Experiencing a Check Engine Light (CEL) along with “Check VSC” and “VSC Off” warnings on your Lexus can be unsettling. Recently, I encountered this very issue on my Lexus, and after some investigation with an OBDII code reader, the fault code P0135 appeared. This code points to an issue with the Oxygen Sensor Bank 1 Sensor 1, which is typically understood to be the upstream oxygen sensor located on the passenger side exhaust manifold (in UK/right-hand drive models).
My first instinct, and a common approach, was to replace the Bank 1 Sensor 1 oxygen sensor. Fortunately, having a collection of used parts, I proceeded to swap out this sensor. However, even after replacement, the engine didn’t run quite smoothly. It exhibited a low idle and a slight misfire. Upon re-scanning with the OBDII reader, the P0135 code persisted and couldn’t be cleared, indicating the problem was not resolved. Suspecting a faulty replacement sensor or a misdiagnosis, my friend suggested checking the oxygen sensor on the opposite bank.
Following this lead, I tackled the oxygen sensor on the other side of the engine. This sensor proved to be more challenging to access due to its proximity to the steering column shaft and various pipework. Despite the difficulty, I managed to replace this second oxygen sensor. Starting the engine, there was a slight improvement, but the engine still wasn’t running perfectly, and after a couple of days, the dreaded CEL and VSC warnings reappeared.
Frustrated and determined to resolve the issue, I delved into online forums, particularly those frequented by Lexus owners in America. My research led me to consider the Engine Temperature Sensor as a potential culprit. Lexus engines, like the 1UZFE and VVTi variants, typically have two temperature sensors located near the thermostat housing. One sensor provides a reading to the instrument cluster, while the other feeds data to the ECU. The ECU uses this information to determine the engine’s temperature and adjust air and fuel mixture accordingly for optimal cold and warm running conditions.
To investigate this further, I contacted Lexus for a quote on a new Engine Temperature Sensor. While aftermarket options were available at a lower price, I had read reports of some aftermarket sensors causing high idle and other running issues. To ensure reliability and rule out sensor incompatibility, I opted to explore my spare parts again. Luckily, I found a temperature sensor still attached to its aluminum water channel pipe. Still not completely certain this was the root cause, but with a readily available spare, I decided to replace the existing temperature sensor.
After ensuring the engine was cool and removing the expansion cap, I carefully unscrewed the old sensor (using a 19mm extended barrel socket). Anticipating coolant spillage, I had rags ready to minimize mess. As expected, some coolant escaped, but I quickly installed the replacement sensor. Following standard procedures, I ran the engine and bled the cooling system.
The outcome was positive. The engine now runs smoothly, the idle is correct, and the previous rough running issues have disappeared. Anecdotally, I also observed improved fuel economy, although further driving is needed to confirm this. During the period with the initial problems, fuel consumption was noticeably higher than usual.
The key takeaway from this experience is that while OBDII code readers are valuable diagnostic tools, the fault code they present may sometimes indicate a symptom rather than the actual root cause. In this case, the P0135 code, suggesting an oxygen sensor issue, was ultimately triggered by a faulty Engine Temperature Sensor. This highlights the importance of considering broader diagnostic possibilities and not solely relying on the initial OBDII code reading when troubleshooting engine issues.
