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Diagnostic Trouble Codes

Every code on this site is the SAE J2012 generic definition — the wording the standard itself uses, not a paraphrase. Where a code is defined by the manufacturer rather than by SAE, the page says so, because a definition found for the wrong marque is worse than no definition at all.

Where these definitions come from

Every generic code on this site carries the SAE J2012 definition, cross-checked against two independent public code tables before publication. Where those tables disagreed, the code was left out rather than guessed at.

That sounds like a small thing until you see what the widely-copied lists actually contain. The dataset that ranks first for "OBD trouble codes" on GitHub — and which has been forked into commercial products — has its descriptions shifted against their code numbers: it gives P0420 as a secondary air injection relay fault and P0300 as a cylinder 12 contribution fault. Both are wrong, and both would send someone to replace a part that is working.

How to read a code number

The first character gives the area: P powertrain, B body, C chassis, U network. The second says who defined it — 0 and 2 are SAE definitions, identical on every vehicle; 1 and 3 are the manufacturer's own, and the same number can mean unrelated things on two marques.

The third character narrows the system, and the last two identify the specific fault. So P0301 is a powertrain code, SAE-defined, in the ignition and misfire group, for cylinder 1.

What P20A9 means

P20A9
Reductant Injection Air Pressure Control Valve Circuit High
a generic powertrain code — powertrain
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What the code meansSAE J2012 definitionReductant Injection Air Pressure Control Valve Circuit High
CodeP20A9
Areathe engine, transmission and the emissions equipment attached to themPowertrain
SystemPowertrain
Defined bythe second character is "2", which marks a generic codeSAE — the same on every make
Warning lighta steady light means have it looked at, not stop immediatelySteady, if lit at all

Notes

  • A stored code names the circuit that reported a problem, not the part that failed. P20A9 tells you which reading fell outside its expected window; the cause can be the sensor, its wiring, its connector, a vacuum or exhaust leak upstream, or the condition the sensor is correctly reporting. Replacing the named component first is the most common and most expensive diagnostic mistake.
  • This is a generic code, so the definition above is the same on every make. What differs between manufacturers is not what the code means but what commonly causes it and how the fault is confirmed — which is why the marque-specific service information still matters even for a code any scanner can read.
  • Read the freeze frame before clearing anything. The module stores the conditions at the moment the code set — engine speed, load, coolant temperature, fuel trims, vehicle speed — and that snapshot is usually more diagnostic than the code itself. Clearing the code erases it, along with the readiness monitors, and a vehicle with incomplete monitors will fail an emissions test even with no light on.
  • One code often causes another. A single upstream fault — an intake leak, a weak fuel pump, low coolant — can set several codes across different systems. Where more than one is stored, the oldest and the most upstream is the one to work from, not the one with the most alarming description.

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OBD-II trouble code by code number

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