Komatsu WA270-8 Recurring NOx Sensor Faults

Confirmed cause: A failing turbocharger with worn internal seals was allowing a small, almost undetectable amount of engine oil to leak into the exhaust stream. This oil was contaminating the downstream aftertreatment components, including the outlet NOx sensor, causing it to report incorrect values and trigger fault codes. Repair performed: The turbocharger was replaced with a new unit. The exhaust gas recirculation (EGR) cooler and all connecting pipes were inspected for oil residue and cleaned thoroughly. The DPF/SCR catalyst was also inspected with a borescope for signs of oil saturation or face plugging; minimal contamination was found and it was deemed acceptable for continued service. A new outlet NOx sensor was installed as a precaution, given the history of repeated faults. Verification: After the repair, a service regeneration was initiated and monitored. Outlet NOx readings now dropped to and maintained near-zero levels as expected. The machine was operated for over 50 hours under various load conditions with no recurrence of the NOx sensor fault codes. Technician notes: - This was a difficult case because the amount of oil leaking from the turbo was not enough to cause visible smoke or significant oil consumption, which are the typical signs of turbo failure. - This scenario highlights how a mechanical engine component failure can manifest as a complex aftertreatment electronic fault. - Always suspect exhaust stream contamination when NOx sensors fail repeatedly without a clear electrical cause.