2005 Chevy Uplander P0420: How to Confirm Closed Loop Operation and Catalyst Efficiency

1) "Is it in closed loop if I erase the P0420 code and the check engine light stays off for two days?" >>> No — the system is not considered to be in closed loop for the entire duration. Closed loop operation depends on real-time sensor feedback, not just the absence of a code. 2) "Would the ECM detect abnormal O2 sensor signals individually?" >>> Yes — if the O2 sensors are malfunctioning or sending inconsistent data, the ECM will typically log this information in live data, which can be viewed via an OBD-II scanner. 3) "How do I know if I'm in closed loop?" >>> Your OBD-II scanner should display whether the system is in open or closed loop mode — this is a direct readout from the engine control module (ECM). 4) "Is a clean tailpipe a good indicator of optimal fuel economy?" >>> I disagree. A clean exhaust does not guarantee efficient combustion. An overly lean air-fuel mixture can produce high emissions and poor fuel economy without showing visible soot or smoke. Suggestions: - Avoid repeatedly erasing codes unless necessary — this resets the system's learning process and may mask underlying issues. - Closed loop operation means the ECM is actively adjusting fuel trim based on real-time sensor input. It typically occurs when the engine is warm, sensors are functioning properly, and feedback is stable. - P0420 codes are like a temperature gauge — they indicate overheating or inefficiency, not a permanent failure. Before replacing components, investigate whether there's an underlying issue (e.g., fuel trim problems). Your 'gauge' here is the catalyst efficiency. Can we assist with further troubleshooting?

Background: I replaced my original engine in a 2005 Uplander with a 3500 Lx2 V6 that had only 10 km (about 6 miles) of use and was stored for five years. Before starting, I sprayed a lithium-based lubricant into the spark plug holes to prevent ring galling after long-term storage. After about 2000 miles, the catalytic converter became clogged. The vehicle struggled to move at 125k miles — clearly a failure of the catalyst and possibly the O2 sensor. I removed the catalytic converter media and now the van runs smoothly with no check engine light. The P0420 code remains, but the car performs well. I suspect that the lithium spray caused damage to both the catalyst and the oxygen sensors. I’ve purchased an ELM Bluetooth OBD-II dongle (ELM 327 HH Advanced), but it came with no software or instructions. I received no support from the seller — they were unhelpful. This product is essentially useless without proper setup. My next steps: - Confirm that all traces of lithium have been fully purged from the engine and exhaust system. - Replace the catalytic converter with an aftermarket unit and install new Delphi O2 sensors. - Connect my analog oscilloscope to the O2 sensor signal wire (with 10 MΩ impedance) to check for sensor poisoning — I wish I had a storage scope. I originally intended only to protect cylinder walls using lubricant, not to coat them heavily. The goal was to evenly fog the combustion chamber walls without causing hydraulic lock or excessive oil buildup. The van runs well now, but I still don’t know if it’s in closed loop mode. The only code is P0420 — no other codes. Fuel economy is stable, though slightly lower than before. My main takeaway: To determine closed loop status, I need a data scanner to access real-time sensor readings. Question: If the ECM detected unusual O2 sensor signals, would it trigger an individual O2 sensor fault code? Note: Since no such code appears, I’m inclined to believe the system is in closed loop. Is this conclusion reasonable as I work on restoring the exhaust system? I’ve already addressed the core issue — getting the exhaust system back in order.

The vehicle will enter closed loop mode when engine conditions (such as ECT temperature and pre-cat O2 sensor readings) are favorable. Regarding your question: "Would the ECM trigger an O2 sensor fault code if it detected abnormal signals?" >>> No — the O2 sensor is just a signal transmitter. The PCM detects abnormal readings from the post-catalyst O2 sensor and interprets them as poor catalyst performance, which results in the P0420 code. In professional diagnostics, a stacked waveform of pre- and post-cat O2 sensor voltages is the standard method to assess catalytic converter efficiency. A quick Google search for 'cat efficiency waveforms' will show several examples. You're overcomplicating open vs closed loop — focus on real-world performance and sensor data instead.

Thanks — I appreciate the point about overthinking closed loop. Since the vehicle runs smoothly, I’ll leave it as is for now until I can get a reliable scanner. I'm currently trying to get my ELM Bluetooth OBD-II dongle (ELM 327 HH Advanced) working. It arrived without any software or instructions — no way to activate or use it. I may have to purchase a different device. It seems many users face the same issue: receiving hardware with no usable software or setup guidance.

My suburban driving has improved from 17 to 18 MPG since I stopped resetting the ECU. The only code remaining is P0420. This may suggest that the system is in closed loop — if fuel trim is actively adjusting, it could indicate stable feedback control. I plan to get a dedicated OBD-II scanner soon for real-time data. I believe fuel economy calculations are based on algorithms and not actual fuel measurement — do you agree? This might explain why MPG readings don't always reflect true consumption.

Agree — fuel economy estimates are algorithm-based, not real-time measurements. These displays can be misleading or 'feel-good' numbers that don’t represent actual fuel usage.