[Ratfactor]

I'm suspecting that I need to replace the ECM on a 2002 forester. Does anyone know if it will require flashing or reprogramming? Talking about a unit pulled from same year/model forester.

 

[bobk25]

I don't think so. Someone on here even installed a slightly different board

ECU got wet, what replacement will work? - SOLVED!


If you match board numbers you should be good to go.

Sent from my SM-J700F using Tapatalk

 

[Ratfactor]

OK. I picked one up at Pick-N-Pull from a forester of the same year (02). Works fine. Did not fix my problem though.

 

[bobk25]

Pray tell - what vexes thee on thine fozzie?

 

[Ratfactor]

O2 heater circuit malfunction. Originally, it was p0141 but it has mutated into p0037. Pretty much the same thing as far as I can tell. It's the downstream O2 sensor. Changed it a while ago and has been fine until recently. All tests I have done on the sensor indicate that it is good. Analyzer detects NO power at that sensor, multimeter says otherwise. Began to suspect the computer, hence the swap. Stumped! :crying:

 

[dave5358]

Put in the $5 fix. For your vintage vehicle, the downstream O2 sensor is truly along for the ride. If the $5 fix doesn't solve your problem, you're only out $5 - which, in Subaru land, is not even worth mentioning.

 

[2.5x_sleeper]

@dave5358 I believe the rear O2 sensor has to be functional, only a P0420 CEL, for the $5 fix to work. If there's are other CEL codes it's not going to clear those codes.

@Ratfactor more poop on the CEL codes here:

• P0141 - O2 Sensor Heater Circuit Malfunction Bank 1 Sensor 2
• P0037 SUBARU - HO2S12 Heater Control Circuit Low

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Bobby...

['07 FSXT MODding Journal] ['03 X MODding Journal]

 

[Ratfactor]

What's the $5 fix? Anyway, mine has a p0141 code.. Guess i'll have to keep poking around in the wiring. The only reason I am trying to fix this is to pass emission test for plate renewal. Runs fine.

 

[2.5x_sleeper]

@Ratfactor click on the link I embedded in my previous post. :wink:

The $5 fix is a not a "fix", but a workaround to prevent the infamous P0420 CEL code.

Bobby...

['07 FSXT MODding Journal] ['03 X MODding Journal]

 

[Ratfactor]

Have been checking things off the list. Still no luck getting the heater circuit working. Heater circuit of downstream o2 sensor. The ground is good but not getting power. I do get power to the sensor part but not the heater (white wire). The Main relay is supposed to supply the power but may rely on a signal from elsewhere to actuate. Does anyone have experience with this issue? That thing is a ***** to get to and I'm not all that flexible anymore. I have checked the wiring between the sensor and ECU and all is good except that no power is coming from the pin that supplies it to the heater circuit. I have swapped ECU's with no change. That leads me to the relay, as that is where the power is routed from. I'll get that thing out when I get a chance but any advice would be welcome.

 

[2.5x_sleeper]

A couple questions:

[1] Are the O2 sensors Denso OEM/factory or Denso OEM fit aftermarket?
[2] Are you trouble shooting using a wiring diagram?

Bobby....

['07 FSXT MODding Journal] ['03 X MODding Journal]

 

[Ratfactor]

sensors are factory units. and they check out... heater circuit has continuity and reads around 6ohms cold.

Yes, I am using the diagram. I downloaded the service manual.

 

[Ratfactor]

From the ECU, connector 136, pin C18 is the power supply for the heater circuit. Even if my sensors were out of spec, I should see power from that pin. I'll be heading over to a friends house (heated garage) for further investigation, in a little while.

OK, back from several hours of poking around with the wiring and such. When following the diagnostic procedure of the service manual, it indicates a bad ECM/ECU. The results are the same with both of the units I have so I don't think that's it. The pin from the ECM that provides power to the heater circuit is only putting out .3v. Since this power is regulated by the ECM based on coolant temp, I suspect the problem lies between the coolant sensor and the ECM. The coolant sensor has 3 wires, one of which can be traced through the gauge and the data link connector. Another traces straight to the ECM. That one seems to be the likely culprit. Will check that next.

 

[Ratfactor]

Ok, so that last concept was somewhat flawed . Seems the temp sensor is not involved at all. On the upside, I know how to get to the temp sensor if it ever needs to be changed!
I did find what seemed to be excessive resistance in the heater circuit when checked from the ECM. I clipped the ground wire just ahead of the connector to the sensor and ran a direct wire to chassis ground. I'm not overly optimistic, but we'll see. CEL has not come on yet but some more driving required to know if that did any good.

 

[2.5x_sleeper]

The grounding is deficient & prone to failure in the earlier Foresters. This can be an major issue if your State salts the roads in the winter! Have you verified all the ground are corrosion free & tight?

Bobby...

['07 FSXT MODding Journal] ['03 X MODding Journal]

 

[Ratfactor]

The grounding is deficient & prone to failure in the earlier Foresters. This can be an major issue if your State salts the roads in the winter! Have you verified all the ground are corrosion free & tight?

Bobby...

['07 FSXT MODding Journal] ['03 X MODding Journal]

I verified all those that I could find. The ground for the o2 heater circuit passes through a connector I could not locate. It's supposed to be near the ECM but seems to be buried somewhere on the firewall. It definitely has a good ground now as I clipped the wire and ran a new wire directly to a ground up by the battery.

I do live in the rust belt. We get lots of snow, humidity, and salt.

 

[Ratfactor]

I feel pretty silly, I checked my order history at Amazon and it turns out I did put the cheapo o2 sensor in the downstream position. I put the better quality in the upstream. I ordered the upgrade and will install it in a couple of days.
The light was back on today and I have exhausted every other possibility that I could come up with. I am still puzzled by the ground circuit, I have checked at the connector where the Sensor lead connects and found a good ground, then the same check yields a different result. That ground is listed as a "harness ground" as opposed to a "chassis ground". Is it possible that that circuit goes to ground under some conditions but not others?