P1091 — 5 documented meanings by make

P1091 does not mean the same thing on every car — manufacturers reuse the number for different faults. 8 makes document it: Audi, BMW, Honda, Renault, SEAT, Skoda, Subaru, Volkswagen. Find your make below.

P1091 on Audi, SEAT, Skoda, Volkswagen

On Audi, SEAT, Skoda, Volkswagen vehicles P1091 is documented as: Mixture regulation, bank 2, range 1, rich limit exceeded. P1091 is stored when the ECU sees below threshold on the circuit named in the code (bank 2). 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, Volkswagen codes:

P1091 on BMW

On BMW vehicles P1091 is documented as: Pre-catalytic converter fuel trim system too rich, bank 1. P1091 points at catalytic converter (bank 1). 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: Converter degraded with age or mileage; Engine running rich and destroying the substrate; Oil or coolant burning and poisoning the converter; Rear oxygen sensor reading inaccurately; Exhaust leak upstream drawing in air.

Check first, cheapest first:

1. Fix any misfire or fuel-trim codes first — they destroy converters, and replacing one without fixing the cause destroys the new one too
2. Check for exhaust leaks before and after the converter
3. Compare front and rear oxygen sensor traces on live data
4. Replace the rear sensor if it is the one reading wrongly — far cheaper than a converter
5. Replace the converter once the engine side is proven healthy

Related BMW codes:

P1091 on Honda

On Honda vehicles P1091 is documented as: Rail pressure control valve, fuel metering unit, open circuit. P1091 is stored when the ECU sees open circuit 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: Broken wire, most often at a flex point such as a door hinge or engine mount; Corroded or pushed-back terminal in the connector; Blown fuse on the supply side; Component failed with an internally open winding or element.

Check first, cheapest first:

1. Check the relevant fuse first — it is the cheapest thing that produces this code
2. Unplug and inspect both halves of the connector for corrosion and terminal fit
3. Continuity-test the wire end to end while flexing the loom by hand
4. Measure the component itself against its specification
5. Repair the break or replace the component, whichever the measurement points to

Related Honda codes:

P1091 on Renault

On Renault vehicles P1091 is documented as: Detection of overheating.

Related Renault codes:

P1091 on Subaru

On Subaru vehicles P1091 is documented as: Tumble generated valve system 1, valve stuck closed. P1091 is stored when the ECU sees stuck or jammed 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: Carbon or oil deposits binding the mechanism; Actuator motor or solenoid failed; Linkage worn or seized; Control side working correctly but the part cannot physically move.

Check first, cheapest first:

1. Command the actuator with a scanner and watch whether it moves at all
2. Remove and inspect for deposits — cleaning frequently fixes this class outright
3. Check the electrical side is delivering the command before condemning the mechanism
4. Inspect the linkage for wear or seizure
5. Replace the actuator if it will not move under command with good supply

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 P1091 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.