Secondary Air Injection (SAI) pumps ambient air into the exhaust stream immediately after cold start. This extra O₂ accelerates catalyst light-off by reacting with unburnt HC and CO to release heat. It's entirely normal behaviour — but on a 5-gas analyzer it looks identical to a vacuum leak.
The "false leak" signature
- O₂ high (often 3–6%) immediately after start.
- CO surprisingly low.
- HC may drop faster than you'd expect.
- Lambda reads lean.
How to tell it's SAI not a leak
- Time it: SAI typically runs for 30–120 seconds from cold start only.
- Listen: the SAI pump is audible as a noticeable "vacuum cleaner" hum.
- OBD: SAI system monitor or live PID shows pump commanded on.
- Warm-engine retest: signature disappears entirely once SAI closes.
// when it matters
If you're testing for emissions pass/fail, let the engine run long enough that SAI has finished (typically 3–5 minutes idle from fully warm). Reading through SAI will always fail CO+CO₂ sanity checks.