hey iv got an 89 bronco that is haunting me.
it keeps throwing the check engine light on with a code 41 always lean or no 02 switching deteced.

iv done a complete tune up checked for vacuum leaks
plugs and wires
cap and rotor
fuel filter and pcv valve
lof and set timming at 10 degrees
cleaned the iac and throttle body
still threw code 41 so i put in a new hego
still did it so i started diagnosing the circuit and beleive i found a problem i know a have source voltage on the heater wire and a good ground onb the ground wire and measured res in the signal wire to the pcm ( disconected) and got .01 ohms so that's good i did find a 4 volt reading on the signal wire odly enough coming from the pcm i even unplugged the wire and put a shot wire in to verify that yes it is outputting 4 volts on pin 29 of the pcm thats from batt neg to pin 29 with the posative lead . it doses this with the o2 harnss conected or disconected it drops to 3 volts with ign on eng off or 4 volts running . ive cleaned the pcm ground on the fender and the grounds on either side of the rad saddle im guessing i have a wiring problem of some sort. i even replaced the pcm with a new reman based on vin number calibration code need help please

 

You need to test your fuel pressure at idle and running with the vacuum hose to the FPR removed and plugged.

 

il do that this weekend again i did that about a month ago but not with the vacuum line off i do know it was in pec around 35 psi close to the same while driving. the pcm shouldnt supply a 4 volt ref to the hego should it? its a 3 wire 02

 

.Be sure you are dealing with the right sensor does your truck have the original engine. does your engine get up to normal temperature and did you clear the codes and drive the truck after replacing the sensor. see if you can get a rich reading from the engine try adding fuel and do you know of any exhaust leaks and how the old sensor looked when you pulled it out. The sensor should make its own voltage

 

i orderd the sensor right from ford with the vin number and the calibration code the old sensore had wires ripped off right at the tip of the sensor . it looked pretty old maybe original . its up to operating temp in about 5 miutes i did the t t stat a month or so ago its a 195 stat i think cant hold the rad hose so i know its good and hot there are no exhaust leaks.

i did the propane test on the 02 sensore in vehicle with engine running introduced propane into the inatke and watched the 02 voltae increse then took it away and voltage droped then created a vacuum leak and watchedd it drop voltage even more...

i did clear the code after replacing the 02 left the battery disconected for a week

 

ok then it sounds like the sensor is working correctly when you did you testes are you reading volts or mv. does the engine run bad at all. so you replaced the sensor in the exhaust pipe below the manifold then. it would be nice to know fuel trim but that is a long shot really. do you have a vacuum gauge and a fuel pressure gauge that would be a great start. if you were worried about reference voltage then you could test off of other sensors. it could be that the engine thinks it lean and it is not or it really is lean or just the opposite start with fuel pressure and vacuum. and if you do have a vacuum leak typically these older engines will idle over desired idle speed any performance issues and sorry for the list of questions just trying to get an idea how did the pintel for the IAC look

 

i was reading on the 1 volt scale . i do have a vacuume gauge and vacuum gun and fuel pressure gauge . the engine runs awsome but is a tad rich do to the fact the pcm always thinks its lean so its gone to full comanded rich condition .



i really think its a wirring problem i do not think there should be voltage on the signal wire not 4 volts anyways and should not be coming from the pcm , voltage should be supplied to the pcm based on oxygen content in the exhaust

 

ok i had the meter on the 1 volt scale while watching the 02 signal directly from the sensor.

and on the 5 volt scale while watching the 4 volts from the pcm . the iac looked pretty good compared to others iv seen

 

if it is a 3 wire o2 and you are reading 4 volts from the engine computer their is your difference vs a regular o2 voltage goes the other way during a rich or lean condition do you have 12 volts at your sensor (not talking about the computer hear but the key switch. what year is the engine in your truck

 

Code 41 for CM or KOER? Knowing that helps get an idea when the problem occurs but sounds like all "CM" based on what you've stated so far.

And yea I agree best bet start with fuel pressure.

4 volts might be nothing more than end result of "margin of error" in the meter you're using, margin of error +/- represents accuracy percentage and near full selected scale. Further away from that you get margin of error increases and reason always want to use lowest range possible for test.

02 sensor operatates between 0 and about 1 volt so even at 4 volts reference its more than enough or should be. 0v = lean 1 volt = rich.

Might be worth a check with a different meter making sure to use lowest scale offered above 5volts (equal scale to first meter first), I would if for no other reason to satisfy my curiosity about reading received.

If meter set to 20volts (0 - 20volts lowest DC range tends be the norm for basic multi meters) and was found to be off by a full volt when testing down at/just under 5 volts? Yea I'd probably pitch it in the trash can.

Run KOER with fuel pressure gauge connected, see if get code 41 if all previous code 41 had been CM and KOER test hasn't been done yet.

If has been a CM code make sure clear codes and then if at all possible put the fuel pressure gauge where can be seen than take it for a drive test see if the pressure drops, thats if its within spec sitting still.

 

yes i have 12 volts on the heater wire from the ign switch down to the 02 the engine is out of a 93 f150 and the 93 engine wiring harness was used and pluged into the stock 89 main harness it was done by the previouse owner did more harm then good lol


and what do u mean the voltage goes the other way??

 

i doubt i had a meter problem im using a snap-on vantage scope that i use everyday at work im a ford service technician and ive talked to fellow mechanics at work and there all stumped so iv turned to the reall experts the guys who drive and work on these trucks on a daily basis lol

 

oh ya the code comes up on cont mem and on demand self test

 

Yea wasn't suggesting it definitely was what you where seeing, same time I realize there are some really cheaply built meters out there these days can't and shouldn't always be taken at face value.

Regardless I wouldn't care who made it is never a bad idea to compare readings check it with another one. Hey two meters say 4volts 4 volts it probably is.

I mentioned it as you seamed genuinely concern with that 4 volt reading.

Intended to ask too as didn't see where you mentioned anything about it, possible I missed it.

Did you get that same 4 volts at all sensors? or only the O2 sensor? Stand to reason you would if everything else in good shape, but be worth a quick spot check picking couple other sensors sharing it at random make sure. Might point to corrosion in that wire for example.

 

Ok yea both of them, computer couldn't swing it rich even when sitting still so yea fuel pressure test if hasn't been done yet be a good idea do next.