Where is the oxygen sensor on a Lexus rx300?
The Lexus RX300 typically uses four oxygen sensors: two upstream (before the catalytic converters) and two downstream (after the catalytic converters); one sensor on each bank of the V6 engine.
In the RX300, which uses the 1MZ-FE 3.0L V6, the sensors are labeled as Bank 1 Sensor 1 (upstream), Bank 1 Sensor 2 (downstream), Bank 2 Sensor 1 (upstream), and Bank 2 Sensor 2 (downstream). The upstream sensors monitor exhaust oxygen content before the cat, while the downstream sensors monitor after-cat emissions to help the engine manage the catalytic converter’s efficiency.
Sensor layout on the RX300
Most RX300 models with the 1MZ-FE engine position the four O2 sensors along the exhaust path: one upstream and one downstream on each bank of the engine. This arrangement allows the ECU to compare pre- and post-cat oxygen readings for precise fuel control and catalytic monitoring.
Before you start, note the general locations: upstream sensors are placed in the exhaust pipes near the exhaust manifolds on each bank, and downstream sensors are placed after each catalytic converter along the exhaust flow. Access points can vary by year and market, but the four-sensor setup is typical for RX300 models around 1999–2003.
In the RX300, these sensors are identified as follows:
- Bank 1 Sensor 1 (B1S1) — upstream sensor on Bank 1, before the catalytic converter.
- Bank 1 Sensor 2 (B1S2) — downstream sensor on Bank 1, after the catalytic converter.
- Bank 2 Sensor 1 (B2S1) — upstream sensor on Bank 2, before the catalytic converter.
- Bank 2 Sensor 2 (B2S2) — downstream sensor on Bank 2, after the catalytic converter.
These four sensors work together to inform the engine computer about exhaust oxygen levels and catalyst efficiency. If you’re replacing or diagnosing, you’ll typically remove and replace the upstream sensors first, then the downstream ones, using the proper O2 sensor socket and anti-seize compound on the threads.
Locating them in practice
Below are practical considerations for locating and accessing the sensors on a typical RX300. Use these tips to prepare for inspection or replacement, and always follow safety precautions when working under a vehicle.
- Consult the service manual or a reliable repair guide for your exact model year to confirm sensor positions. The layout described below generally applies to 1999–2003 RX300 models with the 1MZ-FE engine.
- Upstream sensors (B1S1 and B2S1) are located in the exhaust pipes near the exhaust manifolds on each bank; these are usually accessible from the engine bay or from above the engine depending on engine orientation in your market.
- Downstream sensors (B1S2 and B2S2) are located after the catalytic converters, along the exhaust path toward the mid-pipe or under the vehicle; access often requires a jack and jack stands or a floor jack for safety.
- Disconnect the battery before removing sensors to prevent ECU or sensor faults, and use a proper O2 sensor socket to avoid damaging the sensor’s hex.
After locating the sensors, plan your replacement with the correct horsepower-rated sensors and apply anti-seize compound to the thread sparingly to avoid seizing in the future. If in doubt, consult a professional mechanic for diagnosis and service.
Summary
The Lexus RX300 generally uses four oxygen sensors—two upstream and two downstream—one pair on each bank of the 1MZ-FE V6. Knowing which sensor corresponds to Bank 1 or Bank 2 and whether it is upstream (pre-cat) or downstream (post-cat) helps with diagnosis, replacement, and ensuring the ECU receives accurate exhaust information for optimal engine performance and emissions control.
