P2259: Secondary Air Injection System Control "B" Circuit Low

The first thing this code tells you has nothing to do with what is broken. It tells you about the architecture of the vehicle you are standing next to. A system with only one commanded output has no "B" to report, so the mere existence of this address means the control module drives two separate elements in this system and can fault them independently.

Working out which element is which is the whole first hour, and getting it wrong is how people buy the wrong part. Two arrangements account for nearly all of them. In the first, one output commands the air pump — almost always through a heavy relay, because the motor draws far more current than a module output can carry — while the other commands the air switching or shutoff valve that decides whether that air actually reaches the exhaust. Those are two entirely different devices in two different places with two different price tags. In the second arrangement, found on V-engines, the outputs are split by bank: one switching element per cylinder head, fed from a common pump. Both are real, both are common, and nothing in the letter itself distinguishes them. The wiring diagram for your exact year, make, model and engine is the only authority, and if a shop tells you the letter identifies the part without opening one, that is worth questioning.

There is a fast way to narrow it before you reach for a diagram. These outputs are usually exposed individually in the module's bidirectional actuator test menu, so a scan tool that can command them will list them by name — and the names in a manufacturer-level menu are far more specific than the generic code text. Counting how many secondary air outputs the module publishes also settles a question that occasionally saves the whole job: whether this vehicle really has a second element at all. A generic code reader matching a manufacturer-specific number against a generic definition list will sometimes hand you a "B" circuit on a car that only ever had one. Confirm the reading at manufacturer level before a single part is ordered.

As to the fault itself, these outputs are switched on the ground side, which means the module turns something on by pulling its control line down. A low verdict says the line went down while the module was not asking for it, so something other than the module has completed that path to ground. The candidates are the usual set — a conductor worn through onto chassis metal, a connector bridged by water and road salt, a load with a shorted winding pulling the line down through itself, or an output stage inside the module that has failed in the on state.

But a two-output system carries one possibility a single-output system physically cannot have, and it is the one most likely to be missed. Both control conductors leave the module in the same harness branch and run together toward the same corner of the engine bay. If the insulation between them breaks down — chafe inside a loom, a pinch at a bracket, heat damage where the branch passes the exhaust — then commanding one output puts a path to ground on the other. The signature is unmistakable once you look for it: command output A on the scan tool, watch the B line, and see whether it follows. Two control wires that misbehave together are one damaged section of loom, not two faults.

The code pairing across the two addresses is worth reading before any of that, because it costs nothing and it splits the problem in half. If both the A and B outputs report low, look at what they share — the common ground, the module itself, a single connector that both pass through, a flooded splice. If only B reports low, everything shared is already exonerated and the fault lives in the B branch alone.

On consequences, a great deal depends on which element B turns out to be, and being honest about that is more useful than a single verdict. If B commands the pump, a circuit held low runs a motor that was designed for well under two minutes per cold start, and that matters on a timescale of weeks. If B commands a switching valve, the valve is being held in its energised state continuously, which does not destroy anything quickly but does leave the air path open outside the window it was designed for. Either way the vehicle starts, drives, steers and stops normally, and either way it will fail an emissions inspection.