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How to read a code: P0301

Every character tells you something before you open the code page.

P0301
P0301

Generic or manufacturer-specific

A 0 is a generic SAE code: it means the same on a Toyota as on a Renault. A 1 is specific to the brand and its meaning changes between manufacturers.

P0301

The subsystem

In P codes: 1 air and fuel, 2 injection, 3 ignition, 4 emissions, 5 idle and speed, 6 computer, 7 to 9 transmission.

P0301

The specific fault

The circuit, bank or cylinder. In P0301 it is a combustion misfire in cylinder 1.

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Do you have several codes at once?

They are almost never independent faults. A vacuum leak causes a lean mixture, then misfires and finally a damaged catalytic converter. The free diagnosis sorts your codes from cause to consequence.

Multi-code diagnosis

What to do when a code appears

  1. Read all the codes and save the freeze frame. It is a snapshot of the engine conditions at the moment of the fault, and it is lost when you clear the codes.
  2. Look for the common cause. If there are several codes, one usually causes the others.
  3. Inspect before you buy. Vacuum hoses, connectors, grounds and wiring solve a very large share of faults.
  4. Repair, clear and drive. Complete a mixed drive cycle until the monitors show ready.