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Is the ECM really the culprit?
Posted to Technical Tips Forum on 1/10/2019 13 Replies

Here is just a little reminder to always question the decision to replace an ECM.

I received the above vehicle from our big brother shop with a P0300 and a P0159 B2S2 slow response. It also intermittently surges and performs very poorly very intermittently!

Customer has only owned it for 2 months and its easy to see he bought someone else's headache. B2S1 is new and B2S2 is new along with freshly welded in flex pipes.

I caught a break and did experience the problem when it acted up I saw fuel trim on B1 and B2 or ALPHA @ 75% indicating a rich condition and a 25% reduction of fuel.

I decided to look at the O2 sensors and as intermittent as it was got lucky again. Look at this.

[2004 Infiniti FX45, ECM/Inputs/Outputs Scan Data]

My 02 sensors were heading way north of 1 volt. I missed a little detail at first do you see B1S1 voltage appear to be the first sensor to start to rise? That was a clue.

A look at the wiring diagram showed that all the O2 sensors share a ground on ECM pin 78 labeled ground O2. A volt meter was taped into the easiest O2 sensor on the black and red ground wire. Damn .02 volts was my reading at first however the more it ran it starting creeping up and dropping down sometimes along with a surge.

The ground reached a full 1.46 volts over time and would elevate all the O2 sensors to 2 volts plus intermittently and of course surge and hesitate something fierce.

Pull down the ECM and verify the same result at the module. I thought for sure I would have a bad ground but a voltage test of the 3 ground wires at the ECM showed .02 volts.

OK ECM is bad I concluded and I left for the night as it was quitting time.

Somehow Scott Shotton entered my sub conscious and I heard his comment design the experiment and run the test which he says with such authority! Also an earlier phone call to my buddy Bryan Tenkman who I use for moral support so often said something to effect of big deal run your own ground. I called to have him help me figure out if these were indeed O2 and not AFR sensor duh Mike look at the emission decal under the hood. Oh yeah

I ran the experiment while monitoring all 4 O2 sensor on a graph. Yes of course it did what vehicles do and acted totally normal for 35 minutes. Then here they all start rising AHAAAAH here's my chance to run a redundant ground as I did (sorry for the no picture) every O2 sensor went to 0 volts except B1S1 it stayed elevated over 2 volts.

A not so quick (as this one is buried) unplug of B1S1 dropped my elevated ground wire I had over 1 volt on to .02 volts.

Here is the culprit B1S1 O2 sensor all bulged out. [O2 sensor] possibly bleeding voltage from the heater circuit.

I'm a wanna be mobile guy and routinely program modules for other shops including ours and find out somebody goofed! It happens a lot and I almost bombed this one big-time.

I suppose I posted this as a subtle reminder to not be lazy, design experiments, and be persistent in your thinking things through. Always question the bad module that easily appears as the bad guy. Also I posted this in an attempt to pull me out of the mud for previous negativity in the forums =)

This would have been an expensive mistake for me. Nobody walks on water in this business however some swim better than others. I almost drowned on this one and hopefully this reminder helps you tread water when you really need it.

Mike from Ohio

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car Vehicle Data

2004 Infiniti FX45 4.5L

Engine4.5 L
Trans5-speed Automatic (Electronic)

car Vehicle Data

2004 Infiniti FX45 4.5L

Engine4.5 L
Trans5-speed Automatic (Electronic)