P0491: Secondary Air Injection System Insufficient Flow (Bank 1)

What is the engine computer reporting with P0491?

This code is a more specific version of the general secondary air flow code and sets when the system runs during a cold start but the flow reaching bank 1 of the exhaust comes in below what the computer expects. It points to a partial restriction or a weak pump rather than a complete system failure.

The code sits in the secondary air injection area and is reported by the secondary air injection monitor. It names the circuit or the condition where the computer saw a reading outside its expected range. It does not name a failed part, and several genuinely different faults set this same code, which is the whole reason the causes below are ordered rather than listed.

What people usually notice

• Failed emissions test
• Check engine light appearing shortly after a cold start
• No noticeable drivability symptom

What is actually behind P0491?

The causes below are ordered by how often they turn out to be the answer, cheapest and most common first. That order matters more than the list itself: the expensive component named in the code text is very rarely the first thing to replace, and working down from the top is what stops a secondary air injection fault becoming an expensive guess.

Partially clogged air injection hose to bank 1 is the one to rule out before anything else. Carbon or debris can restrict flow without blocking it entirely.

How do you diagnose P0491?

Start with freeze frame. Coolant temperature at the fault confirms whether the system was tested in its correct cold start window. That snapshot is taken at the instant the code set, and it is the single most useful piece of evidence you have, which is also why clearing the code before reading it is such a costly habit.

Diagnosing this one properly means commanding a component and watching whether it responds, which a read-only tool cannot do at any price. An actuator test that commands the pump and switching valve while measuring actual airflow to bank 1 shows how far short of target flow fell.

The order worth working in

• 1. Partially clogged air injection hose to bank 1: Carbon or debris can restrict flow without blocking it entirely.
• 2. Weak or aging secondary air pump not producing full flow: Pump output can decline gradually before it fails outright.
• 3. Partially stuck switching valve limiting flow to that bank: Carbon buildup can restrict the valve without jamming it shut completely.
• 4. Small leak in the air delivery hose bleeding off pressure before it reaches the exhaust: A leak reduces the flow that actually reaches the exhaust even if the pump works fine.
• 5. Check valve stuck partly closed, restricting flow into that bank: This one-way valve can gum up with carbon and restrict flow in the forward direction too.

Can you keep driving with P0491?

Safe to keep driving, since this system only matters briefly after a cold start and does not affect ongoing engine operation.

This is a severity 1 of 4 code. It will not usually strand you, but it will fail an emissions test while it is present, and the underlying fault rarely gets cheaper by being left alone.

What does it cost to ignore P0491?

This system exists mainly to pass emissions and to help the catalytic converter warm up faster, so ignoring it usually means a failed emissions test rather than engine damage.

There is also the diagnostic cost of waiting. Freeze frame data, pending code history and the readiness monitor state all describe the fault as it was when it set, and all of it is lost the moment somebody clears the code to make the lamp go out.