Reading and clearing codes
What the letters mean, pending versus stored, and when clearing is wrong. ← Browse sections
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Code aging: how a fault clears itself after 40 warm-up cycles
Codes do not stay forever. The ECU counts successful warm-ups and eventually forgets — which is useful to know before you buy a used car.
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P1xxx codes: why your scanner says "manufacturer specific" and what to do
The same number means different things on different cars. That is not a limitation of your tool — it is how the standard was written, and there is a way around it.
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Two-trip logic: why the lamp waits before it warns you
A fault detected once does not light the lamp. Understanding that delay explains why a problem can exist for days before you hear about it.
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P1000: OBD readiness not complete and why the test fails
P1000 is not a fault at all. It means the self-tests have not finished since the last code clear, and it is the most common reason a perfectly healthy car fails an emissions test.
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Pending, stored, and permanent: three kinds of code and why it matters
Not all codes are equal: one warns, one has already lit the lamp, and one cannot be cleared at all. The difference and how to use it.
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Clearing codes: when it helps and when it hurts
The clear button looks like a solution while often destroying the evidence. When clearing is appropriate and what disappears with the code.
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Anatomy of a fault code: what every character in P0301 actually means
The five characters are not arbitrary. The letter names the system, the first digit says whether it is standard or manufacturer-specific, the second names the subsystem, and the last two pin it down. After this, codes read without a lookup table.
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When the scanner lies: ghost codes, false triggers, and why "a code" is not "a broken part"
The code points at a sensor and the sensor is fine. The code vanishes and returns a week later. Why that happens, how to tell a real fault from a side effect, and why the freeze frame decides more than the code does.
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Not all DTCs are equal: 4 severity levels (low, medium, high, critical) and how to prioritize
What DTC severity level means, how to tell "light from dust on the MAF" from "stop the engine now", which codes can wait, which are critical right now.