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I have a 1998 Subaru Impreza and I just failed the NOx emissions test. Before the test I replaced the Catalytic Coverter and the O2 sensor,  I replaced the air filter and changed the oil and filter.

My Hc was .3655 the Limit is 1.2

My CO was 7.1301 the limit is 15.00

My NOx was 2.4764 and the limit is 1.5

 

What do I do next? Eric

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High NOx is because of a lean mixture. Check for vacuum leaks, and check all of the breather and PCV hoses for splits.

 

Did you replace the front O2 sensor or the rear sensor?

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So the Subaru does not have a EGR valve correct? There are not a lot of vacuum hoses. I did check the seal on the airfilter to make sure that it was properly fitted.

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The things that I have read says that a high NOx reading is a lean fuel issue and that vacuum hoses clogged breathers might be the cause. The engine has very few vacuum hoses. Another thing says a clogged EGR valve or system. There is suppose to even be a Nox valve. But before I throw money at what could be or what might be. There has to be a process of elimination. 

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I hard wired the O2 sensor. I can go back and check my connections. If it was not wired correctly wouldn't the check engine light come on. I can pull and check my spark plugs but, I would think a fouled spark plug would raise the HC numbers and they are low?  

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So the cel light came on it has 3 codes the MAP sensor the cat system is not working at 100% and the idle air control problems. What do I do first?

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Does anyone have a cheaper place to buy Subaru parts?  The best I can find on line for the MAP sensor is 181.00 that is crazy for a wiring  harness and a couple of vacuum nipples!   

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Map sensors almost never fail ad are very easy to check with a voltmeter.

Check the vacuum hoses to the sensor, check the solenoid next to the sensor for proper operation.

 

Since these never to bad you can gt a used sensor cheap. There are tons in the U-pull junkyards.

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So you were right about the parts from a junk yard and can buy both for $50(the sensor, and solenoid). I had a bottle of Lucas fuel cleaner and the check engine light went off should I try and get it tested again or should I still assume their is a problem and get the parts? 

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You should test the solenoid first.

Make sure the vacuum lines are in good shape. Start at the sensor, check the line to the solenoid, then follow the line to the engine. On some it goes straight to the engine. Others it goes to a T which splits to the intake manifold and to the fuel pressure regulator. Some have a line that goes to the Evap solenoid under the intake on the passenger side.

 

The line to the solenoid usually has a white filter in it. The filter is to prevent condensation or fuel vapors from getting to the MAP sensor. Make sure you can blow air through the white filter. If no air goes through it, replace it. Couple bucks at a dealer.

 

Under the dash by the steering column are two green connectors. Plug those in and turn the Key On. (Do not start it) this sets the ECU in test mode and cycles all of the solenoids and relays on the car.

Make sure the solenoid by the sensor clicks. Then blow air through the port that goes to the engine to make sure air moves through the solenoid.

Air should flow then stop in correlation with the clicking on and off of the solenoid. The solenoid is good.

 

If no air flows, the solenoid is bad or clogged. If air flows all the time the solenoid is stuck partially open by carbon or dirt. Either of these conditions present, replace the solenoid.

 

The port that goes to the sensor is open to the engine or to the small black filter on top, depending on if the solenoid is open or closed. Cover the port that goes to the engine with your finger, then blow air into the port that goes to the sensor. Air should flow out through the black filter on top, then should stop in correlation with the clicking of the solenoid.

If no air flows, pull/twist the filter off and try again. If air flows now, the filter is clogged. You may be able to clean it out with some compressed air.

If no air flows after removing the filter, the solenoid is bad or clogged. Replace the solenoid.

 

 

MAP sensor testing is easy with a digital voltmeter. Just need a small paper clip. Straighten the paper clip and stick the end into the back side of the connector on the MAP sensor between the wire and the small rubber boot. (The connector is still plugged into the sensor)

Set the voltmeter to DC volts, start the engine. Touch the red probe to the paper clip, and the black probe to a good ground. The negative battery post is best if you can reach it. If not, wire brush the top of one of the strut bolts and use that.

One side of the MAP sensor connector will be the 5v feed from the ECU. Anything less than 4.5v on this pin means you have a connection problem between the ECU and sensor.

The opposite side will be the ground for the sensor and should read 0 volts or very close to it. Up to about 0.1V or 100mV is fine. Above that means you have a bad ground connection between the sensor and ECU.

The center pin of the connector is the signal pin. Connect a vacuum hose straight from the engine to the sensor with the engine running. With vacuum attached to the sensor you should have between 1.5 and 2.0V on the center pin. (Or something close to that)

Remove the vacuum hose and you should see voltage jump up to around 4.0V. This means the sensor is good.

 

If the voltage stays low, you could have a poor connection, bad sensor, or bad ECU.

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