Closed loop mode

Can someone please give me a synopsis of what this means? I know it happens when the engine heats up, but what changes when this happens?

Closed loop means once the engine reaches operating temperature, the computer uses the signals from the O2 sensor(s)for fuel control.


So if I understand correctly from my little research, this is better fuel mileage as well? so warming your vehicle before you drive will result in better mileage? Hope this doesn’t sound too stupid.

I can tell you what “closed loop mode” means and I can tall you what happens when an engine heats up, but without some context to put your question into I cannot give you a specific answer.

Closed loop mode means that something causes a specific action that then reacts by affecting the original “something” in such a way that it continues to cause the specific action. Each causes the other, forever unless the loop is broken by an outside force.

An example would be a muscle spasm caused by a pinched nerve in the spinal column. The pinched nerve causes the spasm, which cause the vertebra to keep piching the nerve, which causes the muscle to keep spasming, which causes the vertebra to keep pinching the nerve, which…get the idea?

An engine example might be a blown headgasket caused by overheating. Lets say the engine overheats, say from a stuck thermostat, warping the head and allowing the headgasket to blow. The blown headgasket then allows the combustion gasses to be blown into the water jacket causing the engine to overheat. The overheating then causes further damage to the headgasket, which then allows the engine to keep overheating, which then allows further damage to the headgasket. Until something in the loop is altered, the condition will continue, one problem continuing to cause the other. In this case the original “root cause” may have been repaired but you now have a “closed loop” between the blown headgasket and the overheating. You’d need to fix the headgasket to stop the loop.

No, not a stupid question. But no, you’ll use extra gas warming it up, the car warms up quickly, just get in and go.

Context, eh? well my neighbors 99 explorer has a engine code of - P0125 - Insufficient Coolant Temperature For Closed Loop Fuel Control- From my limited knowledge and research this is either most likely a sensor or the thermostat. it has 137k on it and neither have ever been replaced. Before I go digging into his engine to fix it, I want to make sure it is the correct item to fix.

Have you checked the coolant level?

Got it.

As Oldschool said, check the level. But insufficient coolant temp is almost always a T-stat.

To verify a bad sensor without actually reading its signal simply run it in parallel with a known good sensor and compare.

A T-stat can be checked with a hotplate, a pyrex vessel, and a thermocouple temp measuring system (or a lab thermometer that will be comfortable at high temps). But since that stuff costs much more than a T-stat, I just change the T-stat.

Remember that a T-stat’s function is not to allow an engine to cool, but rather to allow it to reach operating temp. An engine will cool fine without a T-stat, but it won’t heat up without one.

the coolant level is normal. So I will proceed with the thermostat. the engine running cooler means running richer, correct?


When you replace the T-stat, be sure you burp the system. Many vehicles have a burp valve at the cooling system high point to allow the air to purge. Don’t know about your Explorer.

Motorcraft thermostats have a little jiggle valve in them that allows air to get through. Aftermarket thermostats often don’t have this and might not work very well. My wife’s vehicle went through this a few weeks ago. She had to have the car in a hurry so both temp sensor and t-stat were replaced. I will figure out what to throw in the trash when I get around to testing both of them with the already-mentioned hot plate and thermometer.

Is the neighbor due for a coolant change?