P3120 — 2 documented meanings by make

P3120 does not mean the same thing on every car — manufacturers reuse the number for different faults. 4 makes document it: Audi, SEAT, Skoda, Toyota. Find your make below.

P3120 on Audi, SEAT, Skoda

On Audi, SEAT, Skoda vehicles P3120 is documented as: Pump jet valve, cylinder 10, N306, control limit not reached. P3120 is stored when the ECU sees below threshold on the circuit named in the code (cylinder 10). That describes how the circuit is behaving, not which part has failed — the wiring, the connector and the component itself can all produce it, which is why the checks below start with the cheapest.

Usually one of: The monitored part has degraded rather than failed; An upstream fault is loading the part beyond its design; Sensor used to judge the threshold is itself inaccurate; Normal wear reaching the end of its service life.

Check first, cheapest first:

1. Fix any other stored codes first — this class is very often a consequence, not a cause
2. Verify the sensors that judge the threshold before replacing the expensive part
3. Check for leaks or restrictions that would make a healthy part read as failed
4. Compare readings before and after the component
5. Replace the component once it is proven to be the one that has degraded

Related Audi, SEAT, Skoda codes:

P3120 on Toyota

On Toyota vehicles P3120 is documented as: HV main relay.

Related Toyota codes:

Which one is yours

A generic OBD reader gives you the number, not which of these meanings your manufacturer assigned to it. To get to the specific one:

1. Use a brand-specific scanner — VCDS for VAG, INPA or ISTA for BMW, XENTRY for Mercedes, Forscan for Ford. It shows the manufacturer's own text rather than the generic one.
2. Read all stored codes together — the neighbouring ones usually point at which system is involved and narrow the list.
3. Check freeze-frame data: the RPM, temperature and load at the moment it triggered indicate which system was active.
4. If you have no brand scanner, tell me the make and model below and I will tell you which of these applies to your car.

Code family

P3xxx is reserved section used by some manufacturers for specific diagnostics.

Severity

Reserved section — the meaning is manufacturer-defined; a generic scanner rarely gives the full picture.

That is what holds for P3120 in general. From here it depends on your engine — tell me which car it is and what it does, and I will give you the steps for it: real procedures, torque figures and diagrams instead of general advice.