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Diagnosing closed loop fault state

Old 12-04-2017, 11:53 AM
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Diagnosing closed loop fault state

Howdy,

I've noticed that my Goodbox reports a fuel system state of "CLErr" which I interpret to mean closed loop fault (from the list of possible fuel system states in OBD). Fully warmed up, sitting in cruise control on a relatively flat road, it jumps between Open Loop and Close Loop Fault, for about a minute each. In CL Fault, AFR matches target AFR of 14.7, while in OL AFR goes to 13.8 (for roughly the same calculated load). This is weird... I don't have any DTC's and the check engine light is off. I noticed this purely by accident.

Are there good ways to diagnose CL faults?
AFAIK my O2 sensor is working fine, it reports a stable AFR, and the car seems to still use it, given that AFR's meet targeted while it's in CL fault mode.

My other suspicion is that with the Mazmart low-temp thermostat, it might not be reaching sufficient temperature in this cold weather to go open loop. It holds between 76-78C at speed (168-172F). I would expect a DTC of "took long to reach closed loop" if that was playing into it.

Fuel trims are +/-3 at all times, so no obvious problem there.

So, does anyone know what conditions need to be met for the car to go closed loop without faulting, or what conditions cause closed loop fault?

Thanks!
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Old 12-04-2017, 06:58 PM
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It may just be riding on the edge between CL and OL. Mazda kind of f’d up the transition point for US highway cruise speeds. It’s so close to the edge there that the slightest aberation of grade, bumps, wind, etc. can push it over. Of course it’s going to go richer in OL than CL at the same load/speed/etc. IMO it’s one of the reasons it doesnt get good highway mileage. I tried tuning around it some and didn’t make much progress, but at that time my street use was winding down to just performance tuning purposes only and I just never had any real reason to pursue it further.
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Old 12-04-2017, 09:10 PM
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That's interesting, thanks. Might be playing into the better fuel mileage on long trips over 130km/h. So I get the flip-flopping, but in your case, were you seeing just CL or CL Fault?
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Old 12-04-2017, 09:31 PM
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My OBD-II sometimes reports a CL fault when idling after a cold start, but warm enough that the air pump doesn't come on. Usually is replaced by normal CL status after 15-20 seconds. My guess is it has something to do with rear O2 sensor temperature...
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