1994 Toyota Pickup Trouble code 27 and now 21 and 27

1994 TOYOTA PICKUP
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COISC29
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1994 Toyota Pickup 4 cyl Manual 225,00 miles

Check engine light on trouble code 27, replaced front o2 sensor, rear was replaced about 5 years ago, after replacing front sensor I now have 2 codes , 21 and 27; 21 = main oxygen sensor fault, 27 = sub- oxygen sensor signal or heater circuit fault. Would replacing rear sensor fix the problem ? or do I need to do something else....?
Sep 6, 2010 at 12:53 PM
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DOCFIXIT
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Are you using OEM O2s or after market?
Sep 6, 2010 at 1:20 PM
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COISC29
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The 1 front sensor that I replaced was a Walker oxygen sensor (aftermarket).
Sep 8, 2010 at 10:41 AM
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DOCFIXIT
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Did you have to cut wires and reconnect?
Sep 8, 2010 at 3:14 PM
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COISC29
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No the plug fit perfectly.
Sep 8, 2010 at 10:12 PM
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DOCFIXIT
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Several things can cause codes leak in air induction spark plugs injectors fuel pressure distributor wiring see schematic


https://www.2carpros.com/forum/automotive_pictures/198357_Graphic_610.jpg

Sep 9, 2010 at 10:12 AM