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Old 10-05-2018, 07:11 AM   #1
1000000to1
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O2 sensor problem.

I have a 97 Chevy with a 350. Threw a code 0137 so I figured sensor was bad and replaced it. Threw code again so I checked the wiring and all was good. I then swapped sensors side for side. Truck then threw a code 0157 for other side. So I figured a bad sensor and bought another. Threw same code again. Sure I didnít get two new sensors that were bad, so what gives? Any ideas?
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Old 10-11-2018, 02:45 PM   #2
LH Lead-Foot
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Re: O2 sensor problem.

P0157 and P0137 are codes that indicate 02 sensor in front of the catalytic converter. The engine has to be in closed loop, basically run above 1500 PRM's for 2 minutes. Low voltage = lean or way too much oxygen. I taught tech's to think of them as "Hydrocarbon Sensors" that produce higher voltage with high amounts of fuel in the exhaust. They produce voltages from 0.1 v - 1.0 v normally.

If behind the catalytic converters, their signal moves very little to indicate the cat's ability to store oxygen. Are the 02 sensor in front of the cat? If so, that is bank 1 or 2, sensor 1.

You indicated the problem follows the sensor even when replaced. This kinda removes melted wires either PPL or TAN. How are you getting these codes? Scan-tool of parts store plugging in their code reader, because what you are describing is somewhat confusing. These should be 4 wire sensors, with 2 being heaters to quickly get them into "Closed Loop".

Make sure the are putting the correct information into the scan-tool, like Year, Model and VIN 8th digit for engine 5.7L in your case.

Make sure the wire terminals are ok, tight fit, not pushed out & clean. Most schematics show only one connector between the 02 to VCM, which is the sensors connector only. So weird.

One possibility is the controller, in this case called the VCM, is not grounded well to the body, engine and back to the battery. If you have a good digital multi-meter (DMM), Lets do a voltage drop test to check the VCM.

Back probing a ground wire, like the MAP sensor. It has a vacuum hose to it, but 3 wires. One is GRY that measure 5 volts, key on, engine off. The BLK wire is dedicated ground that goes back into the VCM, out thru ground to G100, block, battery cable to battery. The LT GRN is MAP input into VCM...ignore.

Back probe the black wire without damage, using a "TEE" pin (Sold at fabric stored) to DMM, set to DC Voltage, connect other meter led to negative battery, start engine and take measurement. It should be lower than 0.400 volts (0.4). If higher like 1.23 volts, you have a problem.

VCM or PCM (Computer in general) have multiple grounds that exit the module from the Memory, Input, Output, Clock section. They are not soldered together inside the computer and go to G104, G103, which is out of harness to engine. So double check the battery ground cable and clean battery connection...also chassis or body. Grounds everywhere!

Other issues to answer;
#1 Do you have a cat?
#2 Do you have catalytic converters or have they been replaced?
#3 Are these 4 wire heated 02 sensors? (Should be)
#4 Have the sensor(s) been dropped onto hard surface?
#5 Have the sensor wires / harness been soldered?
#6 Did codes get cleared between repairs. Lean for 70 seconds, set code, but is a "Type B" code which means you have to have two (2) key cycles before it will turn on MIL.
The MIL goes off after 3 consecutive drive trips when test runs and passes. What is a "Drive Trip"? Engine coolant temp and intake air temp are within 10 degrees of each other, reaches 172 degrees or higher / also called "Warm-up Cycles".

The sensor's behind or down-stream of the cat, has only reason to be there is to guarantee the catalytic converters condition.
Hope this helps a little.
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I turned around to go back and she asked why? I told her, if I am going to get slapped for something I didn't do, then I am going to look at her...so I know why!
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Old 10-11-2018, 03:16 PM   #3
SunSoaked
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Re: O2 sensor problem.

That was an awesome answer! I appreciate the time and details you provided. Nice to see free " teachable moments".
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Old 10-11-2018, 05:12 PM   #4
LH Lead-Foot
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Re: O2 sensor problem.

I have GM "Electronic Service Information" on a stand-a-lone CD set, that I use on an older Dell XP-Pro. The problem was GM started this in late 1997 and up. Prior to that, all you get is the engine / transmission section. This was paper service manual period / pre-smart phone.

So, I have a few GM paper manuals when electrical was in separate book but nothing on GMT400 / GMT800 as the Buick dealer did not have trucks during the 90's.

If you find a paper service manual, just make sure if "Section 8" is included because that is electrical only.

I taught at UTI before it was sold to Phoenix University, then sold again. I was there after my first two lumbar fusions.
Later, I worked for GM on contract. I held 32 different technical service seminars that lasted 3 3/4 hours for up to 100+ Techs, then full day classes for up to 12 techs that lasted two days each. We only had 26 classes but include lab scopes. But the job kept me in a hotel 242 days each year on average for 7+ years. Brutal.
I used to say that I would email my wife a photo so the dog did not bite me when I came home.

I was shocked to find how many techs did not know how OBD-11 works, since it has been around in 1994.

Each code is broken down into "Enable Criteria" so if certain codes are stored, other PCM testing is aborted. The PCM / VCM is very busy for 5 -6 minutes (Less on some) once you start the vehicle. It has to test each circuit or device using a method that would make sure it works to control emissions. It uses resistance, voltage drop, or turns it on while driving to see a change, then passes or fail. Then only "Type A" codes turn on the MIL on the first fail, "Type B" code require two failures and then "Type C". There are fail records stored that can only be erased with a scan-tool and the TECH2 can get all 5 fail records. This is so once fixed, you can repeat the same conditions to see if it passes or fails testing. It all easy, right?Fun huh?
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While driving one day with my wife, I noticed a candy apple red open wheel roadster with everything chrome from the windshield forward. As it passed in the other lane going the other way, out of no where, my wife slapped me hard. I said...Hey! What was for? She said you where looking at the blond on sidewalk. I said NO, the car!
I turned around to go back and she asked why? I told her, if I am going to get slapped for something I didn't do, then I am going to look at her...so I know why!
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