current engine conditions. Often the sensor is on its way out or contaminated, not yet failed to the point of P0102.

Common causes

• Dirty or oil-contaminated MAF sensor
• Air filter restriction causing altered flow
• Vacuum leak skewing the reading
• Failing MAF sensor approaching end of life
• Aftermarket cold-air intake without recalibration

Drivability