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Hi, I have a 2008 Chevrolet Express 3500 van with a 6.0 motor. Runs perfect except MIL is on. MIL came on after about an hour of driving, so engine was hot.
Code is P0420, and freeze fame data says with both banks were in "open loop due to insufficient engine temperature." Data also said coolant temperature was 196 degrees, catalyst temperature on both banks was 1175 degrees, rpm was 569 and speed was 0 mph.
Reset codes twice, and same results in about 90 miles both times. Looking at scanner the van goes into closed loop a few minutes after startup, and temperature sensor readings on dash gage and on scanner agree and look good while driving.
I don't get why catalyst monitor and code was set at idle, and why it says it was in open loop due to cold engine when temperature was hot.
Freeze frame data, while crucial, is often wrong. I have seen many lean codes set in what freeze frame would have you believe is open loop. (If you weren't trimming fuel how did you know it was lean?)
One thing I really like about working on GM products, you can check which code monitors have run during the current drive. If you are thinking there really is a temperature related issue keeping the computer in OL, I would check to see if p0128 is failing, but not enough to set a light. That is insufficient time/temperature, usually indicating a thermostat that isn't working great.
