OBD2 engine codes explained in plain English

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Check engine light on? Your OBD2 scanner shows a code like P0300 or P0420 — but what does it mean? This page gives plain-language meanings for 144 common generic SAE diagnostic trouble codes (DTCs) covering engine, emissions, and transmission.

These are generic codes (often P0xxx) shared across OBD2 vehicles — not every manufacturer-specific code a car can store. In the Carspel phone app, an engine health check with an ELM327 adapter explains codes on this list; other codes still appear with the code shown and no reviewed description yet.

How to read an OBD2 / DTC code

A diagnostic trouble code is usually five characters. The letter is the system: P = powertrain (engine and transmission), C = chassis, B = body, U = network. A 0 in the second place marks a generic SAE code (same meaning on most brands); a 1 often marks a manufacturer-specific code outside this list.

Common examples: P0300 (misfires), P0420 (catalytic converter), P0457 (often a loose gas cap). The table below adds drive-or-stop style guidance per code — not a shop diagnosis.

Frequently asked questions

What is an OBD2 engine code (DTC)?

A diagnostic trouble code is a five-character code your vehicle's computer stores when it detects a fault. OBD2 scanners and phone apps with an ELM327 adapter read these codes so you can see why the check engine light came on.

Does Carspel explain every engine code?

No. This page covers 144 reviewed generic SAE powertrain codes (engine, emissions, transmission). Manufacturer-specific and other DTCs may still appear in a reading with the code shown and no reviewed description yet.

Can I keep driving with the check engine light on?

It depends on the code. The table below gives a default: safe to drive, book service soon, or stop driving. A flashing light with a misfire means stop. Several codes together can be more urgent than one alone.

How do I read these codes with Carspel?

With a Bluetooth ELM327 OBD2 adapter, Carspel's phone engine health check (Ultimate plan) reads codes and explains ones on this list. You can also share a more detailed PDF with a mechanic: the same plain-language reading, plus technical detail such as stored, pending and permanent codes, battery voltage, coolant, fuel mixture, the VIN the car reports, emissions self-check status, and recent service on the vehicle. The PDF is not a repair record. See the User Guide, Ultimate Section 9, for connect steps.

Guidance here is a default for that code alone, not a diagnosis. Several codes together, or a flashing check-engine light, can mean more urgency. Not every listed code applies to every car (for example diesel soot-filter codes). A flashing light with a misfire means stop driving. How to connect an adapter: User Guide, Ultimate Section 9.

Misfires & ignition

Code Description Guidance
P0300 Engine misfire detected, not narrowed to one cylinder Book service soon — stop if the light is flashing
P0301 Cylinder 1 misfire detected Book service soon — stop if the light is flashing
P0302 Cylinder 2 misfire detected Book service soon — stop if the light is flashing
P0303 Cylinder 3 misfire detected Book service soon — stop if the light is flashing
P0304 Cylinder 4 misfire detected Book service soon — stop if the light is flashing
P0305 Cylinder 5 misfire detected Book service soon — stop if the light is flashing
P0306 Cylinder 6 misfire detected Book service soon — stop if the light is flashing
P0307 Cylinder 7 misfire detected Book service soon — stop if the light is flashing
P0308 Cylinder 8 misfire detected Book service soon — stop if the light is flashing
P0316 Misfire detected in the first moments after starting Book service soon — stop if the light is flashing
P0325 Knock sensor fault (bank 1) Book service soon
P0327 Knock sensor signal is lower than expected Book service soon
P0328 Knock sensor signal is higher than expected Book service soon
P0350 Ignition coil circuit fault, coil not identified Book service soon
P0351 Ignition coil A circuit fault Book service soon
P0352 Ignition coil B circuit fault Book service soon
P0353 Ignition coil C circuit fault Book service soon
P0354 Ignition coil D circuit fault Book service soon
P0355 Ignition coil E circuit fault Book service soon
P0356 Ignition coil F circuit fault Book service soon
P0357 Ignition coil G circuit fault Book service soon
P0358 Ignition coil H circuit fault Book service soon

Engine timing

Code Description Guidance
P0011 Camshaft timing is further ahead than expected (bank 1) Book service soon
P0014 Camshaft timing is further behind than expected (bank 1) Book service soon
P0016 Crankshaft and camshaft signals do not agree (bank 1, sensor A) Book service soon
P0017 Crankshaft and camshaft signals do not agree (bank 1, sensor B) Book service soon
P0335 Crankshaft position sensor fault Book service soon
P0336 Crankshaft position sensor signal is irregular Book service soon
P0340 Camshaft position sensor fault Book service soon
P0341 Camshaft position sensor signal is irregular Book service soon

Fuel mixture & air intake

Code Description Guidance
P0087 Fuel pressure is lower than the engine expects Book service soon
P0100 Airflow sensor reading is unreliable Book service soon
P0101 Airflow sensor reading is out of the normal range Book service soon
P0102 Airflow sensor signal is lower than expected Book service soon
P0103 Airflow sensor signal is higher than expected Book service soon
P0106 Intake pressure sensor reading is out of range Book service soon
P0107 Intake pressure sensor signal is lower than expected Book service soon
P0108 Intake pressure sensor signal is higher than expected Book service soon
P0110 Intake air temperature sensor fault Safe to drive
P0111 Intake air temperature reading is out of range Safe to drive
P0112 Intake air temperature signal is lower than expected Safe to drive
P0113 Intake air temperature signal is higher than expected Safe to drive
P0171 Engine is running leaner than expected (bank 1) Book service soon
P0172 Engine is running richer than expected (bank 1) Book service soon
P0174 Engine is running leaner than expected (bank 2) Book service soon
P0175 Engine is running richer than expected (bank 2) Book service soon

Oxygen sensors

Code Description Guidance
P0130 Oxygen sensor signal fault (bank 1, sensor 1) Book service soon
P0131 Oxygen sensor signal is lower than expected (bank 1, sensor 1) Book service soon
P0132 Oxygen sensor signal is higher than expected (bank 1, sensor 1) Book service soon
P0133 Oxygen sensor is responding slower than expected (bank 1, sensor 1) Book service soon
P0134 Oxygen sensor shows no activity (bank 1, sensor 1) Book service soon
P0135 Oxygen sensor heater fault (bank 1, sensor 1) Book service soon
P0136 Oxygen sensor signal fault (bank 1, sensor 2) Book service soon
P0137 Oxygen sensor signal is lower than expected (bank 1, sensor 2) Book service soon
P0138 Oxygen sensor signal is higher than expected (bank 1, sensor 2) Book service soon
P0140 Oxygen sensor shows no activity (bank 1, sensor 2) Book service soon
P0141 Oxygen sensor heater fault (bank 1, sensor 2) Book service soon
P0150 Oxygen sensor signal fault (bank 2, sensor 1) Book service soon
P0151 Oxygen sensor signal is lower than expected (bank 2, sensor 1) Book service soon
P0152 Oxygen sensor signal is higher than expected (bank 2, sensor 1) Book service soon
P0153 Oxygen sensor is responding slower than expected (bank 2, sensor 1) Book service soon
P0154 Oxygen sensor shows no activity (bank 2, sensor 1) Book service soon
P0155 Oxygen sensor heater fault (bank 2, sensor 1) Book service soon
P0156 Oxygen sensor signal fault (bank 2, sensor 2) Book service soon
P0157 Oxygen sensor signal is lower than expected (bank 2, sensor 2) Book service soon
P0158 Oxygen sensor signal is higher than expected (bank 2, sensor 2) Book service soon
P0160 Oxygen sensor shows no activity (bank 2, sensor 2) Book service soon
P0161 Oxygen sensor heater fault (bank 2, sensor 2) Book service soon

Catalytic converter

Code Description Guidance
P0420 Catalytic converter isn't cleaning exhaust as well as expected (bank 1) Book service soon
P0421 Catalytic converter warm-up efficiency below expected (bank 1) Book service soon
P0430 Catalytic converter isn't cleaning exhaust as well as expected (bank 2) Book service soon
P0431 Catalytic converter warm-up efficiency below expected (bank 2) Book service soon

Fuel tank & gas cap

Code Description Guidance
P0440 Fuel vapor system fault detected Safe to drive
P0441 Fuel vapor system isn't purging correctly Safe to drive
P0442 Small fuel vapor leak detected Safe to drive
P0443 Fuel vapor purge valve circuit fault Safe to drive
P0446 Fuel vapor vent control circuit fault Safe to drive
P0449 Fuel vapor vent valve or solenoid circuit fault Safe to drive
P0451 Fuel vapor pressure sensor reading is out of range Safe to drive
P0455 Larger fuel vapor leak detected Safe to drive
P0456 Very small fuel vapor leak detected Safe to drive
P0457 Gas cap may be loose, damaged, or missing Safe to drive
P0460 Fuel gauge sensor fault Safe to drive
P0461 Fuel gauge sensor reading is out of range Safe to drive
P0462 Fuel gauge sensor signal is lower than expected Safe to drive
P0463 Fuel gauge sensor signal is higher than expected Safe to drive
P0464 Fuel gauge sensor signal is inconsistent Safe to drive

Cooling & temperature

Code Description Guidance
P0115 Coolant temperature sensor fault Book service soon
P0116 Coolant temperature reading is out of range Book service soon
P0117 Coolant temperature signal is lower than expected Book service soon
P0118 Coolant temperature signal is higher than expected Book service soon
P0125 Engine is taking too long to reach normal operating temperature Safe to drive
P0128 Thermostat is stuck open Safe to drive
P0217 Engine coolant temperature is too high Stop driving

Throttle & idle

Code Description Guidance
P0120 Throttle position sensor fault Book service soon
P0121 Throttle position reading is out of range Book service soon
P0122 Throttle position signal is lower than expected Book service soon
P0123 Throttle position signal is higher than expected Book service soon
P0505 Idle speed control system fault Book service soon
P0506 Idle speed is lower than expected Book service soon
P0507 Idle speed is higher than expected Book service soon

Battery & charging

Code Description Guidance
P0562 Charging system voltage is low Book service soon
P0563 Charging system voltage is high Book service soon

Diesel soot filter

Code Description Guidance
P2002 Soot filter is trapping less than it should (bank 1) Book service soon
P2003 Soot filter is trapping less than it should (bank 2) Book service soon
P242F Ash has built up in the soot filter Book service soon
P244A Pressure across the soot filter is lower than expected (bank 1) Book service soon
P244B Pressure across the soot filter is lower than expected (bank 2) Book service soon
P2458 A filter cleaning cycle ran longer than expected Book service soon
P2459 The soot filter is cleaning itself more often than it should Book service soon
P2463 Soot has built up in the filter Book service soon

Transmission & speed

Code Description Guidance
P0500 Vehicle speed sensor fault Book service soon
P0501 Vehicle speed sensor reading is out of range Book service soon
P0700 Transmission computer has detected a fault Book service soon
P0705 Gear selector position sensor fault Book service soon
P0710 Transmission fluid temperature sensor fault Book service soon
P0715 Transmission input speed sensor fault Book service soon
P0720 Transmission output speed sensor fault Book service soon
P0725 Engine speed signal to transmission is faulty Book service soon
P0730 Transmission isn't shifting into the correct gear ratio Book service soon
P0740 Torque converter lock-up fault Book service soon
P0741 Torque converter clutch isn't locking up as expected Book service soon
P0742 Torque converter clutch is stuck locked Book service soon
P0743 Torque converter clutch circuit electrical fault Book service soon
P0750 Shift solenoid A fault Book service soon
P0755 Shift solenoid B fault Book service soon
P0760 Shift solenoid C fault Book service soon
P0765 Shift solenoid D fault Book service soon
P0770 Shift solenoid E fault Book service soon

Engine computer

Code Description Guidance
P0600 Internal computer communication fault Book service soon
P0601 Engine computer memory error Book service soon
P0602 Engine computer programming error Book service soon
P0603 Engine computer memory retention fault Book service soon
P0604 Engine computer working memory (RAM) fault Book service soon
P0605 Engine computer program memory (ROM) fault Book service soon
P0606 Engine computer processor fault Book service soon

Other emission controls

Code Description Guidance
P0401 Exhaust gas recirculation system isn't flowing enough Book service soon
P0402 Exhaust gas recirculation system is flowing too much Book service soon
P0403 EGR valve circuit fault Book service soon
P0404 EGR valve position reading is out of range Book service soon
P0405 EGR position sensor signal is lower than expected Book service soon
P0406 EGR position sensor signal is higher than expected Book service soon
P0410 Secondary air injection system fault Safe to drive
P0411 Secondary air injection flow is incorrect Safe to drive

Engine health check in the User Guide →
Plans and adapter note →
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