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p0170 Bank 1 Fuel Trim Malfunction

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I have been fighting this CEL for 2 months now. I have replaced both o2 sensors. No help. Here's the latest. I got some info from alldatapro.com that said for a p0170 on my 95 Legacy Wagon to first check exhaust for leaks. No problem found. Then the air intake for leaks. No problem found. Then fuel pressure. I don't have a gauge so I skipped that one for now. Now to what I did find. The engine coolant temp sensor. My Haynes book says to test the resistance across the terminals with the engine cold and then when hot. The readings I got were within the range listed. So far so good. Then I am to check the reference voltage to the sensor from the ECU. It should be 5 volts. It said to disconnect the connector to the sensor and with the ignition on and the engine not running I should see 5 volts. I get 145mv. I'm sure I was making good contact. Could this be my problem? There are only 2 contacts in the connector. Could my ECU not be sending the 5 volts or could I have a bad wire between them? Car doesn't run bad. CEL just keeps coming back.

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Yes, verify the wiring integrity.

 

9 out of 10 diagnostic proceedures for each malfunction code involve essentially checking for proper continuity and/or improper short conditions of the associated wiring. :banghead:

 

The wiring and connectors on todays cars are woefully inadequadete. They degrade as a result of heat, chemical (salt, oil), and vibration. :banghead::banghead:

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I'm not sure about Subaru systems, but a fuel trim malfunction indicates that the FI system cannot fully compensate for either a rich or lean condition. On many cars, they use O2 sensors post combustion and some sort of either airflow rate meter or air mass meter pre combustion.

 

How does the car in question determine the incoming air charge? That device would certainly be suspect. Does the OBD-II system on Subarus store adaptation values for fuel trim? If so, read the adaptation values to determine rich/lean condition and start digging from there.

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You might want to have a look at your air flow meter. some times the "hot wire" in the air flow meter can get covered with oil residue and give a false signal to the ECM , which can give you a false trim code

You can try and clean it with isoproply alcohol , but be warned it doesn't take much to damage the wire and Air flow meters aren't cheep!!!

 

SEA#3

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Ah, so that car uses a hot-film sensor system for measuring incoming air charge. Yes, those little wires wear out over time, and with most makes, cleaning is a short term solution. What does a meter for a Subaru cost? The one for my Mercedes is not too bad at about $400. I have seen some makes at $1300.

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If there is someone out there with a similar car, could you measure across the engine coolant temp sensor connector with it unplugged and the key on but engine off and tell me if I should have 5 volts there?

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Yes, verify the wiring integrity.

The wiring and connectors on todays cars are woefully inadequadete. They degrade as a result of heat, chemical (salt, oil), and vibration. :banghead: :banghead:

Yeah, what he said. Be glad you don't have a European car - Bosch connectors are the pits!

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Well I have ruled out the fuel pressure, coolant temp sensor, intake and exhaust leaks. Can anyone tell me how to test my MAF sensor? Maybe I'll get one from 1stsubaruparts.com. About $255.

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I fixed one with this code with the replacement of the MAF as well today.  Thanks for the thread.

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