O2 Sensors

[dcompton02]

I have a 2003 Chevy Tahoe 5.3 with 189k. I went to Autozone and had them look up why my check engine light was on. They said bank 1 sensor 1 was bad. I replaced it then unplugged the battery for a bit to get reset the code. It went off then 30 minutes later the check engine light came back on? It had the same code as before. Any help would he much appreciated.

 

[OR VietVet]

Since when can an auto parts counter person diagnose a trouble code from the scanner. All the scanner tells you is the circuit the problem is in, not the fix. Parts guy made a sale though, didn't he!

First, what is the exact code? Is the code that came back the EXACT same numbered code?

Bank 1 sensor 1 is the front sensor on the drivers side. That can go bad for any number of things. If replacing sensors without any qualified shop diagnostics I would do all 4 of them since they are all the same age. Also, get smart and change the sensors with AC Delco brand only. I have seen numerous times where in fact you do have a failed sensor and replace with off brand sensor and still not fixed till put in an AC Delco brand.

C'mon man!

 

[dcompton02]

It was code P0171. I took the O2 sensor out before hand and it was black almost as if it was burnt. I had an experienced mechanic put it in but we couldn't figure out why the light wouldn't stay off. When I get home I'll look and see what brand it was. I'm pretty sure it was AC Delco.

 

[OR VietVet]

Pay real close attention to this: See list of causes.
The black look can be normal, especially over a long period of time.

Basically this means that an oxygen sensor in bank 1 has detected a lean condition (too much oxygen in the exhaust). On V6/V8/V10 engines, Bank 1 is the side of the engine that has cylinder #1. The P0171 is one of the more common trouble codes.

This code is triggered by the first downstream (front) O2 sensor. The sensor provides a reading of the air/fuel ratio leaving the engine's cylinders, and the vehicles powertrain/engine control module (PCM/ECM) uses that reading and adjusts to keep the engine running at that optimum ratio of 14.7:1. If something is not right and the PCM cannot maintain the 14.7:1 ratio, but rather there is too much air, it triggers this code.

You'll want to also read our article on short and long term fuel trims to help understand the operation of the engine. Note: This DTC is very similar to P0174, and in fact your vehicle may show both codes at the same time.

This diagnostic trouble code (DTC) is a generic OBD-II powertrain code. It is considered generic because it applies to all makes and models of vehicles (1996-newer), although specific repair steps may vary depending on the model.

Symptoms

You will more than likely not notice any drivability problems, although there may be symptoms such as:
• a lack of power
•detonation (spark knock)
•rough idle
•hesitation/surge on acceleration.

Causes

A code P0171 may mean that one or more of the following has happened:
•The MAF (Mass Air Flow) Sensor is dirty or faulty
Note: The use of "oiled" air filters may cause the MAF to become dirty if the filter is over-oiled. There is also an issue with some vehicles where the MAF sensors leak the silicone potting material used to protect the circuitry.
•There could be a vacuum leak downstream of the MAF sensor
•Possible cracked vacuum or PCV line/connection
•Faulty or stuck open PCV valve
•Failed or faulty oxygen sensor (bank 1, sensor 1)
•Sticking/plugged or failed fuel injector
•Low fuel pressure (possible plugged/dirty fuel filter!)
•Exhaust leak between engine and first oxygen sensor

Possible Solutions

A lot of times, cleaning the MAF sensor and finding/fixing vacuum leaks fix the problem. If you're on a tight budget, start there, but that may not be the fix for certain. So, possible solutions include:
•Clean the MAF sensor. Consult your service manual for it's location if you need help. I find it's best to take it off and spray it with electronics cleaner or brake cleaner. Make sure you are careful not to damage the MAF sensor, and make sure it's dry before reinstalling
•Inspect all vacuum and PCV hoses, replace/repair as required
•Inspect all hoses and connections in the air intake system
•Inspect and/or test the intake manifold gaskets for leakage
•Check for a dirty fuel filter and proper fuel pressure
•Ideally you'll want to monitor short and long term fuel trims using an advanced scan tool
•If you have access, you may want to run a smoke test

 

[dcompton02]

Thank you I have heard to clean the MAF and to check for vacuum leaks. I think you're right too about having both codes. I have codes P0171 and P0174 like you said.

 

[OR VietVet]

Your original post did not say you had 2 codes. Read this:

Basically this means that an oxygen sensor in bank 2 detected a lean condition (too much oxygen in the exhaust). On V6/V8/V10 engines, Bank 2 is generally the side of the engine that doesn't have cylinder #1.

Note: This DTC is very similar to P0171, and in fact your vehicle may show both codes at the same time.

Symptoms

You will more than likely not notice any drivability problems, although there may be symptoms such as a lack of power, detonation (spark knock), and/or a hesitation/surge on acceleration.

Causes

A code P0174 may mean that one or more of the following has happened:
•The MAF (Mass Air Flow) Sensor is dirty or faulty. Note: The use of "oiled" air filters can cause the MAF to become dirty if the filter is over-oiled. There is also an issue with some vehicles where the MAF sensors leak the silicone potting material used to protect the circuitry.
•There could be a vacuum leak downstream of the MAF sensor.

Possible Solutions

Possible solutions include:
•In the vast majority of cases, simply cleaning the MAF sensor does the trick. Consult your service manual for it's location if you need help. I find it's best to take it off and spray it with electronics cleaner or brake cleaner. Make sure you are careful not to damage the MAF sensor, and make sure it's dry before reinstalling
•Inspect all vacuum and PCV hoses, replace if necessary
•Check for a dirty fuel filter and proper fuel pressure

 

[dcompton02]

Thank you! I'll let you know how it goes in a few days.

 

[OR VietVet]

Yes. It is always important to post a fix for others to read.

 

[clandr1]

Pay real close attention to this: See list of causes.<snip>

Ron, you always have the best, most helpful posts. Good job, and thanks for this info.

 

[OR VietVet]

I wouldn't say always. Ask some of the guys I light in to that don't share all the info or history. I do try but there is plenty of guys who that do as good if not a better job. But thanks.