1984 Toyota Pickup

My 1984 Toyotoa EXcab pickup, EFI, manual tran, failed the Calif. Smog
vehicle inspection. It first failed the CO % measurement in the 15 mph test,
but passed the 25 mph test. The technician retested it, and it failed in the
25 mph test, but passed the 15 mph test. I first had the Oxygen sensor
replaced (cost $187.00), but it still didn't pass. Now, the mechanic says the
"systems modulator" needs replacing. I went to an auto parts store and the
clerk says he doesn't know if the mechanic means EGR value, or EGR vacuum
modulator. The EGR valuve was $160, but I didn't get a price on a vacuum
modulator.

If the vacuum modulator is the problem, my Toyota manual makes it look easy
enough to replace.

The mechanic wants $297 to replace whatever this systems modulator is.

Any suggestions?