Join Date: Apr 2008
Location: City of Trees
Frenetic1, are the codes you're getting P1128 and P1129, or are they different ones? Yes, the cheap scanners don't read fast enough but, IMO, they still give a pretty good idea of what's happening with the sensor(s). You will get slightly different readings at idle, than when cruising. The upstreams will continue cycling, but maybe not as big swings, and the downstreams will intermittantly drop below .450 mv (lean) at idle. I've seen mine sit at .75 mv for some time, but as soon as I start driving it goes back up above .450 mv, where it should be. It sounds like your description is about normal. How does it read when cruising (above idle) at almost any speed...especially say, above 45-50 mph? Then you should be getting your rapid, deep swings like you're showing above. If your downstreams are indicating like above, at cruise there is definitely a problem as, at cruise, they should be fairly stable somewhere (not that critical) above .450 mv, and not have any crosscounts....switching above and below .450 mv. Yours up around .700 is about where mine run.
If you can read all screens when cold, your before start/immediately after start voltage (I call it "parked") should be ~1.270 v, and the fuel status should show, "open loop" as soon as started. Within ~40 seconds voltage should rapidly drop, the upstreams should start cycling, and the fuel status should change to "closed loop". Not switching to closed loop in the PCM's calculated (based on engine sensor readings) time will trip the 1128-29 codes. As said, other things can cause this, but my limited experience shows the O2 sensor heaters to be the most logical cause, especially in my case, with the use of the "slow to heat" Bosch sensors. As soon as I went with the upstream NTK's, (heated/operated in ~15-20 seconds), the problem went away. I still use the Bosches downstream, but if I were buying them again, they'd all be NTK's.
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