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Discussion Starter · #1 ·
did a motor swap vq40de direct swap put new upstream cat on driver side as well as o2 sensor put up/downstream cat on passenger side with new upstream o2 sensor. after the swap got a p0420 code bank 1 cleared after i installed those new o2 sensors (^^above) few days later get a weird code p1273 cylinder no.3 high to low side open(bank1)..... literally lost called local mechanic said its a exhaust leak or a vacuum where its sucking to much air. have no exhausts leak that i noticed all head bolts are tight to spec as well

my idea since i only replaced upstream o2 sensors could they be conflicting with each other? went with bosch o2 sensors while the downstreams are still the originals

if anyone has a fail proof way to test for exhaust leaks other then looking for black or putting your hand around the connection points that would be appreciated as well

Super Moderator
3,691 Posts
When you replace the air/flow ratio sensors (front O2 sensors), the ECM can trigger this code if it's self-learning memory is not cleared. Try this first and see if it fixes your problem before getting to deep into it:

1-Start engine and warm it up to normal operating temperature.
2-Turn ignition switch OFF.
3-Disconnect mass air flow sensor harness connector, and restart and run engine for at least 3 seconds at idle speed.
4-Stop engine and reconnect mass air flow sensor harness connector.
5-Make sure Detected Trouble Code (DTC) P0102 is displayed.
6-Erase the DTC memory. (This could be done by disconnecting the car battery for 30-45 min or with a scanner)
7-Make sure no codes are stored in the ECM.
8-Run engine for at least 10 minutes at idle speed.
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