Engine idles ok at first start- rough after a few minutes or after you restart the engine. O2 sensor light on (front#1 unit). replaced sensor 4 times. Once with ford part and 3 aftermarket units. smoke tested system and mechanic tested for vacuum leaks and replaced distributor cap in case of crack also replaced mass airflow sensor. May be 2 problems but don't think so. Washed engine off today and truck ran rough. Someone thinks its the air bypass valve but why would the O2 light be on?. low miles on the truck (25000 miles)
What seems to make the problem better or worse? idles better at first start up
How long have you had this problem? 9 months

 

What O2 light?

Do you mean CEL(check engine light)?

When an engine misfires(runs rough) the O2 should set Lean codes, so O2 is not the cause of the misfires, its a symptom

Stop by an auto parts store and see if there will read the OBD2 codes, WRITE them down
There is not A O2 sensor code, there are many
P0130 02 Sensor Circuit Malfunction (Bank I Sensor 1)
P0131 02 Sensor Circuit Low Voltage (Bank I Sensor I)
P0132 02 Sensor Circuit High Voltage (Bank I Sensor 1)
P0133 02 Sensor Circuit Slow Response (Bank 1 Sensor 1)
P0134 02 Sensor Circuit No Activity Detected (Bank I Sensor 1)
P0135 02 Sensor Heater Circuit Malfunction (Bank 1 Sensor 1)

Rough running after a few minutes from cold start, or after warm re-start narrows things down a bit but see if you can get the codes


You can buy a $15 Bluetooth OBD2 reader(ELM327) and $5 TorquePro APP for your phone
Reads and clears codes, and you can watch live data from sensors to see if they are working and correct

 

Yes you are correct its the CEL light. The CO2 sensor indicates P2195 for #1 sensor being to lean. I suspect the fuel to air ratio is off but after taking it to 3 different "QULIFIED" mechanics i am a little scared to go to a 4th.I would think they could see this on a Diag. Computer. How hard is this to correct . Any other possible areas to trouble shoot. thanks

 

Wait, your mechanic replaced the distributor cap on a 2003? There's no such thing. 2003 Rangers are distributor-less. I don't blame you for not wanting to take it to a 4th. lol.

What engine is in the truck?

 

3.0 engine mass air flow sensor also replaced. Air bypass valve changed.The truck was running good and was taken in to the local ford dealer for a routine tune up. When it was picked up it had the CEL on and the dealer said dont worry about it the light will go out. When it was brought back to the dealer the wanted to charge to diagnose the problem. Each mechanic that has worked on the truck has a different idea on what the problem. its getting very expensive. Latest guess was fuel pump. holding off on that. currently working with online nationwide repair pal techs.to see what they think.