I'm helping a friend replace his upstream O2 sensors on a 2006 XJR... About 40k miles ago he changed the downstream O2 sensors, the ones easy to get to after the convertor. He has the old receipt and it says Denso Upstream 2349125. I didn't think the connectors where interchangeable??? He said he never had a problem with the car after changing out the downstream O2 sensors. I'm thinking of buying his car, and wonder what if any harm has come to the car, by using upstream sensors to replace downstream sensors??? The car rides really well and not shooting off any codes.

 

Ngk 94022 for upsteam from G49701 on x356 with vvt
ngk 95766 downstream from G49701 on x356 with vvt

 

Can I assume from this, that installing an Upstream O2 sensor in a downstream slot, is a big deal? I know that downstream O2 is just for monitoring purposes, and car engine/exhaust and electrical system seems fine, after 40k miles. It has passed many state inspections.

2006 - 2007 = x356???

 

the connectors aren't compatible. upstreams are cup type air\fuel sensors the downstreams are zirconia oxygen sensors, completely different

 

X350 is from some in 2002 to mid 2005 G00442 till G49700

x356 is without the sidestrips on the doors but same look as x350 no more chrome trim around windshield .
H0001 from mid 2005 to mid 2007 approx till H17500...

X358 is facelift model thicker bumpers more sporty looking

 

From X356 on also xjr engines have variable valve timing

 

Can I assuming from this, that it's physically impossible to plug a Denso Upstream 2349125 to a downstream connector; and on top of that, go 40k miles without a problem? If this is the case, I'm going to buy my friends XJR, its a sweet ride and well maintained.