ECM Failure? 2009 Subaru Forester XT

Nov 18, 2023
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Hi Everyone,

Been working on this problem for a couple of days. I recently rebuilt my engine in my 2009 Subaru Forester XT F4-2.5L SOHC ej25. After getting everything buttoned up I got a crank no start. Cleared Codes> Codes came back : p0108, p0118, p0123, p0198, p0223, p0328.

p0108- Map Sensor HIGH INPUT
p0118- Engine Coolant Temp Sensor Circuit HIGH INPUT
p0123- Throttle/ Pedal Position Sensor A circuit HIGH INPUT
p0223- Throttle/ Pedal Position Sensor B circuit HIGH INPUT
p0198- Engine Oil Temp Sensor Circuit HIGH INPUT
p0328- Knock Sensor 1 Circuit HIGH INPUT

I got an ECM pinout Voltage Chart, Wiring diagrams for the Engine harness, and I found that all of the sensors with codes related to them use the same ground wire coming from the ECM. A Blue/White wire in the Engine harness. Yellow/Green wire from plug to ECM.

So far I've tested the Sensor Ground wire with the engine harness unplugged ~ 8-9volts (Should be 0v obviously) ,
with the engine harness plugged into the ECM it shows 4.7volts.
Every sensor wire also shows 4.7volts when everything is plugged in.

When Key is turned to ON position MAP Sensor should read 3.4-3.8v
Oil Temp Sensor 1-1.4v
Coolant Temp 1-1.4v
Knock Sensor 2.5v
Electronic Throttle Control Sensor wire 1 0.64-0.72v 3.96v FULLY OPENED THROTTLE
Electronic Throttle Control Sensor wire 2. 1.51-1.58v 4.17v FULLY OPENED THROTTLE

I also tested the two V V L Pressure Switches for comparison and both read normal.
Same with the other Ground Pin connected to the same ECM Plug.

I'm not sure if I can post the wiring diagrams and ECM pinout voltage diagram on here, but let me know. I can attach if needed.
Any other info needed plz ask.
I'm hoping I didn't accidentally fry the ECM by accident when doing the rebuild.
Update! While everything is connected. I twisted the wiring harness, near the main connector plug, until the fan stopped. The fan turns on when the Coolant Temp Sensor wire reads above >4.7 volts I believe.

A lot of the sensors for Subaru stay between .5v - 4.7v, anywhere out of this range will cause a low/high input code.

These are the new readings when I twist the wire and the fan settles.

Ground wire 0v !!!!!!!!!!!!!! Heck yes.

Oil and Coolant temp sensors 3.3v (should be 1-1.4v)
Knock Sensor 3.6v (should be 2.5v)
Map Sensor 3.6v (Correct 3.4-3.8v)
ETC circuit A 0v (should be 0.64-0.72v)
ETC circuit B 1.6v (should be 1.51-1.58v .... pretty close)

So wiggling the harness will bring all of the voltages back to 4.75v and the fan turns back on.
Wiggle some more, fan turns off, and the voltages I just listed appear.

When sensors hit the 4.7v, the DTCs stay the same or come back, but when ground wire hits 0volts and I clear the DTC codes THE CHECK ENGINE CODES ARE CLEARED. (until I shake the harness near the plug again)

I almost want to put the engine back together and see if it will start when this happens.

Something is telling me the harness is shorting out somewhere, a loose connection in the plugs, or a sensor is causing it all to malfunction. Still bamboozled.

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