2002 Mercedes-Benz ML320 P0155 Oxygen Sensor Heater Circuit Failure - Catalytic Converter Not Ready for Smog Test
Hi everyone, I'm currently troubleshooting my 2002 Mercedes-Benz ML320 with 90,000 miles. I need to pass a smog test soon and am facing an ongoing issue with the catalytic converter readiness code P0155. Last week, during an OBD-II diagnostic, the system flagged a failure in the oxygen sensor heater circuit on Bank 2 (specifically sensors 1 and 2). About a year ago, I replaced the front left-side catalytic converter and installed a new Bosch oxygen sensor for Bank 2 Sensor 1. The vehicle ran smoothly at that time with the check engine light illuminated — which was expected since I did not replace the rear Bank 2 Sensor 2 (located on the muffler), as it was considered expensive and not required back then. To pass the smog test, I recently replaced the Bank 2 Sensor 2 and reset the system. After driving approximately 200 miles under normal conditions, the check engine light has turned off. However, when I connect the OBD-II scanner again, the diagnostic still shows that the catalytic converter is not ready — despite no active fault codes. My question: What could be causing this 'catalytic not ready' status? The smog test deadline is approaching, and I need a reliable explanation to resolve this issue permanently. Any insights or recommendations would be greatly appreciated! Thanks in advance for your help!
If no fault codes are present, try searching for a Mercedes-Benz-specific 'drive cycle' — these outline the driving conditions required to activate and complete emission monitors. While I don't have an official MB drive cycle available here, reviewing GM or Chrysler drive cycles can provide useful context. Note: If the vehicle was running normally with the check engine light on, that's highly unusual. In Mercedes-Benz systems, oxygen sensors and catalytic converters are typically paired components — replacing one without the other often leads to system inconsistencies. You may need to replace both sensors or ensure proper integration. Additional note: Bosch sensors are not widely regarded as top-tier in automotive applications; consider evaluating alternatives if possible.