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PO42 SES light.


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P042 (E042) ......................................... Left Oxygen Sensor Signal Lean

I thought they had 4 sensors, but based on this DTC, I would gues yours only has 2. Left would be left bank (by the radiator).

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I believe that they have three, one for each bank and one for the cat. P042 is a lean condition. A lean condition on one bank usually means that you have a vacuum leak. It could just be a bad connection, at the connector for the oxygen sensor, or for the heater line for that oxygen sensor.

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-- Click Here for CaddyInfo page on "How To" Read Your OBD Codes
-- Click Here for my personal page to download my OBD code list as an Excel file, plus other Cadillac data
-- See my CaddyInfo car blogs: 2011 CTS-V, 1997 ETC
Yes, I was Jims_97_ETC before I changed cars.

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I dont really know a piston from a tire here but I thought that there was an O2 sensor before the cat and one after the cat. Is that right? Is it as easy as screwing a new one into the exhaust or is the one im looking for located elsewhere? You are always so helpful. Thanks again guys.

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I think you got me. There are two for the cat. The code you called out is for the left bank, which is the front one on the exhaust manifold just before the flange. It has a pigtail a few inches long and a connector to the wiring harness. The cable to the oxygen sensor has two wires to it, one for the heater and one for the signal. A bad connection on either one will throw a code. For the P042, I would clean the connector and tighten the contacts if necessary, reset the code, and see if it comes back. If it does, I would look for a vacuum leak or a bad vacuum hose before I replaced the sensor.

CTS-V_LateralGs_6-2018_tiny.jpg
-- Click Here for CaddyInfo page on "How To" Read Your OBD Codes
-- Click Here for my personal page to download my OBD code list as an Excel file, plus other Cadillac data
-- See my CaddyInfo car blogs: 2011 CTS-V, 1997 ETC
Yes, I was Jims_97_ETC before I changed cars.

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My '97 had 4. One on each manifold, one pre CAT and one aft CAT. Later, they eliminated the pre CAT. Not sure about the '93, but I would suspect 4. The thing that throws me is the DTC description.

I might be wrong, but I think the 4.9 only had one.

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What do you mean by vacuum leak? That sounds like another problem that wont be as easy as replacing an O2 sensor. I hope it not anything like that. I just paid for all of my classes and im a little light on funds right now. I could swing a $50 part but a shop visit wouldn't be good right now. In fact im not even sure how im going to get to class tomorrow since im a little bit hesitant to drive it. I just got home and the thing felt like it was going to stall when it was at a stop.

EDIT: I was just reading up on vacuum leaks and they mention the EGR valve causing like systoms. I should mention that I have gotten the PO36 code recently and have been getting it off and on for a couple of years now. I usually just do some spirited driving at it takes care of itself for a while.

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What do you mean by vacuum leak?

Your engine is essentially an air pump. It is sucking air into the cylinders through the intake manifold & TB. Since the throttle plate is usually partially closed to throttle down the engine, there is a partial vacuum in the intake. Any compromised seal or crack is a vacuum leak.

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I don't know if a sticking EGR pintle would cause a P042 but it seems like it may be possible. In any case, you have an intermittent P036 so you probably should clean the EGR. The EGR needs cleaning about every 100,000 miles. In the 1992-1993 Northstars the EGR passage is in the heads under the intake manifold, but it is not too big a job. In the process you will clear up any manifold gasket leaks, too, and you have a very good chance for both codes going away.

CTS-V_LateralGs_6-2018_tiny.jpg
-- Click Here for CaddyInfo page on "How To" Read Your OBD Codes
-- Click Here for my personal page to download my OBD code list as an Excel file, plus other Cadillac data
-- See my CaddyInfo car blogs: 2011 CTS-V, 1997 ETC
Yes, I was Jims_97_ETC before I changed cars.

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