Experiencing car troubles can be frustrating, especially when your Lexus throws up a Check Engine Light (CEL) accompanied by “Check VSC” and “VSC Off” warnings. Recently, I encountered this exact scenario, and after some investigation, I want to share my experience which might help other Lexus owners facing similar issues, particularly those encountering the P0135 OBDII code.
Initially, after the lights illuminated on the dashboard, I sought out a reliable OBDII code reader. Fortunately, a friend had a high-quality Bosch diagnostic tool which pinpointed the error code: P0135. According to the reader, this code indicated a problem with the “Oxygen Sensor Bank 1 Sensor 1.” For those unfamiliar, on a UK specification Lexus (right-hand drive), Bank 1 Sensor 1 typically refers to the upstream oxygen sensor located on the passenger side exhaust manifold.
Confident in this diagnosis, and having a stock of used parts from previous Lexus projects, I proceeded to replace the indicated oxygen sensor. However, after the replacement, the engine still didn’t feel quite right. It exhibited a slightly low and uneven idle, almost like a subtle misfire. Upon re-scanning with the Bosch reader, the P0135 code persisted. The fault remained, preventing the code from being cleared. My friend then suggested the issue might lie with the oxygen sensor on the opposite bank.
Undeterred, I tackled the replacement of the other upstream oxygen sensor. This sensor, positioned on the driver’s side, proved to be more challenging to access due to its proximity to the steering column shaft and various pipework. Despite the awkward location, I managed to swap it out. Starting the engine, there was a marginal improvement, but the engine still wasn’t running as smoothly as expected. After a few days, the dreaded CEL, “Check VSC,” and “VSC Off” lights reappeared.
At this point, frustration was setting in. We all appreciate the renowned smoothness of Lexus engines and strive to maintain that perfect running condition. Turning to online forums, particularly those frequented by US Lexus owners, I delved into potential causes beyond oxygen sensors for similar symptoms and error codes. My research led me to consider the Engine Temperature Sensor. Specifically, Lexus models (including those with the 1UZFE and VVTi engines) often have two temperature sensors situated close together behind the thermostat housing. One sensor is responsible for the temperature reading displayed on the instrument cluster, while the other sends temperature data to the Engine Control Unit (ECU). This ECU input is crucial for determining whether the engine is cold or warm, influencing air-fuel mixture adjustments for optimal running under different temperature conditions.
I contacted Lexus to inquire about the Engine Temperature Sensor. The price quoted was £48 plus VAT. While aftermarket options were available for around £20, forum discussions cautioned against potential issues with some aftermarket sensors causing high idle or other running irregularities. Deciding to explore my own parts inventory again, I located a used temperature sensor still attached to its aluminum water channel pipe. Although not entirely convinced this was the definitive solution, I decided to proceed with replacing the temperature sensor.
Ensuring the engine was cool and the expansion cap was removed, I carefully unscrewed the old sensor using a 19mm extended barrel socket. Anticipating coolant spillage, I had rags prepared around the sensor area. As expected, some coolant escaped as the sensor was removed, but I quickly installed the replacement sensor. Following standard procedure, I ran the engine and bled the coolant system to remove any air pockets.
The outcome was overwhelmingly positive. The engine now runs perfectly. The idle is smooth and correct, and the previous rough running issues have vanished. Interestingly, my research also suggested that replacing a faulty temperature sensor can improve fuel economy. While only time will confirm this, I had noticed increased fuel consumption during the period with the engine running poorly.
The key takeaway from this experience is that while OBDII code readers are invaluable diagnostic tools, the indicated fault code may sometimes point to a symptom rather than the root cause. In this case, while the P0135 code correctly identified an issue within the oxygen sensor circuit (heater circuit malfunction), the underlying problem was not a faulty oxygen sensor itself, but a malfunctioning Engine Temperature Sensor impacting the overall engine management system and triggering the oxygen sensor related code. Therefore, when troubleshooting car issues, especially with seemingly straightforward OBDII codes, it’s crucial to consider broader system interactions and not solely rely on the initial code interpretation. This experience highlights the importance of comprehensive diagnostics and understanding the interconnectedness of engine components for effective car repair.
