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P0300: misfire on more than one cylinder
The control unit has detected a misfire in one or more cylinders without being able to tie it to a specific one.
Get it checked soonFault code
OBD-II fault codes, explained
The most common fault codes, sorted by urgency. A code points to a system, not to a part.
Reviewed by Amara MusaMore about Amara Musa
Amara Musa is a light-vehicle technician, technical manager and periodic vehicle inspection controller, with over 20 years of hands-on workshop experience and experience running a workshop. Trained in Norway, with EV training and basic and advanced diagnostics training.
Last reviewed 13 September 2026. Content taken from OBD-II Codes Explained in Plain English and checked against it.
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Quick answers
Every OBD-II code follows the same five-character format. The first letter tells you which system the fault is in.
PPowertrain: engine and transmission
BBody: airbags, climate control, lighting
CChassis: brakes, suspension, steering
UNetwork and communication between the car's computers
The second character: a 0 generally indicates a generic code shared across manufacturers, a 1 typically a manufacturer-specific one. The codes on this page are generic P0 codes.
The car usually still drives, but the fault needs fixing. If the check engine light is flashing with a misfire code such as P0300 or P0301, reduce speed and get it checked immediately.
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P0301 to P0308: a misfire on one specific cylinder
The same fault as P0300, but the car has isolated it to one cylinder. The number in the code tells you which.
Get it checked soonFault code
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P0171: system too lean
The engine is getting too much air relative to fuel. Usually safe to drive in the short term, but have it checked soon.
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P0420: catalyst efficiency below threshold, bank 1
The catalytic converter is not reducing emissions as effectively as expected, judged by comparing the oxygen sensors before and after it.
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P0505: unstable idle
The system that controls idle speed is not behaving as expected. Have it checked soon, especially if the engine sometimes stalls at idle.
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P0700: a fault in the transmission control system
This is a general code. It means the transmission control unit has found a problem and stored more specific codes of its own.
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None of these is an emergency, but they will not clear themselves and each is worth fixing.
Information
P0455 and P0442: a leak in the EVAP system
The system that captures fuel vapour has a leak. By far the most common cause is a loose, cracked or missing fuel cap.
Not urgentFault code
Information
P0401: insufficient EGR flow
The EGR system is passing less exhaust gas than expected. Not urgent for safety, but have it checked.
Not urgentFault code
Information
P0128: the thermostat is stuck open
The engine is taking longer than expected to reach normal operating temperature, usually because the thermostat is stuck open.
Not urgentFault code
Information
P0135: the oxygen sensor heater circuit
The heater that brings the oxygen sensor quickly up to operating temperature is not working as it should.
Not urgentFault code
A misfire often comes down to worn spark plugs or a failing ignition coil, and a loose or missing fuel cap is one of the most common reasons for an EVAP leak code.
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P0300: misfire on more than one cylinder
The control unit has detected a misfire in one or more cylinders without being able to tie it to a specific one.
PetrolFault code
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P0301 to P0308: a misfire on one specific cylinder
The same fault as P0300, but the car has isolated it to one cylinder. The number in the code tells you which.
PetrolFault code
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P0171: system too lean
The engine is getting too much air relative to fuel. Usually safe to drive in the short term, but have it checked soon.
PetrolFault code
Information
P0455 and P0442: a leak in the EVAP system
The system that captures fuel vapour has a leak. By far the most common cause is a loose, cracked or missing fuel cap.
PetrolFault code
P2002 means the particulate filter is not cleaning as expected, and P0380 that the glow plugs or the circuit that controls them are not responding as they should.
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P2002: the particulate filter is not cleaning properly
The car measures the pressure drop across the particulate filter and finds it is not cleaning as expected.
DieselFault code
Information
P0380: a fault in the glow plug circuit
One or more glow plugs, or the circuit that controls them, is not responding as expected. You notice it as hard starting on cold mornings and rough running for the first few minutes.
DieselFault code
Electric and hybrid codes live in the P0A range, and many basic readers show them as a number with no description. P0A80 means the traction battery has degraded past the manufacturer's threshold.
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P0A80: traction battery degraded past the manufacturer's threshold
P0A80 means the traction battery has degraded past the manufacturer's threshold. It is not a sudden fault but ageing that has been going on for a long time.
HybridFault codeEV
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P0A00 to P0A0F: motor, inverter and their cooling
Codes from P0A00 to P0A0F cover an electric car's motor, the inverter that drives it, and the cooling for both.
HybridFault codeEV
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P0A94: DC-DC converter fault
P0A94 means a fault in the DC-DC converter. That is the part that takes power down from the traction battery to keep the 12-volt battery topped up: the alternator's job on an electric car.
HybridFault codeEV
An electric car still has an OBD-II port, but the useful codes live in the P0A range, and many basic readers show them as a number with no description.
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P0A80: traction battery degraded past the manufacturer's threshold
P0A80 means the traction battery has degraded past the manufacturer's threshold. It is not a sudden fault but ageing that has been going on for a long time.
Electric carsFault codeEV
Amber · book a service
P0A00 to P0A0F: motor, inverter and their cooling
Codes from P0A00 to P0A0F cover an electric car's motor, the inverter that drives it, and the cooling for both.
Electric carsFault codeEV
Amber · book a service
P0A94: DC-DC converter fault
P0A94 means a fault in the DC-DC converter. That is the part that takes power down from the traction battery to keep the 12-volt battery topped up: the alternator's job on an electric car.
Electric carsFault codeEV
Fault code guides for hydrogen cars.
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