O2 Sensor - Downstream

Replace the downstream oxygen sensor(s) on a 2009 Aston Martin DB9 6.0L V12, located in the exhaust system after the catalytic converters.

Warnings

Tools required

Parts

• Downstream oxygen sensor (bank 1) × 1 — Use OEM specification
• Downstream oxygen sensor (bank 2) × 1 — Use OEM specification

Preparation

1. Park vehicle on level surface and apply parking brake.
2. Place wheel chocks at front wheels.
3. Allow exhaust system to cool completely (minimum 1 hour after last engine operation).
4. Raise rear of vehicle using jack and support securely on jack stands at designated lift points.
5. Verify exhaust system is cool to touch before proceeding.
6. If sensors have been in service for extended period, apply penetrating oil to sensor threads and allow 15-20 minutes soak time.

Procedure

1. 1

   Locate downstream oxygen sensors
   Position yourself under the vehicle with adequate lighting. The downstream oxygen sensors are located in the exhaust system after the catalytic converters, one on each exhaust bank. Bank 1 and Bank 2 each have a downstream sensor positioned in the exhaust pipe after the catalytic converter outlet; verify bank numbering and exact sensor location against Aston Martin service documentation for this vehicle. Identify the sensor electrical connectors which will be secured to the exhaust system or chassis with a clip or bracket. Note: The downstream oxygen sensors are located in the exhaust system after the catalytic converters, one on each exhaust bank. Verify Bank 1 and Bank 2 side assignment against the factory service information for this engine, as bank numbering follows cylinder layout rather than a fixed left/right rule. Each sensor is positioned in the exhaust pipe downstream of the catalytic converter outlet. Note: Each exhaust bank has a downstream oxygen sensor positioned in the exhaust pipe after the catalytic converter outlet. Confirm Bank 1 and Bank 2 physical side assignment against the Aston Martin service documentation before proceeding, as cylinder-bank orientation determines which sensor corresponds to which DTC. Note: Oxygen sensor socket with wire slot (verify sensor hex size against Aston Martin service data for this vehicle; commonly 22mm or 7/8")
2. 2

   Disconnect electrical connector - Bank 1 sensor
   Trace the wiring harness from the Bank 1 downstream oxygen sensor to its electrical connector. The connector is typically secured with a locking tab. Press the release tab and carefully pull the connector halves apart. If the connector is secured to a bracket or clip, release it from the mounting point. Inspect the connector terminals for corrosion or damage.
3. 3

   Remove Bank 1 downstream oxygen sensor
   Using the oxygen sensor socket with wire slot (to accommodate the sensor wiring), position the socket over the Bank 1 downstream sensor. Attach the ratchet with extension as needed for clearance. Turn counterclockwise to loosen the sensor. The sensor should break free with moderate force; if excessive resistance is encountered, apply additional penetrating oil and wait. Once loosened, unscrew the sensor by hand and remove it from the exhaust bung. Note the condition of the sensor threads and bung.
4. 4

   Prepare exhaust bung - Bank 1
   Inspect the threaded oxygen sensor bung in the exhaust pipe. Use a wire brush to clean the threads, removing carbon deposits and old anti-seize compound. Ensure the threads are not damaged or cross-threaded. If threads show damage, the exhaust section may require replacement or thread repair before installing a new sensor.
5. 5

   Install new Bank 1 downstream oxygen sensor
   Apply a thin coat of nickel-based anti-seize compound to the threads of the new oxygen sensor. Apply only to the threads; keep compound away from the sensor tip (bottom 1/2 inch) and electrical connector. Thread the new sensor into the Bank 1 exhaust bung by hand, ensuring it starts smoothly without cross-threading. Once hand-tight, use the oxygen sensor socket and torque wrench to tighten to the manufacturer-specified torque (verify exact O2 sensor torque spec in Aston Martin DB9 service documentation before installation). Note: Once hand-tight, use the oxygen sensor socket and torque wrench to tighten to the factory-specified torque for this engine (verify the exact value in Aston Martin service documentation before final tightening). Note: Once hand-tight, use the oxygen sensor socket and torque wrench to tighten to the manufacturer-specified value (verify exact O2 sensor torque in Aston Martin service data before final tightening; do not rely on a generic torque assumption for this V12 application).
   Torque spec

   O2 Sensor45 Nm (33 lb-ft)
6. 6

   Reconnect electrical connector - Bank 1
   Route the sensor wiring harness to avoid contact with hot exhaust components or moving parts. Connect the electrical connector halves, ensuring the locking tab clicks into place securely. If a bracket or clip was present, secure the connector to its mounting point. Verify the wiring has adequate clearance and is not under tension.
7. 7

   Disconnect electrical connector - Bank 2 sensor
   Locate the Bank 2 downstream oxygen sensor electrical connector (confirm Bank 2 physical side assignment against Aston Martin service documentation, as bank numbering follows cylinder layout rather than a fixed left/right rule). Press the release tab and carefully separate the connector halves. Press the release tab and carefully separate the connector halves. Release the connector from any mounting bracket or clip. Inspect terminals for corrosion or damage.
8. 8

   Remove Bank 2 downstream oxygen sensor
   Position the oxygen sensor socket over the Bank 2 downstream sensor. Using the ratchet with extension, turn counterclockwise to loosen. Once broken free, unscrew by hand and remove the sensor from the exhaust bung. Examine the sensor condition and note any unusual deposits or damage that might indicate engine issues.
9. 9

   Prepare exhaust bung - Bank 2
   Clean the Bank 2 oxygen sensor bung threads with a wire brush, removing all carbon buildup and old anti-seize. Inspect threads for damage. Verify the bung is in acceptable condition for sensor installation.
10. 10

    Install new Bank 2 downstream oxygen sensor
    Apply nickel-based anti-seize compound to the threads of the new Bank 2 sensor, keeping compound away from the sensor tip and connector. Hand-thread the sensor into the exhaust bung, ensuring proper alignment. Once hand-tight, torque to the factory-specified value using the oxygen sensor socket and torque wrench (verify the exact torque in Aston Martin service documentation).
    Torque spec

    O2 Sensor45 Nm (33 lb-ft)
11. 11

    Reconnect electrical connector - Bank 2
    Route the Bank 2 sensor wiring to avoid exhaust heat and moving components. Connect the electrical connector halves until the locking tab engages. Secure the connector to any mounting bracket or clip. Verify proper wire routing and clearance.
12. 12

    Lower vehicle and clear diagnostic codes
    Carefully lower the vehicle from jack stands and remove wheel chocks. Start the engine and verify it runs normally. The check engine light may illuminate initially due to stored oxygen sensor codes. Using an OBD-II scanner, clear any stored diagnostic trouble codes related to the downstream oxygen sensors. Confirm the specific DTC-to-sensor mapping with the scanner and factory documentation, as code assignments on this V12 may differ from generic bank/sensor code numbers. Allow the engine to reach operating temperature while monitoring for fault codes. Note: Using an OBD-II scanner, clear any stored diagnostic trouble codes related to the downstream oxygen sensors; confirm the specific DTCs and their bank/sensor assignments against Aston Martin DB9 diagnostic documentation. Note: Using an OBD-II scanner, clear any stored diagnostic trouble codes related to the downstream (Sensor 2) oxygen sensors. Verify the specific DTCs against Aston Martin diagnostic data; generic post-catalyst codes such as P0136/P0137/P0138 and P0156/P0157/P0158 may apply, but their exact bank/sensor assignment on this V12 must be confirmed against Aston Martin diagnostic documentation and the scan tool rather than assumed.

Reassembly

1. No disassembly was required for this procedure beyond sensor and connector removal.
2. Ensure all electrical connectors are fully seated with locking tabs engaged.
3. Verify sensor wiring is routed away from heat sources and has adequate clearance.

Verification

• Start the engine and allow it to reach normal operating temperature.
• Verify the check engine light does not illuminate after clearing codes and completing a drive cycle.
• Using an OBD-II scanner, monitor live data for both downstream oxygen sensors to confirm they are reporting voltage fluctuations (typically 0.1-0.9V cycling).
• Perform a test drive including highway speeds to allow the ECU to complete oxygen sensor readiness monitors.
• Re-check for diagnostic trouble codes after test drive; no oxygen sensor codes should be present.
• Visually inspect underneath vehicle for any exhaust leaks at sensor installation points (listen for hissing sounds with engine running).

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