Chapter 52 · Emissions Systems, Diagnostics & Deletes

Rear O2 & Catalyst

Catalyst oxygen storage, the rear-lambda trim loop, and rear O2 elimination.

Calibration symbols

ESKONF CDHSH CDKAT DFPM N249

ECUs covered

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Size

5 symbols · 1 diagram · ~879 words

Catalyst oxygen storage, the rear-lambda trim loop, and rear O2 elimination.

On modern OBD-II compliant vehicles, the exhaust gas aftertreatment architecture relies on a closed dual-sensor feedback loop. On the VAG 1.8T 20V platform (AWP, AWW, AUQ, AUM), the engine incorporates an upstream planar wideband oxygen sensor (LSU 4.2 / Bosch CJ125 ASIC) positioned pre-catalyst in the downpipe, and a secondary planar switching zirconia narrowband sensor (LSF 4.2) positioned post-catalyst.

While the primary role of the downstream sensor is legal compliance—specifically evaluating catalytic converter Oxygen Storage Capacity (OSC) to flag catalyst degradation (P0420)—it also performs **Secondary Lambda Trim Control (Katalysator-Alterungs- und Vergiftungserfassung - KAVS)**. Simply installing physical mechanical spacers (spark plug non-foulers) or blindly deleting DTC codes in flash often corrupts the primary closed-loop fuel trims. Achieving a factory-grade calibration for motorsport test-pipes or high-flow racing catalysts requires de-linking the secondary trim loop and correctly re-configuring readiness monitoring registers.

┌──────────────────────────────────────────────────────────────────────────────────────────────────┐
│                     DUAL-OXYGEN SENSOR CLOSED-LOOP INTERACTION ARCHITECTURE                      │
├──────────────────────────────────────────────────────────────────────────────────────────────────┤
│                                                                                                  │
│   [ Engine Exhaust Gas ] ──► [ Primary Wideband LSU 4.2 ] ──► Fast Closed-Loop Control (λ = 1.0) │
│                                         │                                                        │
│                                         ▼                                                        │
│   ┌─────────────────────────────────────────────────────────────────────────────────────────┐    │
│   │ Catalytic Converter Ceramic Substrate (Cerium Oxide Oxygen Storage Bed: Ce2O3 <-> 2CeO2)│    │
│   └─────────────────────────────────────┬───────────────────────────────────────────────────┘    │
│                                         │                                                        │
│                                         ▼ Dampened / Filtered Gas Stream                         │
│                              [ Secondary Narrowband LSF 4.2 ]                                    │
│                                         │                                                        │
│                    ┌────────────────────┴────────────────────┐                                   │
│                    ▼                                         ▼                                   │
│   ┌───────────────────────────────────┐     ┌───────────────────────────────────┐                │
│   │ Path A: Catalyst Diagnostic Test  │     │ Path B: Secondary Fuel Trim (KAVS)│                │
│   │ Compares upstream vs downstream   │     │ Trims primary wideband reference  │                │
│   │ signal oscillation ratio.         │     │ voltage to compensate for aging   │                │
│   │ IF Ratio > MINKAT:                │     │ and lead/phosphorus poisoning.    │                │
│   │ → Sets DTC P0420 (MIL Lamp ON)   │     │ (Must be decoupled on test-pipes!)│                │
│   └───────────────────────────────────┘     └───────────────────────────────────┘                │
└──────────────────────────────────────────────────────────────────────────────────────────────────┘

52.1. Physics of Catalyst Oxygen Storage Capacity (OSC) & DTC `P0420`#

A functional three-way catalytic converter does not merely oxidize pollutants instantaneously; it acts as a chemical chemical accumulator for gaseous oxygen. The washcoat contains cerium dioxide (\text{CeO}_2):

During brief rich excursions (\lambda < 1.000), cerium releases stored oxygen to oxidize carbon monoxide and unburned hydrocarbons: $2\text{CeO}_2 + \text{CO} \longrightarrow \text{Ce}_2\text{O}_3 + \text{CO}_2$
During brief lean excursions (\lambda > 1.000), trivalent cerium absorbs excess free oxygen from the exhaust: $\text{Ce}_2\text{O}_3 +\frac{1}{2}\text{O}_2 \longrightarrow 2\text{CeO}_2$

The Diagnostic Differential Algorithm#

When the Diagnostic Fault Path Manager (DFPM) initiates the catalyst conversion diagnostic routine (B_kat):

The engine intentionally cycles fuel mixture lean and rich (\pm 3\% lambda oscillation).
On a fresh, high-capacity catalyst, the oxygen buffer absorbs the oxygen swings. Downstream sensor voltage remains rock-steady between 0.65\text{V} and 0.75\text{V}.
On a degraded or catless downpipe, the downstream sensor mirrors the upstream oscillations with zero chemical buffering (0.10\text{V} \leftrightarrow 0.85\text{V}).
ME7.5 integrates the area under the downstream voltage curve relative to the upstream reference signal: $E_{\text{cat}} =\frac{\int |dV_{\text{post}} / dt|}{\int |dV_{\text{pre}} / dt|}$
If E_{\text{cat}} exceeds the calibration limit MINKAT (factory default \approx 0.45), the test fails. After two consecutive driving cycles, ME7.5 triggers DTC P0420 / 16804 (Catalyst System Efficiency Below Threshold - Bank 1).

52.2. The Secondary Lambda Trim Loop (`KAVS`)#

Crucially, Bosch ME7.5 uses the rear O2 sensor voltage to continuously calibrate the zero-point of the front wideband oxygen sensor:

Wideband planar sensors (LSU 4.2) suffer from chemical aging, thermal drift, and reference pump cell leakage over time.
The downstream sensor (LSF 4.2) is a Nernst cell that exhibits an extremely steep, immutable voltage cliff at exact stoichiometry (\lambda = 1.0000 \iff 0.450\text{V}).
The secondary control routine (KAVS) evaluates average post-cat voltage. If the rear sensor sits persistently at 0.78\text{V} (rich) while the front sensor reads \lambda = 1.000, the ECU concludes the front sensor has drifted lean.
It applies a long-term bias trim to the front sensor.
The Pitfall: When a mechanical spacer (non-fouler) is fitted to the rear O2 sensor to extinguish P0420, the sensor is isolated from the main exhaust gas velocity. The trapped, stagnant gas skews rear sensor voltage, causing KAVS to apply unwanted fuel trims of \pm 8\dots 12\% to the primary engine fueling, leading to phantom rich or lean running issues!

52.3. Complete Software Elimination Protocol for Rear O2 Sensor#

To eliminate the secondary O2 sensor cleanly in software without corrupting primary fuel trims or leaving hanging readiness monitors:

Step 1: Disable Master Functional Diagnostic Codewords#

Set the following one-byte configuration switches in flash:

Symbol Name	Flash Offset (032LP)	Factory Value	Tuned Value	Function
`CDKAT`	`0x0181A1`	`0x01`	`0x00`	Codewort für Diagnose Katalysator (Disables P0420 test routine)
`CDHSH`	`0x018197`	`0x01`	`0x00`	Codewort für Diagnose SondenhinterKat-Heizung (Disables rear sensor heater test)
`CDLSH`	`0x0181A0`	`0x01`	`0x00`	Codewort für Diagnose SondenhinterKat (Disables rear sensor circuit test)
`CWKAVS`	`0x0181E0`	`0x01`	`0x00`	Codewort für Katalysator-Alterung und Vergiftung (Disables secondary trim loop!)

Step 2: Neutralize Secondary Lambda Trim Authority#

To ensure zero trim drift, clamp the maximum allowable secondary trim authority:

Set CLAHSH (Limit for secondary trim authority) to 0.00.
Set FKHAB (Correction factor for post-cat heating) to 0.00.

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

When physically unplugging the rear O2 sensor from the underbody chassis harness connector:

The ECU output stage detects an open circuit on the rear sensor heater circuit (Pin 68).
Locate ESKONF Byte 2 / Byte 3 in flash.
Set the bit pair corresponding to ECU Pin 68 to 11b (0x03 = Unused / Diagnostic Masked), permanently preventing electrical DTCs P0140 (No Activity Detected) and P0141 (Heater Circuit Malfunction).

Cross-referenced on shared calibration symbols, not on subject matter — these are the chapters that touch the same maps.