Team420

Hey all, first thread in a while....
On my 91 p/u 3vze.....
Yesterday...I noticed my cel didnt come on with the key, tried pullin the codes, and no flashing cel... so I pulled the guage cluster thinking maybe the bulb burnt out.... nope... there was no bulb, However... there was a bulb for rear abs, which my rig dont have, so I took the bulb out and put it in for cel, turn the key, and bam... now I got code 21.... Ok... right...easy, replace the o2, and all will be well... right? But.... why would someone pull the bulb? It only cost $60 to replace the darn o2, I am wondering as this rig came to me with a new exhaust, aftermarket highflow cat, flowmaster muffler, if that was what caused the cel 21 in the first place, I wouldnt imagine that they would put a new exhaust and re-use the old o2, so my question is.... will I still have this cel when I replace the o2? I think it's a cali truck(4 wire o2).... and I have heard before that using a highflow with cali rigs causes a cel.... any thoughts?

sb5walker

Code 21 indicates bad heater circuit in the o2 sensor - either a bad connection or the heater element in the sensor is burned out. So if the connection is okay, must be the sensor. Get a denso and the truck will run better and the heater circuit probably won't burn out. Check sparkplugs.com. You can also get the denso part # from there & search for "denso" and the part # on amazon - they've been cheap this year.

abecedarian

Code 21 implies a failure in the heater circuit or a fault in the O2 sensor causing a rich reading. This does NOT necessarily mean the O2 sensor itself is faulty. A leaking cold-start injector could cause this code to be thrown as could spark plugs gapped too small, incorrect timing or valve adjustments, coolant temp, throttle position or air-fuel sensor or even a clogged muffler or catalytic converter... or even a clogged fuel return line to the fuel tank.

The code basically means that the ECU has tried to adjust the fuel mixture to compensate for what the O2 sensor says is a lean mixture and the ECU cannot make the mixture any more lean than it has. The heater circuit is most likely to fail, but the O2 sensor element can fail as well. And anything that pushes the fuel mixture too rich can cause to code as well.

Sorry to sound so vague but do not assume a code has any one particular cause. Look at what the code means and what can cause the issue.

Team420

Well, I tested the sensor, per fsm, and it came back fine.(as far as the heater circuit), the thing is... even after I clear the code (pull efi fuse, and have a beer), as soon as I turn the key to on, the cel lights up, and stays on, If I leave terminals, e-1 and te-1 jumped, it starts flashing the 21..this leads me to believe it's a bad wire somewhere....

Team420

You are absolutley right; but.... If the light comes on immediately after the key is turned to on(after clearing the codes) wouldnt that imply a short/ no reading ?

Jay351

Sounds like the PO knew something was up and removed the bulb....

The CEL should always come on for a few seconds when you turn the key to run..

Team420

Ya, but after a couple seconds it should go out, unless there is a code.... and no matter how long I pull the efi fuse/disconnect neg batt cable, I still get this code as soon as the key is turned to on....... The only reason I could see it throwing this code before the engine is run is if there were a short somewhere in the circuit. Please correct me if I am wrong in my assumption.
Today I went out to start it, and guess what....... wont start, unless I crank on it for 30 sec or so...... this is new! She has always started up on the forst bump of the key after sitting a few hours, after warmed tho, I would have to crank for a sec or 2 to get it goin again..... Now I'm thinking I have a combination of issues....Its getting cold here at night(upper 30's).... so I'm leaning toward the intake air temp sensor.... my little multi meter is gonna be workin overtime today.....

Team420

AFM is fine per fsm.... I did notice that the csi was disconnected....I think I did that a while back.... I did get it started after things warmed up a bit, so thats prolly y.

back to the o2 issue.... started doing some testing, and found that the connection from the o2 to engine ground 1 had almost 2k ohms resistance.... my other rig had no resistance, so I wired it straight to the e1 ground on the block.... still code 21.... then, just for poops and giggles, I left the connector to the o2 unplugged and started the truck.... still same code, and runs the same.... now I just need to figure out which wire is no good, and where they go, I tested from ht2 at the ecm to the ht2 connector at the ecm, and its good, then checked from ox1(ecm) to ox1(o2) on the connector and nothing, tried ox2(ecm) to the connector(o2) nothing, so I assume thats my bad wire....

But..... Is the code 21 just for the heater circuit? Or would it be for the ox circuit as well?

Team420

Found a broken wire just before the connector at the o2.... problem is...its the sheilded wire, and and cant figure where it goes.... any one care to point me in the right direction from here?

Team420

Ok guys.... got this thing all wired up... shows continuity to the ecm on ht, ox terminals at the connector for the ecm.... and got good ground where I supposed to for the e1 terminal....still code 21 as soon as I turn the key to the on position.... what gives? Anyone got any idea????

*BTW*

Anyone else thats trying to do this per fsm.... the pinout for a 91 appears different than what the 93 fsm shows....either that or my rig has the wrong ecm........ the heater circuit should show on ht1 at the ecm...on mine it shows up at ht2, and the ox sensor itself should show on ox1.... but mine does not....not even in the same connector.....

Team420

Did some more testing, and found that I get 12v at the red/white wire at the connector for the o2( this would be the heater power wire) however.... I also get continuity to engine ground from this same wire, something like 1945ohms resistance when the ecu is connected, disconect the ecu, and there is no continuity to engine ground.... does this mean I have a bad ecm?

Team420

Well...I took the ecm out of my 93 auto, andf put it in my 91 m/t just to see if it threw the same code or not... I'm happy to say...no cel now.... but...Is it safe to drive it like this till I can find the right ecm?