Code 21 (1 Viewer)

[Firedog]

My '93 has had the check engine light on since i bought it. checking the codes I found a code 21.
The O2 sensors looked like they were original so I replaced them with aftermarket Bosch sensors..... Still had the code 21. Waited a few months and decided to go ahead and order Toyota O2's. Installed the new sensors and.......Code 21
So, I decide to get out the FSM (I know, about time) Went through the test and the O2 sensor was working fine, but the heater on the sensor was inop. All test indicate the ECM needs to be replaced.
The truck runs great and only has 120,000 miles, (although it only gets about 9mpg in town)
Anyone know what the drawback is to not having the heater working on the o2 sensor? I can live with the check engine light (it's been on this long and hasn't bothered me).
Will having the heater on the o2 sensor working properly give me better mileage or performance?
BTW, I still have the "old" computer, never updated it to the '94, but the only code I have is the 21.

 

[Firedog]

Ok, decided to do some research and answer my own question (at least i will post the answer to help someone else)

Pulled from several sources on the net-
"The oxygen sensor must be hot (about 600 degrees or higher) before it will start to generate a voltage signal, so many oxygen sensors have a small heating element inside to help them reach operating temperature more quickly. The heating element can also prevent the sensor from cooling off too much during prolonged idle, which would cause the system to revert to open loop."

Looks like the o2 sensor won't produce enough voltage or a proper voltage fluctuation unless the sensor is over a certain temperture. With the heater not working, it is possible that the sensor could stay in open loop which will cause the loss of fuel economy.

So...... it looks like i will need to talk to Dan .....again

Just think, if I fix this I could even get 10 mpg!!! (don't knock it, it's a 10% increase!

 

[CJF]

So what's your game plan at this point?

 

[Firedog]

So what's your game plan at this point?

To double check all wiring from the ECM to the o2 sensor heater. Check the o2 sensor heater voltage output at the ecm. (the FSM gives a test for this, but the test is done at the diagnostic port, I want to check at the ECM to make sure I don't have an issue between the two)
If all checks out good, I guess I will be hitting Dan up for a new ECM (unless I stumble across one at a bone yard)

 

[CJF]

Keep us posted.

Good luck,

Curtis

 

[Firedog]

DAMN.... typed a nice LONG update to this and the server update ate it!
Here we go again......

Code 21 can apply to either the o2 sensor, OR the heater for the o2 sensor.
I replaced the o2 sensors with new Toyota units and still had the code so out came the FSM. After following the procedures in the FSM the answer was that the ECM needed to be replaced. I called Dan and found that I really wasn't going to replace the ECM with a new factory unit (like $1200!!!). and I really couldn't accept that the ECM was bad. My truck, being a '93, has the "bad" ecm, but code 21 was the only code stored.
I looked through the wiring diagram and found that the heater on the o2 sensor is supplied 12 volts from the B+ relay and the ECM supplies ground at more than 8 times every 10 seconds to heat the o2 sensor. I decided that it should be an easy circuit to trace and the problem was probably somewhere in the wires from the B+ relay. The o2 sensor was the easiest place to start, so I tested for 12 volts on the yellow and red wire at the connection for the #1 o2 sensor and found none! I started to trace the wires back into the right front fender and got lucky, I found a bulge in the wire. I cut the wire and found it corroded and broken under the insulation. Did a quick temporary fix on the wire, pulled the ECM fuse for about a minute, then started the engine.....


NO CODES!!!!!

I plan on doing a permanent fix tomorrow.

I did decide to buy a used computer (the "good" computer from a '94) from "Outback Offroad Gear" here on MUD, he had it, it was a good price, and considering how much a new one cost I figured it would be good to have a spare (knowing my luck, my computer would have gone out next week and no one would have a used one available...)

I plan on running a couple of tanks through to see if I have any mileage / performance improvement and I will update again.

 

[CJF]

 

[landboozer1]

I recently got a code 21 I think I may have a similar problem, will test wiring and check back in