P0101Diagnose promptly
Mass or volume airflow circuit range/performance
The controller judged the measured airflow signal implausible for the expected airflow model or operating condition.
Open evidence guide ↗
View plansThe code is a clue.
Keep the evidence.
Each reference separates the controller’s observation from possible conditions, the next useful evidence, and the safety screen. Start with the full code, status, reporting module, and exact definition.
P0420
Systemdefinition familysubsystem + detected condition
Do not buy a part from a code name.
A DTC identifies where a controller detected a problem. Wiring, power, ground, leaks, mechanical conditions, other inputs, and operating state can produce the same code.
Jump to systemAir measurementAir-fuel feedbackCatalyst monitoringCombustion qualityCombustion sensingEGR emissions controlEVAP vapor containmentElectrical supplyElectronic throttle controlEngine temperature controlEngine timing correlationFuel controlFuel deliveryHybrid battery controlIdle speed controlTemperature sensingTransmission controlVariable valve timingVehicle network communication
Air measurement
1 referenceAir-fuel feedback
2 referencesP0133Diagnose promptly
Oxygen sensor circuit slow response — Bank 1 Sensor 1
The controller determined that the upstream oxygen-sensor feedback changed more slowly than expected during its test.
Open evidence guide ↗P2195Diagnose promptly
Oxygen sensor signal biased or stuck lean — Bank 1 Sensor 1
The controller determined that the Bank 1 upstream feedback signal remained on the lean side or did not respond as expected.
Open evidence guide ↗Catalyst monitoring
2 referencesP0420Diagnose promptly
Catalyst system efficiency below threshold — Bank 1
The Bank 1 catalyst monitor calculated efficiency below its calibrated threshold during a qualifying test.
Open evidence guide ↗P0430Diagnose promptly
Catalyst system efficiency below threshold — Bank 2
The Bank 2 catalyst monitor calculated efficiency below its calibrated threshold during a qualifying test.
Open evidence guide ↗Combustion quality
2 referencesP0300Safety screen first
Random or multiple-cylinder misfire detected
The controller detected crankshaft-speed variations consistent with misfire across multiple or non-fixed cylinders.
Open evidence guide ↗P0301Safety screen first
Cylinder 1 misfire detected
The controller attributed a qualifying misfire pattern to Cylinder 1.
Open evidence guide ↗Combustion sensing
1 referenceP0325Diagnose promptly
Knock sensor 1 circuit — Bank 1 or single sensor
The controller detected an electrical or performance problem in the Knock Sensor 1 path for the identified bank or single-sensor application.
Open evidence guide ↗EGR emissions control
1 referenceP0401Diagnose promptly
Exhaust gas recirculation flow insufficient detected
The controller did not observe the expected system response when exhaust gas recirculation flow was commanded or tested.
Open evidence guide ↗EVAP vapor containment
3 referencesP0442Condition dependent
Evaporative emission system leak detected — small leak
The controller detected a small loss of vacuum, pressure, or sealing performance in the evaporative-emissions system during its leak test.
Open evidence guide ↗P0455Safety screen first
Evaporative emission system leak detected — gross leak or no flow
The controller detected a large sealing failure or an expected flow/pressure change that did not occur during the evaporative-system test.
Open evidence guide ↗P0456Condition dependent
Evaporative emission system leak detected — very small leak
The controller detected a very small sealing loss during the evaporative-system integrity test.
Open evidence guide ↗Electrical supply
1 referenceP0562Safety screen first
System voltage low
A controller detected supply voltage below its calibrated threshold for the operating condition and duration.
Open evidence guide ↗Electronic throttle control
1 referenceP2135Safety screen first
Throttle/pedal position sensor A/B voltage correlation
The controller detected disagreement between redundant throttle-position or pedal-position signals beyond its allowed relationship.
Open evidence guide ↗Engine temperature control
1 referenceP0128Diagnose promptly
Coolant thermostat temperature below regulating temperature
The engine did not reach or maintain the controller’s expected coolant temperature within the modeled time and conditions.
Open evidence guide ↗Engine timing correlation
1 referenceP0016Safety screen first
Crankshaft/camshaft position correlation — Bank 1 Sensor A
The controller detected a crankshaft-to-camshaft timing relationship outside its expected window for Bank 1 intake camshaft sensing.
Open evidence guide ↗Fuel control
2 referencesP0171Diagnose promptly
System too lean — Bank 1
The controller reached a calibrated level of positive fuel correction on Bank 1 while trying to maintain its target mixture.
Open evidence guide ↗P0172Diagnose promptly
System too rich — Bank 1
The controller reached a calibrated level of negative fuel correction on Bank 1 while trying to maintain its target mixture.
Open evidence guide ↗Fuel delivery
1 referenceP0087Safety screen first
Fuel rail/system pressure too low
The controller detected fuel pressure below the expected or commanded level for the operating condition.
Open evidence guide ↗Hybrid battery control
2 referencesP0A80Safety screen first
Replace hybrid battery pack
The hybrid control system detected battery deterioration or imbalance meeting its calibrated threshold; the standardized label contains a repair instruction, but diagnosis still requires exact evidence and procedure.
Open evidence guide ↗P0A7FSafety screen first
Hybrid battery pack deterioration
The hybrid control system calculated battery-pack deterioration beyond a calibrated threshold.
Open evidence guide ↗Idle speed control
1 referenceP0507Diagnose promptly
Idle control system RPM higher than expected
The controller observed idle speed above its commanded or expected range under the qualifying condition.
Open evidence guide ↗Temperature sensing
1 referenceP0113Condition dependent
Intake air temperature circuit high input
The controller saw intake-air-temperature circuit voltage corresponding to an implausibly cold value or an open/high-input condition.
Open evidence guide ↗Transmission control
1 referenceP0700Safety screen first
Transmission control system malfunction indicator request
The powertrain controller received a request from the transmission controller to illuminate the malfunction indicator lamp because the transmission system stored a qualifying fault.
Open evidence guide ↗Variable valve timing
1 referenceP0011Diagnose promptly
A camshaft timing over-advanced or system performance — Bank 1
The controller detected that the commanded and observed intake-cam timing relationship on Bank 1 was too advanced or did not respond as expected.
Open evidence guide ↗Vehicle network communication
1 referenceU0100Safety screen first
Lost communication with ECM/PCM A
A controller reported that expected communication with the engine or powertrain control module was lost for a qualifying period.
Open evidence guide ↗NEED THE METHOD FIRST?Understand how
Understand how
DTCs mature.
Stored, pending, and permanent status changes what the next scan can tell you.
Read the status guide ↗