SAP Off — disabling the secondary air injection system

SAP Off disables the secondary air injection system (SAP/SAI) in the ECU software of a petrol engine. The ECU stops running the pump and valve after a cold start and no longer monitors the system, so P0410, P0411, P0491 and related faults do not come back. For tuners: send us the original file and get a ready-to-flash file with corrected checksums, on average in 30–60 minutes.

What the secondary air pump is for

A cold petrol engine needs a rich mixture to start, λ < 1. The catalytic converter is not doing its job yet: it only works efficiently from about 300–350 °C. According to Hella, up to 80% of the emissions of a whole drive cycle are produced during the cold start.

The secondary air pump is a blunt but effective fix. Right after the start it blows fresh air into the exhaust port just behind the exhaust valve. CO and unburnt hydrocarbons finish burning there, and the heat from that afterburning brings the catalyst up to temperature sooner. In MIT tests (2010) this cut hydrocarbon emissions during the cold start by 46–88% and CO by 37–93%.

The pump comes on after a cold start, usually for a minute to a minute and a half. Pierburg, which makes these pumps, says it should not run for more than 90 seconds. Once closed-loop lambda control kicks in, the ECU switches the pump off and the valve closes so hot exhaust gas cannot reach the pump.

System components:

• Electric air pump (V101 on VW and Audi). It draws air from the air filter housing or through its own filter. Power comes through a relay (J299 on VAG) that the ECU switches.
• Combi valve on the cylinder head or manifold. The shut-off side only lets air through during warm-up, the check-valve side keeps exhaust gas and condensate away from the pump. Early valves are opened by vacuum through a solenoid valve (N112 on VAG), later ones by pump pressure, and electric valves appeared around 2007.
• Air passages in the cylinder head or manifold that carry the air to the exhaust ports.
• Monitoring sensors: the upstream O2 sensor, and on some cars a secondary air pressure sensor (G609 on Audi).

How the ECU knows the system has failed

According to Pierburg, European EOBD only checks the electrical circuits of the pump and valves: open circuit, short to ground, short to battery voltage. US OBD II also checks that the system actually works. On older cars the ECU ran the pump once per trip on a warm engine and watched the O2 sensor: if the air arrives, the sensor has to show a lean shift. From the 2006 model year, California's CARB rules require the flow to be checked during the warm-up itself, and systems with a pressure sensor measure the pressure in the air line.

That has a practical consequence. A resistor in place of a removed pump only satisfies the circuit check. If the ECU checks flow through the O2 sensor or pressure, the fault comes back after another cold start. Owners who tried it report on forums that the light returned within a few weeks.

Why secondary air pumps fail

The usual complaints are a noisy pump or a pump that does not run at all. The most common causes:

• Moisture and condensate in the pump: corrosion, wear, seizure.
• A combi valve stuck with carbon or leaking.
• A cracked or detached hose, or a vacuum leak. Pierburg specifies at least 390 mbar of control vacuum at the vacuum valve; below that, look for a leak.
• Carbon-blocked passages in the cylinder head, which Ross-Tech lists as one of the causes of P0491.
• The pump relay, fuse, connector or wiring.

Pierburg makes a specific point: most pumps die because a leaking check valve lets exhaust condensate in. Replace only the pump and the new one will soon go the same way.

Which fault codes the ECU sets

Typical secondary air codes per SAE J2012. Manufacturers also have their own codes for the same faults: on VAG, for example, 16795 is P0411.

What SAP Off changes in the software

Clearing the code with a scan tool achieves nothing: on the next cold start the ECU tests the system again. So the logic in the file itself is changed.

The rest of the calibration is unchanged and checksums are recalculated. SAP Off only deals with secondary air: the raised idle and retarded ignition during warm-up stay. If those bother the owner too, add Cold Start Off to the same order. SAP Off is often combined with CAT/Lambda Off and EVAP Off.

What to do with the pump and valve

Removing the hardware is not required: once the file is flashed, the ECU simply never switches it on. One catch: if the combi valve is stuck open, the O2 sensor may read lean and the ECU logs a mixture fault (fuel trim limit). That belongs to another system and SAP Off does not remove it. Replace such a valve or blank off the port.

If you remove the system:

• Close the valve port on the head or manifold with a blanking plate and gasket, otherwise exhaust will leak.
• Cap the outlet on the air filter housing that fed the pump.
• Cap the vacuum lines that went to the solenoid valve: an open fitting on the intake means a vacuum leak and new mixture faults.

How to order the file

1. Read the original file via OBD or on the bench with the tool you use. We need an unmodified stock file.
2. Upload it to the portal and select SAP OFF. Other options can go in the same order.
3. Get the finished file with recalculated checksums, on average within 30–60 minutes.
4. Flash the file, clear the faults and do a cold start: the pump should stay off and secondary air faults should not return.

Supported ECUs are listed below. If yours is not there, send the file with a request and we will check whether there is a solution for that software.

Disabling secondary air injection increases CO and hydrocarbon emissions after a cold start and changes the vehicle's emissions class. In the EU, the UK, the US and a number of other countries this is prohibited on public roads and may cause an inspection failure. The service is intended for tuners and is used where the law allows it: motorsport, off-road, export, vehicles not used on public roads. Responsibility for using the file lies with the vehicle owner.