Fault code 4 - Intermittant

[Antony]

My '95 wagon had the check engine light come on.
I took the fault codes from the box near the battery, and the only code was #4 - secondary air injection.
I reset the code, nothing came back on re-start. When the light was on, there were no "driveablity" problems. Since reset, the light has not returned.
Has anyone had this occur? And secondly, what might one look for in the "secondary air injection" system as the source of the fault? Thanks for any input.

[Arthur Dalton]

Two things:
First possible is the vac line under the front engine cover may be cracked. This line is plastic and they dry out. It goes to the AIR switch-over-valve. Common fault, easy remedy.
Second , is a possible fading 02 sensor..
The sensor looks for a lean condition when the pump is energized
and if it does not see it , it pops a code 4. This lean condition requires a sensitive sensor as the voltage requirement is very low to trigger the test .
So, a slow/dirty sensor will not verify the test and the CE will light.
A good run on the highway in 3rd gear sometimes will clear up a dirty/carboned sensor.
This is assuming the AIR pump is coming on, of course...
If not , check the fuse..

[Antony]

Thank you for your insites; very helpfull. I'll check the vac hose, and run it along the HW as suggested (it's all around town mormally). Great advice and thanks again.