P1591 — 3 documented meanings by make

P1591 does not mean the same thing on every car — manufacturers reuse the number for different faults. 7 makes document it: Audi, Ford, Mazda, SEAT, Skoda, Subaru, Volkswagen. Find your make below.

P1591 on Audi, SEAT, Skoda, Volkswagen

On Audi, SEAT, Skoda, Volkswagen vehicles P1591 is documented as: Boost pressure control solenoid, N75, no function. P1591 points at turbocharger or boost control. The standard definition names the system involved; which part inside it has actually failed is what the checks below establish, cheapest first.

Usually one of: Boost hose split or clamp loose; Wastegate or variable-vane mechanism sticking; Boost pressure sensor reading wrongly; Turbo bearing worn; Intercooler leaking.

Check first, cheapest first:

1. Pressure-test the boost pipework for leaks — a split hose mimics a failed turbo and costs a fraction to fix
2. Inspect the actuator and linkage for free movement
3. Compare the boost sensor reading with the commanded value on live data
4. Check the turbo for shaft play and oil in the intake only after the pipework is proven sealed

Related Audi, SEAT, Skoda, Volkswagen codes:

P1591 on Ford, Mazda

On Ford, Mazda vehicles P1591 is documented as: Cruise control decrease distance signal. P1591 points at cruise control. The standard definition names the system involved; which part inside it has actually failed is what the checks below establish, cheapest first.

Related Ford, Mazda codes:

P1591 on Subaru

On Subaru vehicles P1591 is documented as: Neutral position switch, low input. P1591 is stored when the ECU sees signal too low on the circuit named in the code. 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: High resistance in the supply or ground path, dragging the signal down; Corroded connector adding resistance the module reads as a low reading; Sensor drifted out of tolerance with age; Partial short to ground somewhere in the signal line.

Check first, cheapest first:

1. Read the live value on a scanner and compare with a known-good reading
2. Clean and re-seat the connector, then re-test before replacing anything
3. Check the ground point for that circuit — a poor ground causes this far more often than a bad sensor
4. Measure the sensor against specification with the connector back-probed
5. Replace the sensor only once the wiring and ground have been ruled out

Related Subaru 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

P1xxx is manufacturer-specific code (VW/Audi, BMW, Mercedes, Toyota etc. have different meanings for the same code number).

Severity

Manufacturer-specific codes need a brand scanner (VCDS for VAG, INPA for BMW, STAR for Mercedes).

That is what holds for P1591 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.