('07 Cobalt, Ecotec 2.2L)

I came on the text below in the AllData (paywalled) service manual. It seems like they're saying that ambient air comes into the exhaust 'oxygen' sensor (pic below) ... through the wires? Through some 'gap' between the wire and its surrounding insulation? ("Sharp bends or kinks could block the reference air path through the lead wire.")

This seems so outside of anything I've every heard of that I wonder if I'm just reading it wrong?

Heated Oxygen and Oxygen Sensor Notice

Notice: Do not remove the pigtail from either the heated oxygen sensor (HO2S) or the oxygen sensor (O2S). Removing the pigtail or the connector will affect sensor operation. ...

Do not repair the wiring, connector or terminals. Replace the oxygen sensor if the pigtail wiring, connector, or terminal is damaged.

This external clean air reference is obtained by way of the oxygen sensor signal and heater wires. Any attempt to repair the wires, connectors, or terminals could result in the obstruction of the air reference and degraded sensor performance.

The following guidelines should be used when servicing the heated oxygen sensor:

  • Do not apply contact cleaner or other materials to the sensor or vehicle harness connectors. These materials may get into the sensor causing poor performance.

  • Do not damage the sensor pigtail and harness wires in such a way that the wires inside are exposed. This could provide a path for foreign materials to enter the sensor and cause performance problems.

  • Ensure the sensor or vehicle lead wires are not bent sharply or kinked. Sharp bends or kinks could block the reference air path through the lead wire.

  • Ensure that the peripheral seal remains intact on the vehicle harness connector in order to prevent damage due to water intrusion. The engine harness may be repaired using Packard's Crimp and Splice Seals Terminal Repair Kit. Under no circumstances should repairs be soldered since this could result in the air reference being obstructed.

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  • I had understood it to come in from the butt end of the sensor and not the wires themselves. Wires are wires, which means they have metal coated with PVC (or whatever they use). Not much air traveling through there in my approximation. I don't have any hard evidence or links to prove anything, so leaving this as a comment. Mar 23, 2023 at 15:21
  • I remember reading that the reference air travels down the wires before. It will be interesting to hear from someone who knows.
    – HandyHowie
    Mar 23, 2023 at 17:35

2 Answers 2


According to Auto Service World, oxygen sensors requiring reference air can get it in several ways:

Some O2 sensors were designed with laser welded “waterproof” housings and thereby seal the reference air inside. More advanced sensors use housings designed with a breathable membrane to keep water and contamination out and let fresh air into the element.

but the Bosch Oxygen Sensor Brochure indicates that their sensors do draw reference air through the wires:

Oxygen sensors need a reference air sample to compare against the exhaust gas to be able to produce a signal. This sample is drawn into the oxygen sensor from the outside, but there are different methods to get this air into the sensor. Bosch Oxygen Sensors are the only sensors that draw air from the connector and through the wires to ensure the reference air sample is clean and to keep contaminates out.

This Divize Oxygen analyser sensor page explicitly states that reference air is obtained through the "cable shroud", which I think makes it clear what is happening. The reference air for these types of sensors is drawn in through the gaps between the wires in the wire bundle that are wrapped in a presumably impermeable shroud. If the wire bundle is bent too sharply or compressed, the wires will be distorted and the gaps may be reduced impeding the air flow.

  • The last part was actually my understanding. Seems you have some other references as well which tells a lot! :o) Mar 24, 2023 at 15:50

My brother suggested I look in Wikipedia. (Duh) Not an explanation, but at least corroboration:

... In theory, the sensor wire harness and connector are sealed. Air that leaches through the wire harness to the sensor is assumed to come from an open point in the harness – usually the ECU, which is housed in an enclosed space like the trunk or vehicle interior.

If I find more, I'll post here.

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