Persistent O2 Sensor Fault Codes P0135, P0147, P0155 on 1999 Cadillac DeVille - Inspection Failure in NY State
I brought my 1999 Cadillac DeVille in for a check engine light diagnosis. A GM-certified mechanic replaced three of the four oxygen (O2) sensors, at a cost of approximately $700. However, the same fault codes—P0135, P0147, and P0155—returned after just 50 miles. My own OBD-II scanner confirmed the issue persisted. The mechanic claimed the codes were 'bogus' and attributed the problem to a loose wire, charging an additional $200 for diagnosis (which reportedly took three hours). Despite this explanation, the same codes reappeared about a week later. I'm now questioning whether the O2 sensors are actually faulty or if the codes are being misinterpreted—especially since the vehicle fails emissions inspection in New York State. Given that these codes are persistent and consistent across multiple visits, is it possible the issue lies elsewhere (such as power feed or ground connection) rather than a sensor failure? Or has the mechanic made an error in diagnosis?
Did they install GM-brand O2 sensors? If not, that raises concerns about part quality. If they did use OEM parts and the codes persist, it's likely there's a problem with the power supply or ground connection in the O2 sensor circuits—these are common failure points in 1999 Cadillac DeVille models.