I've had long-standing trouble with code 21 in my 1988 FJ62 that I finally decided to approach systematically and mostly due to all the accumulated wisdom from others on this forum I was able to solve it today. Background is that for the last 10 years or so the check engine light would transiently illuminate when under hard acceleration. As soon as I let up on the gas, the light would turn off. I would always find code 21 for #1 O2 sensor. I replaced both O2 sensors but no change. Over the years it slowly worsened, coming on more easily and sometimes staying on although invariably did not come back on with an engine restart. Still always code 21. The truck always ran pretty well and except for a failed fuel pump recently was doing well for having 230k miles on the original engine.
About six months ago the check engine light came on immediately after starting and stayed on. Still only code 21 and would come back immediately after clearing the code. Since it was still running well I just lived with it knowing that I would eventually need to get it fixed before my required CA smog check. Finally decided to try to solve the puzzle and started studying my factory service manual and searching every thread I could on both the 60 series and 80 series forums. Here's the story:
I didn't think the #1 O2 sensor was the issue but I pulled it off and tested the heater resistance between terminal +B and HT on the O2 sensor connector. Came in within spec at 6.0 ohm. On the truck side connector I confirmed I had 12 volts at the +B wire (yellow with red stripe) so it seemed like the heater circuit was probably not the issue. I did the FSM procedure to check the O2 sensors (shorting terminals TE1 and E1 while connecting my digital volt meter across VF1 and E1 on the check connector) and while it would probably have been easier to count the voltage fluctuations with an analog meter it seemed to be more than 8 times in 10 seconds. I was stumped.
About six months ago the check engine light came on immediately after starting and stayed on. Still only code 21 and would come back immediately after clearing the code. Since it was still running well I just lived with it knowing that I would eventually need to get it fixed before my required CA smog check. Finally decided to try to solve the puzzle and started studying my factory service manual and searching every thread I could on both the 60 series and 80 series forums. Here's the story:
I didn't think the #1 O2 sensor was the issue but I pulled it off and tested the heater resistance between terminal +B and HT on the O2 sensor connector. Came in within spec at 6.0 ohm. On the truck side connector I confirmed I had 12 volts at the +B wire (yellow with red stripe) so it seemed like the heater circuit was probably not the issue. I did the FSM procedure to check the O2 sensors (shorting terminals TE1 and E1 while connecting my digital volt meter across VF1 and E1 on the check connector) and while it would probably have been easier to count the voltage fluctuations with an analog meter it seemed to be more than 8 times in 10 seconds. I was stumped.