1988 FJ62 3FE - Codes 21 & 28 after fixing grounds and exhaust leaks

[ChaleTito]

Looking for some guidance from the 3FE experts.


My ’88 FJ62 is currently throwing Code 21 (O2 sensor) and Code 28 (AFM).


History:

• Originally had codes 14, 21, 22, and 28
• Found and cleaned a heavily corroded intake manifold ground strap
• After cleaning the ground, codes 14 and 22 disappeared
• Found exhaust leaks upstream of both O2 sensors and repaired them
• AFM was replaced during previous troubleshooting
• O2 sensors are installed and connected
• Engine runs reasonably well overall

One thing that may be relevant: the exhaust has been modified by a previous owner. The factory dual-cat setup was changed and the truck now runs a single catalytic converter.


My questions:

1. Has anyone seen Code 21 persist on a 3FE with a single-cat setup?
2. Could the modified exhaust configuration contribute to Code 28, or should I focus elsewhere?
3. Are there specific AFM signal, E2 ground, or ECU tests you would recommend before I start throwing more parts at it?

At this point I’m trying to determine whether I’m dealing with:

• A remaining AFM/wiring issue
• An O2 sensor signal issue caused by the exhaust modification
• Or another shared sensor ground/reference problem

Any input from those who have chased Codes 21 and 28 on a 3FE would be appreciated.

 

[Lead Head]

Code 28 is issue with #2 oxygen sensor, not AFM. How has the exhaust been modified? Where are the O2 sensors now? You can directly test the O2 sensor voltages up at the diagnostic plug.

 

[ChaleTito]

The O2 sensors are still in their original locations. The exhaust was modified to a single-cat setup, with both sides merging into a Y-pipe before the catalyst.

I’ll check the O2 voltages at the diagnostic connector. Any chance you can point me to the correct terminals and expected readings?

 

[MWM1]

Not to derail your work, but I cleared several related codes just by replacing ALL the vacuum hoses. The existing ones may look good, but could be frayed at the ends.
Good Luck!

 

[Lead Head]

The O2 sensors are still in their original locations. The exhaust was modified to a single-cat setup, with both sides merging into a Y-pipe before the catalyst.

I’ll check the O2 voltages at the diagnostic connector. Any chance you can point me to the correct terminals and expected readings?

Do you still have the cover with the diagram over the connector? There are two pins, labeled Ox and Ox2. Those are direct connections to the oxygen sensor signal wires.

After the engine has been running for a little while, they should alternate between 0.1 - 0.8v. Here's a good video, skip to about 5 minutes:

 

[cruisermatt]

Do a good inspection of all o2 sensor wiring. Pretty common for it to get burned on exhaust pipe from improper installation or pinched between engine block and bellhousing