O2 Sensor Reading 0.5v.. Bad ECU??

Post by **Darkfleet** » 31 May 2017, 16:41

A few weeks ago I noticed though my Bluetooth obd2 reader that the voltage output for my front o2 sensor is reading a flat 0.5mv and barely fluctuating (pretty sure it should be cycling with fuel trim).

: O2 Readings

: What is should be; o2chart.gif (10.22 KiB) Viewed 5875 times

Thinking my o2 sensor was bad a ponied up and got myself a new sensor after install the problem still persists.

I started probing wires at the o2 connector to ensure that proper ground is there .. turns out i had very high resistance at the connector. Found it hard to believe that the wires are bad but decided to make sure I had continuity to the ECU connector. Probed all 4 wires and have 0 ohms between the o2 sensor connector and the ECU connector. This leads me to believe something has failed in the ECU.

My ECU sticker has unfortunately faded out and I cannot read the PN. I know that the engine management is Denso and my wiring lines up with the Denso wiring diags. Anyone know what ECU will mate up with mine? Is this just straight swap or is flashing involved? are there any alternatives?

Appreciate any input in going forward,

Car is a 1999 V70 N/A Manual

Post by **bmdubya1198** » 31 May 2017, 17:57

Being that your car is a '99, the ECU is VIN-specific, so it will need to be flashed. Although, I don't think this is your ECU causing this problem. Do you have any codes or a CEL?
My cousin recently had a problem with his upstream O2 sensor on his '99 S70, along with a couple other things. We replaced the O2 sensor, and it was still running rich and idling like crap. Turned out to be the emissions systems fuse. Changed it out, CEL went out almost instantly and it ran great again. Double check all your fuses first.

Post by **wizechatmgr** » 31 May 2017, 18:34

Happen to check for the +12v to the heater on the O2 sensor? Could be it just never comes up to temp. What is the refresh rate on the scanner you're using? It doesn't look sufficient for troubleshooting this. I'd tap the sensor signal wire and check with a multimeter or preferably an oscilloscope.

I know VIDA gives me 2 channels for the front and 2 for the back. Only the first channel of each is used. I would look to see what happens on the rear while in neutral accelerating and decelerating. It should fluctuate and so should the front if you just quickly blip the throttle.

Make sure the car's hot before doing this. Some ECU's run in a open/closed loop fashion - where they substitute values from a table until the sensor gives a usable (expected) reading.

Post by **Darkfleet** » 31 May 2017, 18:47

Hey thanks for the replies!

I checked all my fuses and nodice they are all still good. I looked back through my codes and did see that the car threw a P1017 a few months ago which I believe is a ECM malfunction on fuel air. I've also gotten a p1032 "Air Fuel Ratio Sensor 1 Heater Control Circuit High Bank 1"

I have actually tracked down 12V to the heater and sure enough it is there - my o2 sensor heater also gets a pass on DTC. So I've been doing exactly what you say with the throttle and what i notice is taht the fuel trim seem to affect the rear o2 while the front o2 stays idle at 0.5mv. Sometimes blipping the throttle the front one will react.

Unfortunately I don't have access to a scope - might be able to see if I can borrow one from work and report back if I get to it. I tried the obd adapter I am using on an 850 and it showed the o2 bouncing up and down like it should. Pretty sure its just reading what the ecu tells it and the ecu has enough refresh rate to catch the change. I've got a feeling its the ECU seeing as its a brand new O2 and the wires are good all the way to the ECU.

I should also mention I've been getting a knock sensor code as well.. figured it was due to the engine running lean at times which I can see on the rear o2.

Post by **Darkfleet** » 31 May 2017, 18:58

More pics.

Post by **wizechatmgr** » 31 May 2017, 19:33

Occasionally bad O2 sensors do make it to market. The "reference" or "tested" ones are usually 2-3 times as much as the ones you typically see. To me it looks like either an ECU/signal issue or the O2 sensor itself.

Try checking it with a multimeter with the connection NOT made to the car wiring harness...

Post by **FLXC90** » 01 Jun 2017, 02:34

I don't know if the 99 and up work like the 98 and down so forgive me if this is dumb! Since you changed the sensor, have you done a hard reset, disconnect battery, deplete energy, reconnect and drive cycle ? Is that just a cable throttle thing?

Post by **Darkfleet** » 01 Jun 2017, 16:55

I did do a fuel trim reset - no change.

So I did some multimeter probing - the heater circuit is getting 12V but funny enough 3V is showing up on the 02 signal line from the ECU. When probing the O2 sensor with the car running I get very similar readings that show up on the rear o2 sensor on my obd tool.

That 3V on the signal line from the ECU seems odd...

Post by **wizechatmgr** » 01 Jun 2017, 18:11

The 3V could actually be a temp sensor signal from the O2 sensor (just thinking off the top of my head, haven't checked the schematic)
Would next suggest removing the ECU and testing for continuity (and resistance) from the O2 sensor harness connections to the ECU harness connections.

If that all checks out, more likely than not the ECU has a minor fault.

Was this a direct replacement O2 sensor or did you have to splice a connection on?

Post by **wizechatmgr** » 01 Jun 2017, 18:13

Disconnect the battery before doing anything I just listed...