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Discussion Starter #1
If you have a 2006+ 2.5 engine with the variable valve lift (not the variable valve timing) mechanism, and a digital multi-meter (Ohmmeter) I'd appreciate if you could measure the resistance of the related oil pressure switches when the engine is off and post the results here.

Background: There's a couple of current threads that involve DTC P0026 and/or P0028. These codes are related to the pressure control of the variable valve lift system found on 2006+ 2.5 engines. The system uses oil pressure to switch one of the intake valves on each cylinder between low and high lift, the oil pressure being controlled by a solenoid valve driven by a duty cycle signal. The OBD uses a separate oil pressure switch to verify that the correct pressure (low, or high) is being applied to the lift mechanism.

When the ECM is commanding a low mode, the pressure should be low, and the oil pressure switches should be closed, that is, low resistance between their connector terminal and ground. When the ECM is commanding a high mode (usually at higher engine speeds), the switch should be open.

If a switch internal contact becomes poor, such that there isn't a low resistance across it when the pressure is low, the OBD will trigger one of the two DTCs because an open switch contact should occur only when the ECM is commanding a high mode, whereas the faulty open switch contact would exist when the ECM is commanding a low mode.

On my 2007, with roughly 56,000 miles, I found the resistance across both oil pressure switches to be around 10 Ohms. This seems somewhat high for a switch, but might be normal for these. If others find more or less the same resistance across their switches, it could set a benchmark for checking for a defective switch when a P0026 or P0028 is triggered.

For example, if a good number of measurements reveal a maximum resistance of about 25 Ohms (with no trouble codes being generated), then if someone is troubleshooting and finds the resistance of their switch to be 100 Ohm, then it might well be the switch that is bad. And if changing the switch corrects the problem, we would know that at that resistance the OBD will trigger a code, and this could be something that could be quickly checked when experiencing a P0026 or P0028.

I've attached some photos to identify the switches. They are located at the upper front of the left side head, and the upper back of the right side head.
 

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Also getting P0026 code

I am getting the P0026 code, when I checked the resistance on the switch, the multimeter was reading 29.8.
Our car was showing P0026 and we were experiencing a slow idle speed. The RPM would drop when stationary almost to the point of stalling. The code went away on its own for about a month and a half, but the idle issue remains intermittently.

The CEL came on again yesterday - I assume it is P0026 again but will have to confirm. I'll check the resistance if you can give me some more guidance. I have a multimeter, but have never tried to use it to diagnose an auto electrical issue.
 

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Discussion Starter #4
Perhaps if you could provide some idea of how you have used the multi-meter in the past it could help focus the guidance we provide. Nevertheless, in the meantime, here's an outline:

Your multi-meter will have one or more resistance (Ohm) ranges. It's preferable if the multi-meter is a digital type, and not the analog that has a moving pointer. The latter can work, but some don't have a resistance range that will read out low resistance with good reliability. You want to choose a range that will show resistance between close to zero and, say, 200 Ohms.

The P0026 is related to the right (passenger) side VVL system. The oil pressure switch of concern is the one shown in the third photo in post #1 above. It's the part with the white connector. This is at the upper back of the right side head.

Remove the white connector. (Might have to work your hand in from the back, under the air duct.). When the connector is off, the black plastic top of the switch connector should be visible. (See the middle photo showing the left side switch.) Inside there's a single, thin, metal terminal.

Connect the negative (black) test lead of the meter to a good engine ground. Test the ground by touching the other (red) test lead probe to another spot on the engine. The meter should read less than 1 Ohm if the connections are good. Then move the red test lead probe to the switch terminal. The meter should now indicate the resistance of the switch. (Repeat the last two tests to be sure the readings are reliable.)

Because your symptom appears to be intermittent, it's possible that measuring the resistance in this way might not be conclusive. The reading might be low at this particular time, as would be expected with a good switch. But if it reads very high, I would think that is a positive test.

You can also pull the white connector on the left side (near the front of the head), and measure the resistance of the other switch, using the same technique. This would provide a comparison, which might be indicative.

It should be noted that the code along with poor idling could also be due to a malfunctioning VVL solenoid valve.
 

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Resistance measured at oil pressure switches

plain OM, thanks for your very detailed instructions. I was able to measure the resistance at each switch.

Driver side: 13.3 ohms
Passenger side 40.7 ohms.

The passenger side has been giving me issues (code P0026). This was all read without the car running. Obviously there is a discrepancy there, but not sure how telling that is.

I suppose there would not be any value to doing something similar with the engine running?
 

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Diagnostics steps from service manual (EN-91)

I also wondered if you've run through this diagnostic from the service manual, or if you think it would be valuable?

 

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Discussion Starter #7 (Edited)
I did look at that diagnostic tree, but noticed that nowhere does it check the oil pressure switch condition. Yet the switch is critical to the way the engine control module verifies if the hydraulic pressure to the valve lift cams is correct. However, if you want, you could go through the sequence, although I suspect the suggested measurements will be fine.

If you have the full service manual, look at the engine General Description section. There's a section for the diagnostic codes (starts on page GD(H4SO)-8). In this section there's details of what criteria are used to generate a code. This is where the function of the oil pressure switch is described, and how its position -- on or off -- leads to a code.

There's not much more you can do with the engine running unless you have a scanner that can display the state of the VVL oil pressure switches. If you do, then with the engine idling, both switches should be closed, or "on". But I suspect that when the code is set (CEL on), the high resistance across the switch (~40 Ohms) will be read as "off".

In this regard, when the engine is off or at lower engine speeds, the oil pressure to the variable lift cam is low. It's only when the engine gets up to cruising-type speeds (and depending on some other factors) that the engine control module will command a switch from the low lift cam to the high lift cam. This is effected by changing the signal to the switching solenoid from a low duty cycle to a high duty cycle. More oil pressure is routed to the cams. The oil pressure switch, which is normally closed (there's continuity across it) when the pressure is low, will then go open. But I believe that's the problem with your passenger side switch - the resistance across the switch contacts is already too high -- e.g. your measured 40 Ohms -- and read by the ECM as "off". This is inconsistent for the diagnostic system; the ECM is commanding a "low", yet the switch, showing high resistance, is indicating a "high". This is not correct and that, I believe, triggers the code. [This is still a hypothesis, yet to be verified by experiences reported here.]

The fact that your code is for the passenger side, and your driver side switch is still down at around 13 Ohms (and no code), seems consistent with what I expected. The measurements start to provide a range -- 13 Ohms is still good, but 40 is going to be problematic.

I recently re-checked the two VVL oil pressure switches on my 07 and they hadn't changed -- still around 10 Ohms.

Thanks for the data; adding to the knowledge base.

If you decide to replace the switch yourself, be very careful not to over-tighten. There have been instances where the block, in the area of the switch, cracked. There's not much "body" around the threaded hole for the switch. (Not meaning to discourage you with this, but here's one example: http://www.subaruoutback.org/forums/66-problems-maintenance/45237-just-messed-up-big-crap.html)

p.s. If you do have the switch replaced, measure the resistance of the new one. Again, this would provide a reference for what it should/could be.
 

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Very detailed reply - thanks! My inclination was to swap the right hand and left hand switches to see if eventually it returned the P0028 code. I assume they are interchangeable. However, reading that horror story definitely has me concerned! I'll have to read up more on the procedure and see if I feel comfortable doing that work. My mechanical skills are pretty modest.
 

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Discussion Starter #10
Thanks gotwww. That's what I would expect, or even lower.

Were you getting one of the two trouble codes? How high is the resistance of the old switch?
 

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Thanks gotwww. That's what I would expect, or even lower.

Were you getting one of the two trouble codes? How high is the resistance of the old switch?
Actually, I was getting P0028 (driver's side) and the resistance on that was all over the place (not consistent) but when I went to replace it this morning I saw what looked like water in the connector base that I think may have been causing it. I blew the water out and so far no codes.
 

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Discussion Starter #12
Please let us know if it holds, or if you end up replacing, and if you do, and have the change, the resistance readings. ("All over the place" is a valid reading for this purpose!)
 

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P0028

. on my 2009 Forrester,
P0028 (comes and goes, all around town driving though, seldom highway speeds)
left /driver side ~150 ohms (give or take, can;t remember exactly)
right/passenger side ~15 ohms (started on 20 then settled at 15)

seems consistent with what you are saying, huh?
having trouble finding part numbers though, ideas? parts diagrams available?
 

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both switches replaced

I ended up replacing one of the pressure switches in August. Was getting P0028 (driver's side). Measuring the old one I found that when the switch should have been closed the resistance was low, but not zero, but the longer I held the OHM meter on it the higher it went but it never stabilized out. This stopped the P0028 and it never came back.

Two weeks ago I started getting P0026 (passenger side) and since I had already had to replace one I figured for $12 ( Amazon.com: Beck Arnley 201-1954 Oil Pressure Switch with Light: Automotive ) it was worth replasing the second one also. Again, solved the problem completely. The second one measured 112 ohms when supposed to be closed.

2006 Tribeca 3.0L 170k
 

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Discussion Starter #15
. on my 2009 Forrester,
P0028 (comes and goes, all around town driving though, seldom highway speeds)
left /driver side ~150 ohms (give or take, can;t remember exactly)
right/passenger side ~15 ohms (started on 20 then settled at 15)

seems consistent with what you are saying, huh?
having trouble finding part numbers though, ideas? parts diagrams available?
http://www.subarupartsforyou.com/cp_partdetail.php?partid=20385
p/n 25240AA060

Should be the same for the VVL oil pressure switch at the back of the right (passenger) side head.
 

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thanks, any reason to replace the passenger side unit at the same time if the reading is 15-20 (and there is no P0026 error) do you think?
***edit** just saw gotwww's post, I may just go ahead and replace both. I saw one comment about cheap oil filter causing these errors, maybe the filter/dirty oil affects the oil switch?
I sure hope this fixes the problem, cause the solenoid bolt on the left/driver side is blocked in by the timing belt cover.
Did I understand correctly that P0028 is left/driver side and P0026 is right/passenger side?
 

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Discussion Starter #17
From the reports above, 15-20 Ohms doesn't seem to be problematic if it's stable and repeatable. However, if it's higher, or unstable, then if it isn't causing problems now, I suspect it probably will sooner rather than later.

P0026 is right side, P0028 is left side.

Please be careful when changing the sensor -- see the "Just messed up . . ." thread (linked) in post #7 above.

And, let us know how it works out.
 

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installed the new Beck/Arnley sensor on driver side, P0028,
cel has not come back,
gas mileage has improved back to 20/28 or so
bad sensor once removed read 20 ohms
then after I blew the oil out it didn't register at all.

If I get around to replacing the passenger side I will take a new reading on both installed.
 

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Discussion Starter #19
Thanks for the update. It's adding relevant information about these switches and the trouble codes.

Be interested in how the other sensor works out, and the readings on the new ones after installation.

Thanks again.
 

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repeated VVL oil pressure switch failure

plain OM I have a 06 Sub impreza OBS, I've owned the car for 3.5 years and have replaced the VVL oil pressure switch 4 times now (always the left one) and it needs to be replaced again!

What happens is the CEL comes on (throws P0028) and the car runs a little rough, I notice oil drops on the garage floor, and when I check the white switch itself, it's all covered with oil.

My mechanic wants to try using a Subaru Genuine part this time. I am wondering if we should be looking for the cause of this parts failure. You seem to know a lot about this little part, any thoughts? :)

Should I have him replace the valve itself too?

Thanks for any input you might have!
 
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