2004 Mercury Mountaineer Bank 1 Sensor 1 Heat Circuit Fail.

ELDERBAUM

MEMBER
2004 MERCURY MOUNTAINEER
4.0L
6 CYL
AWD
AUTOMATIC
207,000 MILES

Ok, I know what this is, but my question is I Put a new motor in this truck back in July. 3 weeks ago It started making a ticking clanking sound, at idle while at operating temp. Engine being under full warranty it was replaced. I had no trouble codes before it went in to the shop, it ran and drove fine, it was loosing oil pressure at the top end of the motor. I picked it up today and after the new engine was installed and headed out and my check engine light can on. Prompting me with the O2 sensor. It wasn't on during the time after the engine was installed and their test drive and driving it. It came on after I picked it up. I am kind of irritated because it wasn't on before and now it throws it. Could it be from the install of the new engine?

1 Reply

CARADIODOC

MECHANIC
33,871 POSTS

There are dozens of fault codes related to the oxygen sensors, but one of the easiest to diagnose is the heater circuit. The most likely cause, given the recent history, is a stretched or corroded terminal in the connector, but normally that would set the code right away, not minutes or miles after you picked up the car.

There are, however, a couple of points to be aware of. First, if the connector was overlooked, there are other codes that would set related to the sensor's performance, and those may not set until the coolant temperature reached a certain value. That's the signal that tells the Engine Computer to start watching the O2 sensors' readings. Until then, a performance code may not set.

Second, there is always a long list of conditions that must be met for a code to set. Among those, certain other codes must not be in memory already if they're related to things the computer uses for a reference. Only about half of the over a thousand potential fault codes turn on the Check Engine light, so there could have been a code in memory all along that you didn't know about, and that could abort some self-tests run by the computer. Now that the original code has been erased, the self-tests can resume, and that could be why the O2 sensor code first set now.

I don't THINK that is the case here since a heater code is pretty straight-forward, but it can explain why some codes aren't set when a problem occurs. I'm leaning toward a poor connection between mating terminals in the O2 sensor's connector.