P0158 and P0160 OBD2 Codes on 1999 Infiniti QX4: Causes and Solutions After Timing Belt Service
My 1999 Infiniti QX4 with a 3.3L 6-cylinder engine and 4-wheel drive has failed an emissions test due to OBD2 diagnostic codes P0158 and P0160. The inspection station initially suggested replacing the oxygen sensor as the fix, but I was advised at an auto parts store that many people have replaced O2 sensors without success—indicating a deeper issue. The vehicle currently has 107,000 miles on it. I'm seeking reliable advice on possible root causes for these codes. Additionally, I understand my Infiniti QX4 is equipped with four oxygen sensors—where are they located, and which specific sensor(s) are associated with P0158 and P0160? For context: the vehicle recently underwent a comprehensive service including timing belt replacement, belt tensioner, water pump, radiator, hoses, belts, and spark plug replacement. Could these recent repairs be contributing to or triggering the O2 sensor codes?
After a major service like timing belt replacement, it's not uncommon for OBD2 codes such as P0158 and P0160 to appear. These codes may indicate issues with the exhaust system or sensor readings post-repair—especially if there are leaks or misalignments in the intake or exhaust components. It's important to verify that all repairs were completed correctly and that no vacuum or manifold leaks exist before assuming a faulty oxygen sensor.