CEL - P0174 code

[caverman]

my bad......I meant the front one.

Mine is tuned....and of course the front sensor is a bit more expensive.

[MARINE1991]

Oh ok cool... I had to replace both of mine at different times, but I still get a CEL when I run it hard, stays on for a awhile then goes off, guess it gets too hot or somthing....

[JANNETTYRACING]

Originally Posted by caverman

Ok...so this error code has been coming back on me. It's happened twice now since I last posted this thread.

I've check my exhaust and the connnection of the sensor and it seems to be good. I have a feeling it's either a bad sensor or cat. I say that because it just randomly comes on. Prime example is that it's been off for a while and last Friday I took it out to the drag strip. After a good 4 or 5 runs I had no light. Then two days later I start it up and the light comes on again. I was initially thinking that there was some kind of exhaust leak and it wouldn't show it's ugly head until I really went WOT on a cool night or something. Well, that theory went out the door.

So...what would you do? Should I go buy another sensor (assuming I can and hopefully they aren't a ton of money) and give that a try? Then if that doesn't work look at replacing the cat....which is probably alot more expensive?

BTW: Do you think the dealer is the only place to get a sensor from?

Your Tune is off, P0174 system lean means your fuel trims are reaching +25% correction at some point while Starting, Idling, Driving etc.

Ted.

[JR-Vette]

The PCM trips a lean error code not at above +25 lean but +13%
CATs convert and if EPA allowed the PCM to accept +25 the CAT would convert and send out exhaust way beyond the EPA allowed emissions plus cook the CAT.
Being GM by law has to warranty CATs for 8 years they are not going to allow one side to get as much as +25% lean.

Also might not be the CAT itself but rather the onboard PCM 02 test is out of bounds

Being this started after exhaust was taken apart

1. For 02s to function correctly they need outside reference air so the wires of 02 sensor are very special and have a air channel
Many times people remove or install 02s with the wiring still attached to the main wiring harness. This prevents the wires from turning as the 02 is turned and the 02 wires spin around themselves, crimp and cut off the air path.
Always have the wires unconnected and allow wires to freely turn with the 02 and not twist and crimp up

2. Tests for the 02s are done on cold start, the colder the weather is the more apt with long tube headers for the tests to falsely fail.
This is because with long tube headers the 02 sensors are located further downstream or the headers are not allowing the exhaust or 02s to get hot enough.

3. check to assure the wire for the heater of 02 is supplying 12 volts to 02 as if not then without 02 heater the 02 cannot heat up fast enough for the cold start 02 tests and trip false lean code.
Check 02 heater fuse being good.
The 02 tests are comparing front to rear 02 values if one 02 on a side is colder then the other then DTC is tripped no matter if valid or not.

4. The feds keep adding more and more ethanol into gas, approved few months ago by feds is from 10% to 15% ethanol. Ethanol causes engine to run leaner and could trick PCM into thinking AFR is lean when it is not.

Check 02 wiring, use a thermal gun and see if exhaust is getting hot enough.
Prove what is wrong by using a full OBD scanner and seeing if in fact that side of engine has leaner long term fuel trims then the other side of engine.

IF the fuel trims are in fact not balanced left/right then suspect some type of unmetered air leak or exhaust leak before the 02 sensor.

Never replace 02 of one side as that means 1 02 operates quicker then the older 02 on other side of engine.
Also do not mix brands of 02 sensors and do not get sucked into buying "generic" brand 02s as they may not function correctly with the quad driver hardware of PCM

Originally Posted by caverman

I've had my car tuned for about 5k miles now with no CEL. All of a sudden I got a CEL that I noticed last night. Ran by O'Reilly this afternoon and the scan tool said "P0174 - System Too Lean (bank 2)".

Now, I will also say that on Monday I had some diff bushings put in and we had to remove the exhaust to allow the diff to come down. My cat-back is tack welded so we removed it from the LTs back. In order to do that we removed the O2 sensors out of the cat and left them hanging until we were done.

I cleared the code with the scan tool and it didn't come back but I've only driven it maybe 10 miles and didn't get on it or anything.

Anyone know what that code is saying. Doesn't it have to do with the O2 sensor for the cat?

[JANNETTYRACING]

Originally Posted by JR-Vette

The PCM trips a lean error code not at above +25 lean but +13%
CATs convert and if EPA allowed the PCM to accept +25 the CAT would convert and send out exhaust way beyond the EPA allowed emissions plus cook the CAT.
Being GM by law has to warranty CATs for 8 years they are not going to allow one side to get as much as +25% lean.

Also might not be the CAT itself but rather the onboard PCM 02 test is out of bounds

Being this started after exhaust was taken apart

1. For 02s to function correctly they need outside reference air so the wires of 02 sensor are very special and have a air channel
Many times people remove or install 02s with the wiring still attached to the main wiring harness. This prevents the wires from turning as the 02 is turned and the 02 wires spin around themselves, crimp and cut off the air path.
Always have the wires unconnected and allow wires to freely turn with the 02 and not twist and crimp up

2. Tests for the 02s are done on cold start, the colder the weather is the more apt with long tube headers for the tests to falsely fail.
This is because with long tube headers the 02 sensors are located further downstream or the headers are not allowing the exhaust or 02s to get hot enough.

3. check to assure the wire for the heater of 02 is supplying 12 volts to 02 as if not then without 02 heater the 02 cannot heat up fast enough for the cold start 02 tests and trip false lean code.
Check 02 heater fuse being good.
The 02 tests are comparing front to rear 02 values if one 02 on a side is colder then the other then DTC is tripped no matter if valid or not.

4. The feds keep adding more and more ethanol into gas, approved few months ago by feds is from 10% to 15% ethanol. Ethanol causes engine to run leaner and could trick PCM into thinking AFR is lean when it is not.

Check 02 wiring, use a thermal gun and see if exhaust is getting hot enough.
Prove what is wrong by using a full OBD scanner and seeing if in fact that side of engine has leaner long term fuel trims then the other side of engine.

IF the fuel trims are in fact not balanced left/right then suspect some type of unmetered air leak or exhaust leak before the 02 sensor.

Never replace 02 of one side as that means 1 02 operates quicker then the older 02 on other side of engine.
Also do not mix brands of 02 sensors and do not get sucked into buying "generic" brand 02s as they may not function correctly with the quad driver hardware of PCM

Great Detailed Information JR, Thanks for your time.

I never let my fuel trims get outside of + or - 4% so I didn't know where the Threshold was.

When I see the code on cars that come in the fuel trims are usually + or- 25%

Ted.

[caverman]

Originally Posted by JR-Vette

The PCM trips a lean error code not at above +25 lean but +13%
CATs convert and if EPA allowed the PCM to accept +25 the CAT would convert and send out exhaust way beyond the EPA allowed emissions plus cook the CAT.
Being GM by law has to warranty CATs for 8 years they are not going to allow one side to get as much as +25% lean.

Also might not be the CAT itself but rather the onboard PCM 02 test is out of bounds

Being this started after exhaust was taken apart

1. For 02s to function correctly they need outside reference air so the wires of 02 sensor are very special and have a air channel
Many times people remove or install 02s with the wiring still attached to the main wiring harness. This prevents the wires from turning as the 02 is turned and the 02 wires spin around themselves, crimp and cut off the air path.
Always have the wires unconnected and allow wires to freely turn with the 02 and not twist and crimp up

2. Tests for the 02s are done on cold start, the colder the weather is the more apt with long tube headers for the tests to falsely fail.
This is because with long tube headers the 02 sensors are located further downstream or the headers are not allowing the exhaust or 02s to get hot enough.

3. check to assure the wire for the heater of 02 is supplying 12 volts to 02 as if not then without 02 heater the 02 cannot heat up fast enough for the cold start 02 tests and trip false lean code.
Check 02 heater fuse being good.
The 02 tests are comparing front to rear 02 values if one 02 on a side is colder then the other then DTC is tripped no matter if valid or not.

4. The feds keep adding more and more ethanol into gas, approved few months ago by feds is from 10% to 15% ethanol. Ethanol causes engine to run leaner and could trick PCM into thinking AFR is lean when it is not.

Check 02 wiring, use a thermal gun and see if exhaust is getting hot enough.
Prove what is wrong by using a full OBD scanner and seeing if in fact that side of engine has leaner long term fuel trims then the other side of engine.

IF the fuel trims are in fact not balanced left/right then suspect some type of unmetered air leak or exhaust leak before the 02 sensor.

Never replace 02 of one side as that means 1 02 operates quicker then the older 02 on other side of engine.
Also do not mix brands of 02 sensors and do not get sucked into buying "generic" brand 02s as they may not function correctly with the quad driver hardware of PCM

So....based on this it still sounds like it could be the 02 sensor. My car was tuned by Andy @ ADM so I know it was tuned correctly as well as I never had a CEL until after the exhaust came off.

I do remember seeing the O2 sensors hanging down which tells me they weren't disconnected prior to installing them back. It makes complete sense that if they weren't disconnected they could have been twisted up during reinstall.

Do you think I could un-plug the sensor, remove it, reinstall, then replug and I might be okay? Or, once they get twisted they are toasted? And, if I had to replace one 02 you saying I should really replace both?

[jeff4946]

I have heard that the threads on these o2 sensors are easy to bugger up. You need to really be careful when you pull em, or they won't seal good again on reinstall.

[JANNETTYRACING]

Originally Posted by caverman

So....based on this it still sounds like it could be the 02 sensor. My car was tuned by Andy @ ADM so I know it was tuned correctly as well as I never had a CEL until after the exhaust came off.

I do remember seeing the O2 sensors hanging down which tells me they weren't disconnected prior to installing them back. It makes complete sense that if they weren't disconnected they could have been twisted up during reinstall.

Do you think I could un-plug the sensor, remove it, reinstall, then replug and I might be okay? Or, once they get twisted they are toasted? And, if I had to replace one 02 you saying I should really replace both?

I would agree that once they are twisted they are Junk.

[bluetorp]

It's only throwing a code for one bank, so it's unlikely to be the tune, especially since it only manifested after messing with the sensors/exhaust.

O2s are cheap. Yank it and replace it.

[caverman]

Originally Posted by bluetorp

O2s are cheap. Yank it and replace it.

Cheap is relative. The front sensors are basically $100 each. So, if I have to replace both then it's $200. That will almost buy me the line lock I was wanting. And, there is no gurantee that's the issue. I could just be tossing the money at nothing.

Although I'll say it sounds like more than likely it's the sensor.

[caverman]

Ted or JR.....if I do need to replace my sensor, do you think I need to do both at the same time?

Maybe I'll replace the one and then if the CEL comes back I can check it and it should say that it will have an issue with Bank 1 that time (theoretically). Then replace the other.

I just hate to toss money at this not knowing if it's even the issue. Guess I don't really have a choice though. Any suggestion on the best place/price for a front 02 sensor?

[bluetorp]

You are correct. Cheap is relative.

You shouldn't need two, just the one for the bank throwing the code.

Also, I'd say chances are pretty high that someone in the DFW area has HPTuner and would probably be willing to spend a couple minutes to run a scan and see what the O2 waveforms look like for free, or for lunch or a couple beers.

[JR-Vette]

Originally Posted by caverman

So....based on this it still sounds like it could be the 02 sensor. My car was tuned by Andy @ ADM so I know it was tuned correctly as well as I never had a CEL until after the exhaust came off.

I do remember seeing the O2 sensors hanging down which tells me they weren't disconnected prior to installing them back. It makes complete sense that if they weren't disconnected they could have been twisted up during reinstall.

Do you think I could un-plug the sensor, remove it, reinstall, then replug and I might be okay? Or, once they get twisted they are toasted? And, if I had to replace one 02 you saying I should really replace both?

Consider there is a air channel between the wire and the insulation, once the wires get twisted the insulation may not return back to shape and the air path is still cut off.
You might check them and if twisted ( normally right at the end of the 02) correct that and then test.
Keep in mind the 02 tests are only done at cold start and the DTC might not trip for a few days so do not be fooled.
Best again is with a OBD scanner as you will instantly see what the 02 sensors report and if one side is reporting much leaner then the other side.

Also common with use of headers were wires for 02 sensor get hot enough causes insulation to get too soft and deform enough to shut that air path down.

Always in taking 02 sensors on or off is leave wiring disconnected from wiring harness so that when 02 is turned the wires freely turn with it.

Consider 02s live in about 900 plus degrees, the older they get, the more they age out the slower they function.
If the car does not have too many miles on it then you could get away with replacing just one but if older then best to replace as a pair so if it is one of the fronts replace both fronts so that they both are switching at the same rate. If not then the older one will degrade more and report AFR differently then the side with newer 02.

Check the wiring closely first and see if that solves the issue.
Again if in doubt check to see if the fuse for 02 heaters are not blown because if 02 heater does not work then on cold start when PCM does 02 tests the 02 tests would fail or not test fast enough and cause PCM to see values from the front and rear 02 out of spec and trip a false lean error code

A good OBD scanner has the federal 02 test function when commanded with scanner instantly would test and inform if 02s are within spec and if balanced left to right
Also if the tune is not nuts on the cold start AFR could be outside the window and fool PCM tests and trigger false DTC.

Use a thermal tool and measure the temps of exhaust primaries, temp right before the CAT, right after if and then at muffler tips
That would tell you if both sides balance and in fact if AFR is really 25% too lean or not
Remember a tune that was good in hot weather might not be good enough to handle colder weather where 02 tests are more prone to fail as exhaust, CATs and 02s just a bit too cold.

[JR-Vette]

Originally Posted by bluetorp

You are correct. Cheap is relative.

You shouldn't need two, just the one for the bank throwing the code.

Also, I'd say chances are pretty high that someone in the DFW area has HPTuner and would probably be willing to spend a couple minutes to run a scan and see what the O2 waveforms look like for free, or for lunch or a couple beers.

You cannot make that a general statement as makers of 02 sensors themselves spec 02 sensors to work well below 30,000 miles.
You cannot expect living in 900-1200 degrees not to degrade and since 02 sensors only function from 0 to 1 volt ( mostly around 0.45 mVolt) it does not take much for them to slow down.
If 02s have 20,000 plus miles it might be less costly long run to swap both as another issue can be the newer 02 has different specs and outputs differently then older original one and why cars are seen with a AFR imbalance left/right so safer to use same brand for both sides.

Add ethanol in gas today does not do favors for 02s and long tube headers forcing downstream placement of them that small voltage operating range does not take much to cause them to lie about the true AFR.

Use OBD and do the 02 tests and being the output voltage of 02 looks like a A/C sine wave see if older 02 swings the same rate as newer one.

Owner needs to first see if 02 wiring got twisted, wires too hot or when exhaust was put back that there is a leak before 02 that then is seen by the sensor.
Also common with long tube headers not of good quality that over time or taking exhaust apart that header welds crack and suck in air to fool a 02 sensor.
I have smoke tested many cars with headers to find small weld cracks you cannot see well while installed in car

Lastly many times in design of headers causing 02 sensors to be moved the owner are forced to use 02 with longer wires, too long and they bundle up the excess wire, tie wrap, etc and have shut down the reference air path.