Chapter 46 · Emissions Systems, Diagnostics & Deletes

Secondary Air Injection

SAI chemistry, the diagnostic flow test, and a three-layer software elimination.

Calibration symbols

ESKONF CDSLS DFPM J299 KFZW N112 N249 FAW FST G70 SLS

ECUs covered

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Size

11 symbols · 2 diagrams · ~1,143 words

SAI chemistry, the diagnostic flow test, and a three-layer software elimination.

The Secondary Air Injection (SAI / Sekundärluftsystem - SLS) is an active emissions control subsystem designed to accelerate catalytic converter light-off during the cold-start phase. On factory VAG 1.8T vehicles (AWP, AWW, AUQ, AUM), secondary air injection operates for 60\dots 90\text{ seconds} following an engine start below engine coolant temperatures of approximately 35\text{°C}.

While essential for original EPA Tier 1 / ULEV and Euro 4 cold-start hydrocarbon compliance, the SAI hardware is notoriously prone to mechanical failure (pump bearing seizure, rivet failure on pump housings, combi-valve diaphragm rupture, and cracked plastic corrugated plumbing). For motorsport, track-day, and simplified engine bays, complete removal of the system requires precise calibration adjustments across the Bosch ME7.5 Diagnostic Fault Path Manager (DFPM) to prevent recurring check engine lights (P0411, P1423) and rough cold-start idling.

┌──────────────────────────────────────────────────────────────────────────────────────────────────┐
│                   SECONDARY AIR INJECTION (SAI) ELECTRO-PNEUMATIC ARCHITECTURE                   │
├──────────────────────────────────────────────────────────────────────────────────────────────────┤
│                                                                                                  │
│   [ Airbox / Intake Filter ]                                                                     │
│               │                                                                                  │
│               ▼ Fresh Filtered Air                                                               │
│   ┌───────────────────────┐         12V Switched Power                                           │
│   │ Secondary Air Pump    │ ◄────────────────────────────── [ SAI Pump Relay J299 ]              │
│   │ (V101 Electric Motor) │                                            ▲                         │
│   └───────────┬───────────┘                                            │ Pin 65 (Low-Side Drive) │
│               │ High-Volume Low-Pressure Air                           │                         │
│               ▼                                               ┌────────┴────────┐                │
│   ┌───────────────────────┐                                   │   Bosch ME7.5   │                │
│   │ Combination Valve     │                                   │       ECU       │                │
│   │ (Vacuum-Actuated)     │ ◄─────────────────┐               └────────┬────────┘                │
│   └───────────┬───────────┘                   │ Vacuum Command         │ Pin 114 (PWM/Low-Side)  │
│               │ Injected Secondary Air        │                        ▼                         │
│               ▼                               │             ┌───────────────────────┐            │
│   ┌───────────────────────┐                   └─────────────┤ SAI Solenoid Valve    │            │
│   │ Cylinder Head Exhaust │                                 │ (N112 Electro-Pneum.) │            │
│   │ Ports (Behind Valves) │                                 └──────────┬────────────┘            │
│   └───────────┬───────────┘                                            │                         │
│               │ Exothermic Post-Combustion Oxidation                   ▼                         │
│               ▼                                            [ Intake Manifold Vacuum ]            │
│   ┌───────────────────────┐                                                                      │
│   │ Catalytic Converter   │ ◄── Rapid Thermal Light-Off (> 350°C in < 30 seconds)                │
│   │ (Exhaust Downpipe)    │                                                                      │
│   └───────────────────────┘                                                                      │
└──────────────────────────────────────────────────────────────────────────────────────────────────┘

46.1. Chemical Reaction & Catalytic Light-off Physics#

During cold start, engine friction is high, and fuel atomization on cold intake manifold walls is severely degraded (requiring heavy fuel enrichment via FAW and FST, resulting in raw fuel equivalence ratios of \lambda \approx 0.75\dots 0.85). This produces immense concentrations of unburned hydrocarbons (HC) and carbon monoxide (CO) in the exhaust ports.

The secondary air pump (V101) delivers ambient oxygen directly into the cylinder head exhaust passages immediately behind the exhaust valve stems:

Exothermic Thermal After-Burning: In the presence of excess atmospheric oxygen and exhaust gas temperatures exceeding 600\text{°C}, spontaneous thermal oxidation occurs inside the exhaust manifold and turbocharger turbine housing: $2\text{CO} + \text{O}_2 \longrightarrow 2\text{CO}_2 + \Delta H \quad (\Delta H = -283\text{ kJ/mol})$ $\text{C}_n\text{H}_m + \left(n + \frac{m}{4}\right)\text{O}_2 \longrightarrow n\text{CO}_2 + \frac{m}{2}\text{H}_2\text{O} + \Delta H$
Thermal Energy Transfer to Catalyst: The chemical energy released by post-combustion oxidation heats the exhaust gas stream entering the catalytic downpipe to over 700\text{°C}. This forces the catalytic converter ceramic substrate past its operational activation threshold (T_{\text{cat}} > 350\text{°C}) within 20\dots 30\text{ seconds} of initial crank, reducing cold-start cumulative emissions by up to 80\%.

46.2. Closed-Loop Diagnostic Monitoring & Mass Airflow Modeling#

Bosch ME7.5 does not utilize an inline physical mass flow sensor inside the secondary air plumbing. Instead, the ECU executes an algorithmic diagnostic validation routine (DFPM test B_sls / B_slse) utilizing signals from the pre-catalyst wideband oxygen sensor (LSU 4.2 / CJ125) and the mass airflow sensor (G70 / HFM5):

┌──────────────────────────────────────────────────────────────────────────────────────────────────┐
│                             BOSCH ME7.5 SAI DIAGNOSTIC FLOW TEST                                 │
├──────────────────────────────────────────────────────────────────────────────────────────────────┤
│                                                                                                  │
│   Engine Running in Secondary Air Phase:                                                         │
│   1. ECU Commands N112 Open + J299 ON                                                            │
│   2. ECU Observes Upstream Lambda (US):                                                          │
│      - Normal Cold Start without SAI: λ ≈ 0.80 - 0.85 (Rich)                                     │
│      - Normal Cold Start WITH SAI Active: λ > 1.25 - 1.40 (Extreme Lean Deflection)               │
│                                                                                                  │
│   3. Plausibility Evaluation:                                                                    │
│      IF (Measured Upstream Oxygen Concentration < Threshold MSLRE):                              │
│          → Diagnostic Failure Counter Increments                                                 │
│          → If Fault Persists for 2 Consecutive Driving Cycles:                                   │
│              MIL Illuminated: DTC P0411 (16795) - Incorrect Flow Detected                        │
│              MIL Illuminated: DTC P1423 (17831) - Bank 1 Flow Too Low                            │
│                                                                                                  │
│   4. Electrical Load Verification:                                                               │
│      Output Stage Diagnostics monitor Pin 65 (J299) and Pin 114 (N112):                          │
│      - Unplugged N112: DTC P1421 / P1425 (Short to Ground / Open Circuit)                        │
│      - Unplugged J299: DTC P0418 / P1435 (Secondary Air Relay Open / Malfunction)                │
└──────────────────────────────────────────────────────────────────────────────────────────────────┘

The diagnostic evaluation relies on two primary calibration thresholds:

MSLRE (Mindest-Sekundärluftmasse für Erkennung): Minimum secondary air mass flow modeled by the ECU required to pass the plausibility check. If the observed lambda deflection indicates a secondary air flow lower than MSLRE, a flow fault is flagged.
MDLRE (Mindest-Dauer Sekundärluft-Diagnose): Minimum operational duration the secondary air pump must run under steady-state engine speed before the diagnostic test concludes.

46.3. Complete Software Elimination & Calibration Decoupling#

When physically removing the secondary air pump, combi-valve, and solenoid from a vehicle, performing a proper software delete requires addressing three distinct architectural layers in the binary:

Layer 1: Diagnostic Fault Path Master Deactivation (`CDSLS` / `CWSLS`)#

To permanently halt the execution of the SAI diagnostic test routine, set the primary configuration codewords to zero:

Symbol Name	Flash Offset (032LP)	Factory Value	Modified Value	Description
`CDSLS`	`0x0181A3`	`0x01`	`0x00`	Codewort für Diagnose Sekundärluftsystem (Master diagnostic switch)
`CWSLS`	`0x0181A4`	`0x01`	`0x00`	Codewort Konfiguration Sekundärluftsystem (Operating mode configuration)
`MSLRE`	`0x0181AA`	`0x18` (12.0 kg/h)	`0x00` (0.0 kg/h)	Minimum Secondary Air Mass threshold for detection
`MDLRE`	`0x0181AC`	`0x32` (5.0 s)	`0x00` (0.0 s)	Minimum diagnostic test evaluation duration

Layer 2: Decoupling Catalyst Heating Airflow and Timing Retard#

During the SAI phase, ME7.5 triggers the Catalyst Heating Routine (B_kh), which intentionally retards ignition timing into the negative range (-10.0\text{°}\dots -15.0\text{° KW}) via KFZWS and increases idle target airflow to supply high thermal mass into the exhaust.

If the pump has been removed but the catalyst heating timing tables remain active, the engine will suffer from severe cold-start bucking, popping in the downpipe, and erratic idle speed fluctuations.
Correction: Neutralize the catalyst heating ignition delta map KFZWS (Zündwinkel-Spätverstellung bei Katalysatorheizen) by setting all cells to 0.0° KW. This ensures cold-start ignition timing follows standard baseline timing (KFZW), eliminating rough cold starts.

Layer 3: Hardware Output Driver Masking (`ESKONF`)#

When the physical solenoid valve N112 and relay J299 are disconnected from the wiring harness, the Bosch output power stages detect an open-circuit load condition, generating electrical DTCs (P0418, P1421, P1425).

Modify the ESKONF (Endstufenkonfiguration) hardware mask bytes in flash (as detailed in Section 7.2) to disable load monitoring for these pins:
- Locate ESKONF Byte 4 and Byte 5.
- Set the bit pairs corresponding to N112 (ECU Pin 114) and J299 (ECU Pin 65) to 11b (0x03 = Unused / Output Stage Diagnostic Disabled).
- This completely suppresses open-circuit DTCs without requiring resistor trick boxes in the harness!

46.4. Physical Elimination & Hardware Guidelines#

When deleting the secondary air system on a 1.8T 20V engine:

Combi-Valve Port Block-Off: The combi-valve is mounted to the rear of the cylinder head (exhaust side above the transmission bellhousing). Remove the combi-valve and install a CNC-machined billet aluminum block-off plate equipped with a high-temperature Viton O-ring or graphite-impregnated gasket. Torque M6 fasteners to 10\text{ Nm} with threadlocker.
N112 Vacuum Line Routing: The N112 solenoid is mounted to the underside of the intake manifold bracket alongside N249. Disconnect the vacuum supply line originating from the intake manifold and cap the manifold vacuum barb with a reinforced silicone vacuum cap and pinch clamp.
Chassis Weight & Clutter Reduction: Removing the cast iron mounting bracket, V101 air pump, corrugated plastic plumbing, and combi-valve removes approximately 4.2\text{ kg} (9.3\text{ lbs}) of parasitic weight from the front overhang of the vehicle.