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I have a 6 year old Goodman GMVC96 60K BTU furnace with a 2.5 ton A-coil. The behavior of the 2-stage gas valve puzzles me. After about 5 minutes on LO fire and before the thermostat has been satisfied, the gas valve CLOSES and the blower SPEEDS UP. At that moment, the control board displays an E3 error code. After a few more minutes circulating unheated air, the blower spools back down and stops. Almost immediately thereafter the E3 error code is canceled and the start sequence resumes successfully but now the gas valve is on HI fire. The furnace continues to run in this mode until the thermostat has finally been satisfied.

I understand that the E3 error code indicates an open limit circuit. I believe this is most likely the high limit in my system due to insufficient conditioned air over the heat exchanger. I changed the air filter and verified that all supply registers were open and that no return registers were blocked but the abnormal behavior persisted. During a service call a few years ago, the HVAC tech advised me that our system had markedly insufficient return air. With that observation in mind, I just had an additional 12" flexible return duct installed. Although that may have helped somewhat, it did not eliminate the behavior I've described.

Thanks in advance for any guidance!

Regards,
John

Measure the temp rise and adjust the fan speed.  Chances are the manifold pressures are way off.  Most Goodman's are way off from the factory.

Thank you! I will perform the troubleshooting you have suggested and report back to this forum.
After the furnace restarts, the fan speed is much faster, presumably to accommodate the HI fire gas valve 2nd stage. Does it make sense that this greater movement of conditioned air over the heat exchanger is what prevents the high limit circuit from opening again?

Regards,
John

The gas pressure is likely too high in low fire.  You need to lower it to 1.9"wc.

Or increase the fan speed to lower the temp rise.

This is an LP conversion so I believe the LO fire pressure should be 6.0" w.c. Is that correct?

Regards,
John

Yes, that's right.

Thank you Admin! The LO fire propane pressure was indeed too high (9.6 wc"). We decreased that to 6.0 wc" and also increased the blower speed slightly. So far, so good. No more E3 error codes. I truly appreciate your help pointing me in the right direction!

Regards,
John

Great to hear!