Emission control codes
P04xx and P2xxx: catalyst, EVAP, EGR, DPF and oxygen sensor monitoring. ← Browse sections
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P2004 to P2010: intake manifold runner control stuck and circuit faults
Flaps inside the intake manifold change how air swirls into the cylinder. On a diesel they seize with carbon, and the failure has a mechanical worst case worth knowing.
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P2016 and P2017: intake manifold runner position sensor
The feedback half of the intake flap system. Without it the ECU is commanding flaps blind, and it will not do that for long.
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P2031 to P2036: exhaust gas temperature sensor bank 1 sensor 2
A modern diesel carries several exhaust temperature sensors along its after-treatment system. Which one has failed determines what stops working.
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P2074: MAP/MAF and throttle position correlation
Three sensors that should agree about how much air is entering the engine. When they do not, the ECU cannot tell which one is lying — but you can.
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P2088 and P2089: camshaft actuator A circuit low and high
The electrical half of the variable valve timing system. Low or high circuit means a wiring problem — but the reason it failed is usually still the oil.
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P2098 and P2099: post-catalyst fuel trim bank 2 lean and rich
Bank 2 versions of a code that catches people out, because it is about a correction the ECU makes after the catalyst — not about the catalyst itself.
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P2200 to P2204: NOx sensor circuit on a modern diesel
The most expensive sensor on a modern diesel, and the one that stops AdBlue dosing when it fails. Neither of those facts is obvious from the code.
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P2237 to P2240: wideband oxygen sensor pump current circuit
Wideband sensors work differently from the switching sensors most guides describe, and testing them the old way gives an answer that looks alarming and means nothing.
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P2279: intake air system leak — the code that names the fault
Most codes describe a symptom and leave you to find the cause. This one states the cause outright, which makes the search unusually direct.
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P2452 and P2453: DPF differential pressure sensor circuit
The whole DPF strategy rests on one pressure measurement. When that sensor is wrong, the ECU either regenerates constantly or never regenerates at all.
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P2459: DPF regeneration frequency — the code that says you drive wrong
Not a component failure. The ECU has noticed the filter is regenerating far more often than it should, and that is usually about journeys rather than parts.
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P0400 and P0403: EGR flow malfunction and control circuit
P0400 says the exhaust gas is not flowing. P0403 says the valve is not being commanded properly. One is usually carbon; the other is usually electrical.
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P0404: EGR position sensor range/performance explained
P0404 compares commanded EGR position against measured position. Carbon, not electronics, is behind most of them — and cleaning beats replacing far more often than parts sellers admit.
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P0405 and P0406: EGR position sensor A low and high
These are the feedback side of the EGR system. Without a trustworthy position signal the ECU is commanding a valve blind, and it will not do that for long.
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P0411 to P0414: secondary air injection flow and valve circuit
The system only runs for a minute after a cold start, which is why the fault is nearly invisible and the code arrives with no symptom at all.
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P0421 and P0422: warm-up catalyst efficiency below threshold
A separate code for the close-coupled catalyst, and a separate meaning: this one is usually about how the engine behaves in the first minute, not about the converter.
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P0440 and P0446: EVAP purge and vent valve faults
These two codes point at the valves that seal and breathe the fuel vapour system. A stuck purge valve is one of the few EVAP faults you can actually feel while driving.
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P0441: EVAP incorrect purge flow — the valve that never opens
Not a leak code. This one says the purge valve is not moving the vapour it should, which is a different repair from every other EVAP code.
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P0442: small EVAP leak — finding a hole you cannot see
P0442 means a leak around 1 mm across somewhere in the fuel vapour system. Guessing costs more than testing, and the fuel cap is innocent more often than the internet claims.
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P0443 to P0445: EVAP purge valve control circuit
Electrical codes for a valve that is easy to reach and easy to test. This is one of the more DIY-friendly items on the EVAP system.
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P0447 to P0449: EVAP vent control circuit and the solenoid under the car
The vent valve is the forgotten half of the EVAP system, mounted underneath in the salt spray. Corrosion, not electronics, is what usually kills it.
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P0451 to P0453: EVAP pressure sensor range and circuit
This sensor is the instrument that finds EVAP leaks. When it is wrong, the system reports leaks that are not there — or misses ones that are.
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P0456: very small EVAP leak and the loose fuel cap myth
P0456 chases a hole around 0.5 mm across. It is the most frequently misdiagnosed emissions code on the road, and the fuel cap is responsible far less often than everyone assumes.
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P0460 to P0463: fuel level sensor circuit faults
A wrong fuel gauge is annoying. What people miss is that the EVAP monitor needs a correct fuel level to run at all, so this code quietly blocks an emissions test.
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P0457: EVAP leak detected, fuel cap loose — the cheapest code there is
A code that names its own fix. Tighten the cap, drive for a few days, and it clears itself — unless the cap seal has hardened, which is the other half of the story.
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P0480 and P0481: cooling fan relay control circuit
A cooling fan that does not run produces a very specific pattern: fine on the motorway, overheating in traffic. That pattern is the diagnosis.
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P0496: excessive EVAP purge flow during non-purge conditions
P0496 means vapour is being drawn into the engine when it should not be. Unlike most EVAP codes it has real drivability symptoms, and a stuck purge valve causes almost all of them.
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P2002 and P2463: DPF efficiency below threshold and soot accumulation
One code says the filter is not filtering, the other says it is too full. Both usually trace back to regeneration that keeps starting and never finishes.
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P2096 and P2097: post-catalyst fuel trim too lean or too rich
These codes come from the downstream sensor asking the ECU to correct the mixture. They are frequently mistaken for catalyst failure when the real cause is an exhaust leak.
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P2270 and P2271: O2 signal stuck lean or rich after the catalyst
The downstream sensor should sit steady but still respond when the mixture is forced. These codes mean it stopped responding at all, and an exhaust leak imitates the symptom perfectly.
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P0455: EVAP large leak — 10 reasons why "just the gas cap" isn't always right
Deep dive into P0455: how OBD-II detects a large leak, difference from P0442/P0456, EVAP smoke test, why gas cap first, what to do with the charcoal canister.
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P0430: catalyst bank 2 — why the "other side" of the engine fails separately and how to diagnose
Difference between P0420 and P0430, when both appear together vs separately, how OBD-II sees bank 2, why V-engines fail asymmetrically, and repair cost.
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P0420: Why the Catalytic Converter "Fails"
A detailed breakdown of code P0420: what "catalyst efficiency below threshold" means, common causes, how to diagnose it, repair costs, and whether you can keep driving.