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1997 Ranger with a 2.3

Can an oxygen sensor be defective and the truck not have a CEL?

Can the heater circuit not work properly and still not have a CEL?

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well, ive never seen a bad 02 sensor that didnt cause a CEL. heater circuit? i dont know what you mean by that. if the engine doesnt come up to temperature because of a stuck-open (or missing) thermostat you wont get a CEL. it will just assume the engine is in "cold" status and adjust idle-air valve and fuel delivery to a rich condition. eventually it may trip the CEL, it will certainally burn more fuel that way.

 

There is a heater circuit in the oxygen sensor that will warm the sensor so the engine can go into closed loop.

When my engine is cold in open loop it will run fine, but as soon as it goes into closed loop I have some idle and driveability problems. These problems go away as the engine warms up to operating temps.

I'm pretty sure that my intake air temp and coolant temp sensors are working fine.

I'm wondering if the oxygen sensor heater circuit is messing up, and the cool exhaust can't make the sensor read right. Then when the exhaust gets hot enough, the sensor starts working better.

 

460: do you perhaps have TWO coolant temp sensors? One for the gauge and one for the ECM? Some do, or did at one point in time.
A HEGO is heated just as you note, so it can 'read' the mix and go into closed loop. Most times, the open loop mode sets a richer mixture than closed, which jibes with your driveability experience.
If your ECT was a bit over zealous in reporting warm temperatures, or you had a hole in the ducting that pre-heats the incoming air, or a lazy vacuum motor that positions the heated air flap, you could get what you report...
tom

 

The answer is in short, YES. The computer looks for certain response times in order to condemn the O2 sensor. The computer may still report that the sensor is good, until the fuel trim hits a certain critical cutoff point, about 15% I think on Fords. When that point is reached, the computer has to make a decision about what is wrong. If the O2 sensor still has a good response, It will report the failure as a lean or rich code. If it does not have a good response, it will report back as a sensor voltage too high or too low.

In either case, these are both extreme codes. Because the accepted input parameters are so wide, it is recommended that O2 sensors be replaced every 60,000 - 80,000 miles. They should be replaced before they fail, because other things can be damaged if they fail. For example the catalytic converters can be damaged by any fuel trim of greater than 5% over a long period of time.

That one is harder to answer, but the heater circuit is only a small part of the sensor. The computer wont reject the heater unless the heater either registers as a short, or an open circuit. If you are having problems getting the O2 sensor to warm up, or to have the vehicle get into open loop mode, a thermostat is more likely the ticket.

But as I mentioned earlier, if the O2 sensors have over 80,000 mile on them, it is still a good idea to replace them.

 

I looked at the O2 sensor today, and it doesn't look like it's ever been replaced. The truck just turned 180,000 miles.

The truck engine warms up fine and goes into closed loop normally. I'm using a ScanGauge II that plugs into the OBD-2 port under the dash, so I'm guessing it "sees" the same data as the EEC would see. If so, the air intake and coolant temp sensors both report normal temperatures and appear to be working fine. If there is a separate coolant temp sender for the gauge, as there often is, I don't think it runs through the OBD-2 port.

I'm going to take Bear's advice and try a new O2 sensor just because the one in there looks to be real old.

We'll see...



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You do have 2 coolant sensors - one sends a signal to the temp. gage and the other sends a signal to the ECM. I agree with your decision to replace the O2 sensor.