Crazy FTs

Ray Diagnostician Ontario Posted   Latest  
Tech Tip
2003 Nissan Xterra SE 3.3L (VG33E) 4-spd
P0300 — Random/Multiple Cylinder Misfire Detected
Rough Idle

The customer complaint was that the engine idled rough, the CEL was on, and so the shop replaced the spark plugs and the plug wires

The shop owner checked the vacuum hose routing for the egr system.

I graphed the 02s and FTs and one bank's front 02 was stuck near 750mv

That bank's STFTs would climb up from 0% to +25%, then would start over at 0%

The other bank's front 02 was near 100mv and the STFTs would go from 0%, climb down to -25%, then start over again at 0%

All 4 02 connectors are on the right side intake, valve cover.

The front 02s have 3 wires each, 2 for the heater and 1 each for the signal to the ecm

I printed a wiring schematic and I noticed that the front 02 signal wires to the front 02s were switched bank to bank.

So I switched the front 02 connectors back to where they should be and I disconnected the battery so the ecm could relearn the FTs.

The Fts are now close to 0% and the engine now runs smooth.


Michael Mobile Technician

Wow, It sounded like there was something backward going on there. Good find. It is always good to be the hero.

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Geoff Diagnostician

That's a classic one Ray. They are teaching the "reversed wiring" problem in Denso sponsored classes now. Where's the awesome PICO pictures for this one? :-) For anyone unfamiliar with Ray, he has a huge library of BEAUTIFUL PICO scope files for all sorts of engine problems. A true asset to our community! (I've been following his work for years over on Scott B's first website.)

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Ray Diagnostician

Thank you Geoff! And I always read all of YOUR excellent posts! I didn't use the scope on this one, but I used the scanner to graph the 4 02s and the FTs. I used propane to ck the front 02 that was stuck at near 100mv. Ray

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Sean Owner

I ran into something similar about ten years ago. A 1960's Ford panel van with a 1996 Crown Victoria drivetrain with ron francis wiring. It had been to several shops, and it was my first week at a new job, in a new state, Utah. I looked at the data for a minute at idle, and noticed that it was doing the exact same thing, one back full rich, and being commanded so, the other bank exactly

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