I’ve got a 92 F150 with a 302. I’ve done a mass air conversion & swapped in an Explorer intake, along with a few other mods. The check engine light came on & I was getting codes 41, 91, & 33. From an A9L computer that’s No Right HEGO switching detected always lean, No Left HEGO switching detected always lean, and EGR valve opening not detected. The EGR valve is not connected to the engine right now, so I’m not worried about that one. After reviewing some wiring diagrams, I figured out there was a mistake in the O2 sensor wiring. I got that straightened out, but I still get code 91 indicating the driver’s side is running lean. I’ve disconnected the battery several times to clear the codes. I’ve read voltage from the sensors & I get around 0.050 volts from the driver’s side. The passenger side runs around 0.85 volts. I’ve swapped the O2 sensors with each other and the driver’s side always reads lean regardless of which sensor is in it. I’ve checked for grounds on the circuit & it seems okay. I’ve pressurized the intake to about 3 psi & repaired a few minor vacuum leaks. I installed an engine vacuum gage & the vacuum is steady at 19”. Vacuum that high indicates to me that I don’t have any vacuum leaks. I’ve pulled all the plugs on the driver’s side & none look abnormal to me (all just slightly tan, no mechanical damage). I’ve listened to the injectors with a stethoscope & they all seem to be firing. I haven’t checked fuel pressure, but I figure that would affect both banks & also be evident while driving at high RPMs. I’ve also cleaned the MAF sensor & replaced the fuel filter. I’m out of ideas. Anybody got anything that might help?<o></o>

 

Make sure you have the O2 sensors connected to the correct inputs, pin 29 is right O2 and 43 is left O2.

 

I checked that & that's where they're going. I've checked the voltage from the sensor & it shows lean at the sensor.

 

O2 sensor voltage varies all the time.. or at least it should, if you have one that doesn't suspect the heater circuit or the O2 wiring. You could also try patching the good sensor into both computer inputs, then if the computer still doesn't act right it becomes suspect.

 

Thanks, I'll give that a try.