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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P0506: idle speed lower than expected and the stall that follows
The ECU is holding the throttle where idle should be correct, and the engine is turning slower than that. Something is taking torque, or something is restricting air.
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P0507: idle speed higher than expected — hunting the vacuum leak
P0507 means the engine idles faster than the ECU commanded and it has run out of authority to bring it down. Almost every case is unmetered air entering somewhere.
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P0520 to P0523: oil pressure sensor circuit — sensor or actual pressure?
This is the one category where guessing wrong destroys the engine. Treat every oil pressure warning as real until you have proved it is the sensor.
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P0606: PCM processor fault — when the computer accuses itself
P0606 is the ECU reporting its own internal self-test failure. Before condemning a control module, rule out the power supply and grounds that produce identical symptoms for a fraction of the price.
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P0700: the transmission code that is only a pointer
P0700 is not a fault. It is the engine ECU telling you the transmission module has stored something, and that you are reading the wrong module.
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