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MY99 CEL P1123 P1125

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Hi Guys

Thought I would share my CEL story. This is just the first installment...

I have a 99 Boxster that I bought used, it now has 73k miles (I added the last 17k). It came down with a CEL ten days ago which the dealer diagnosed as codes P1123 and P1125. I've seen the codes mentioned here before and so have a good idea what's going on. Anyway the dealer sold me an induction cleaning which they thought might clear it up - it did, but only for about two days.

My understanding is that the engine is trying to run leaner, but can't lean itself out enough. I'll mention here that I actually had a bad feeling when I filled it up right before the CEL. I grabbed a pump with a hand written sign on it that said 'No auto shut-off, low pressure'. I used the pump anyway and wonder now if I got some crud from the bottom of their tank into my engine...

Anyway when the CEL came back on I refilled the tank, added some Techron, reset the CEL by disconnecting the battery and then took off for some highway driving. 300 miles later, no CEL. But when I got back to city driving the CEL came back on pretty quick.

Next step was to take a look at the MAF and try cleaning it. It's a Bosch, so I assume not original. Basically I just dumped some tape head cleaner on it and put it back in. Started the car up the next morning, drove a bit with everything OK, and then after it warmed up it wouldn't hold idle speed. Just kept dropping to zero. Gave it one more crank in the garage after I limped home and of course then it -did- hold idle for a bit, just testing my sense of humor I guess.

Anyway the engine has otherwise seemd to run fine this whole time. Maybe not as much pep as I would like, but it's been that way since I've had it.

Guess tonight I'll borrow a buddy's tester and see what kind of codes I've got. Then I'll clear those out and try running with the MAF disconnected for a while and see what codes result from that.

I'll keep you all posted.

One last question - I can display my ignorance a bit - how would a faulty gas cap contribute to P1123 and P1125? I actually don't know what proper operation of the gas cap provides - an air tight seal (I assume) or a certain amount of ventilation into/out of the gas tank?


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how would a faulty gas cap contribute to P1123 and P1125? I actually don't know what proper operation of the gas cap provides - an air tight seal (I assume) or a certain amount of ventilation into/out of the gas tank?
Per the OBD II manual:

In order to prevent fuel vapors from entering the atmosphere, an EVAP canister is connected to the tank ventilation line.

Fuel vapors are collected in the EVAP canister. As the EVAP canister can only accommodate a certain amount of fuel vapors, the fuel vapors must be taken in and combusted during driving.

For this to occur, the EVAP canister purge valve opens. Due to the vacuum predominating in the intake manifold, air is sucked up via the purge air line through the EVAP canister. The air sucked up in this way draws the collected fuel vapors with it.

American law requires the tank ventilation system to be checked for leaks.

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Thanks Loren.

What do you think about the following...

I 'cleaned' the MAF sensor and got codes p1125 p1123 (o2 lean threshold) p0102 (no MAF). Then I disconnected the MAF sensor and got codes p0102 (no MAF) and p0112 (no air intake temp sensor). Is the air intake temp sensor part of the MAF unit?

I thought the car ran great with the MAF disconnected. But I hadn't replaced the carpet/padding over the engine and probably just liked the sound!!!

Anyway I'm inclined to order a new MAF sensor.

Also the OBD2 box was giving me readings that were all over the map on one bank of o2 sensors. I would get readings of maybe .04, wait a few seconds and get .7, etc. Is it normal for these guys to jump around like that?

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Well a new MAF and almost 24 hours w/no CEL - we'll see how it goes. Iwould say the car did not run any worse w/out the MAF. And the air intake temp readings are kinda suspicious - 96.8 always - yeah right.

Anyone know what might affect o2 readings for just one side of the engine? O2 sensors 1 and 1b read abnormally low sometimes...

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  • 7 months later...

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