Oxygen sensor error code

I have a 1999 Suburban. Service engine soon light came on and error code is O2 sensor efficiency below threshold. I take this to mean I need to change an O2 sensor. It looks like my rig has 4 of them. Is there any way to tell which one is generating the error code or do i have to change them all? I have minimal experiance so any help will be greatly appreciated. Thanks for your help.
Mike N.

I have a 1999 Suburban. Service engine soon light came on and error code is O2 sensor efficiency below threshold. I take this to mean I need to change an O2 sensor. It looks like my rig has 4 of them. Is there any way to tell which one is generating the error code or do i have to change them all? I have minimal experiance so any help will be greatly appreciated. Thanks for your help.
Mike N.

You didn't mention how many miles you had on the rig. Rule of thumb is 60k miles for O2 sensors. I doubt that you can find which one is at fault and there are probably at least two different physical styles (pre cat and post cat?) amongst the four.

If you are over the 60k, replace them all. It's the price we have to pay for all the green weenies that you all allowed a big voice in our affairs.

Before you go replacing all of them, take the rig somewhere and get the codes ran. The code will tell you which O2 sensor is at fault. This will save you about $180-$200 as opposed to replacing all of them..

Welcome to your first post on AF.
As Corning-d3 says you need to get the code no and post back.
Also the scanner usually points to which 02 sensor the code is from like what bank and what sensor.
But remember a 02 code does not say replace the sensor it says check it out.
Many other things can cause a 02 sensor code.
And they are not like spark plugs or replace ever so many miles.
Only replace if they go bad. And make sure they are bad before replacement.
Some of them will outlast the car.
Get it scanned and get the code no and post back the code no.
MT

Hey guys thanks for all of the good info thus far. I went and got the code information like you suggested. "0420 CATLIST EFF BELOW THRESHOLD BANK 1" was the result. Does this shed any light on how I should proceed? Thanks again Mike N.

Oops. This rig has just over 100K in it. 1500 suburban 5.7L 4X4 Auto Transmission I think that covers it.

Code 420 is not a 02 sensor code and it can be caused by many things.
First thing make sure tune up is up to par.
How many miles on your plugs and tune up?
Here is a link on 420/430 codes.
http://www.troublecodes.net/articles/catfailure/
Also some info.
DTC P0420, P0421, P0430 & P0431: Check Possible Cause Of Misfire DTC P0420 and P0421 indicates bank one catalyst system efficiency is minimum requirement. DTC P0430 and P0430 indicates bank 2 catalyst system efficiency is minimum requirement. Possible causes are as follows: Use of leaded fuel. Oil contamination. Cylinder misfire. Fuel pressure too high. HO2S sensor improperly connected. Damaged exhaust system component. Faulty ECT sensor. Faulty HO2S. Ensure ignition timing is correct. Retrieve all Continuous Memory DTCs. If misfire code(s) is not present, go to next step. If misfire code(s) is present, isolate cylinder and repair as necessary. Check HO2S Monitor DTCs If DTCs P0136, P0138, P0140, P0141, P0156, P0158, P0160, or P0161 were present in step 1), service as necessary before continuing. If none of these codes are present in step 1), go to next step. Check ECT Sensor DTCs If DTCs P0117, P0118, P0125 or P1117 were present in step 1), service as necessary before continuing. If none of these codes are present in step 1), go to next step. If any codes except P0420, P0421, P0430 and/or P0430 were present in step 1), service as necessary before continuing. If no codes except P0420 and/or P0430 were present in step 1), go to next step. Check Rear HO2S Wiring Harness Turn ignition off. Ensure HO2S wiring harness is correctly routed and connectors are tight. Repair or replace as necessary. If wiring harness and connectors are okay, go to next step. Check Fuel Pressure Turn ignition off. Release fuel pressure. Install fuel pressure gauge. Start engine and allow to idle. Note fuel pressure gauge reading. Increase engine speed to 2500 RPM and maintain for one minute. For fuel pressure specifications, see FUEL PRESSURE SPECIFICATIONS article. If fuel pressure is as specified, go to next step. If fuel pressure is not as specified, go to CIRCUIT TEST HC . Check For Exhaust System Leaks If exhaust system leaks, it may cause catalyst monitor efficiency test to fail. Inspect exhaust system for cracks, loose connections or punctures. Repair or replace as necessary. If exhaust system is okay, go to next step. Check For Exhaust System Restrictions Inspect exhaust system for collapsed areas, dents or excessive bending. Repair or replace as necessary. If exhaust system is okay, go to next step. Ć Check Manifold Vacuum Install tachometer. Connect vacuum gauge to intake manifold vacuum source. Start engine and raise engine speed to 2000 RPM. Manifold vacuum should rise to more than 16 in. Hg. If manifold vacuum is okay, go to next step. If manifold vacuum is low, go to step 11). Leave tachometer and vacuum gauge connected. Start engine and raise engine speed to 2000 RPM. On a non- restricted system, manifold vacuum should quickly rise to normal range as increased RPM is maintained. On a restricted system, manifold vacuum will slowly rise to normal range as increased RPM is maintained. If manifold vacuum is okay, no indication of exhaust leak or restriction has been detected and testing is complete. If manifold vacuum is low or slow to respond, go to next step. Leave tachometer and vacuum gauge connected. Remove exhaust pipe from exhaust manifold. Start engineand raise engine speed to 2000 RPM. If manifold vacuum is now okay, fault is downstream from exhaust manifold. Reconnect exhaust pipe to exhaust manifold and go to next step. If manifold vacuum is still low or slow to respond, fault is in exhaust manifold or intake manifold gasket. Repair or replace as necessary and repeat QUICK TEST. Leave tachometer and vacuum gauge connected. Disconnect muffler/tailpipe assembly from rear of catalytic converter. Start engine and raise engine speed to 2000 RPM. If manifold vacuum is now okay, fault is in muffler/tailpipe assembly. Repair or replace as necessary and test drive vehicle to verify elimination of symptom. If manifold vacuum is still not okay, fault is in catalytic converter. Repair or replace as necessary. Check tailpipe/muffler assembly for debris from catalytic converter. Test drive vehicle to verify elimination of symptom.
Let us know how it goes or what you find.
MT

PO420 seems more like an 02 sensor code. Hook up scanner and read the data. You should be able to see the voltage fluctuating for that O2 sensor. At closed loop voltage for a pre cat O2 should fluctuate between .150 mv - .850mv. Low voltage = Lean, High Voltage = Rich

19Malibu70
Code 420 is not a 02 sensor code.
What sets a code 420 is the pcm does not see any difference between the the pre and post cat 02 sensors.
It can be a weak sensor or weak cat or any number of other things.
Read the test I posted for testing for the code and you will get a idea of what sets that code.
MT :lol: :grinyes: :lol2:

Hey all: Thanks for all of the sound advice. I will be honest. This all seems a bit overwhelming for a newbie. At the outset, I thought this was going to be a simple job but now I think it might be more than I can handle. What do you think should a neophite tackle this or spend the money (which is tight) and take it to a pro? Thanks in advance. Mike N.

it is not an o2 sensor code. when an o2 fails it either reads high or low, if its fluctuating voltage its not broken. the cat efficiency codes means, like mt said, that the pre and post cat o2 sensors are reading the same which can only mean the cat is borked. now the problem is, why is the cat out? misfire? vacuum leak causing overly rich condition? you can get teh cat replaced and see how logn it lasts or you can take it to a shop and have it checked out to make sure everything is kosher.

"Topping" off the gas tank will set the code off too.Do not top off a gas tank in an OBD II vehicle,a Vetronix rep my father knows that does help him out on car computer headaches said this.

i dont see the correlation between topping off the gas tank and a catalyst efficiency code.