Chrysler Repair: possible relay, big rig, maf sensor

Question
hey roland. i don't have a chrystler but a 2000 focus. i think it's either electrical or fuel and emissions but your the expert. when i get up to 60+mph's i here a clicking noise sounds like a relay opening and closing but the car seems to run ok. it doesnt click all the time just periodically at high speeds. it seems to do it more in warm weather than in cool weather maybe since cool air is more dense. when i pass a big vehicle oncoming and get a air push it does it every time. when i'm on the interstate for 50 or 60 miles the engine light comes on. i've had it cleared around town it will never come on. p0171 changed the o2 sensor, cleaned the maf sensor, and fuel filter. i don't know if the clicking noise has anything to do with this or not but the car doesnt stall. thanx for your time.

Answer
Hi Johnny,
I haven't come across a flakey relay sound as you describe.
I suspect that if you had a diagnostic box that read out the status of relays in live time you could determine which one it was as you were driving. Those cost more than the code reader only type, unfortunately. But if you had a cooperative mechanic you might get him to come driving with you and put it thru its tricks while he watches the box. The other possibility is look at those relays that are on the firewall as being the ones you could here more easily and see if those would have a basis for cycling on and off. The ones located in closed boxes or on the fenders would be less likely to be audible to you at highway speeds.
The pressure increase on big rig oncoming suggests it has to do with the MAF sensor or something that it controls.
Otherwise, I can't think of anything else to suggest.
Let me know when you figure out what it is.
Roland