Thread: P0171 after installing headers

Car/mods in my signature. No vacuum leaks that I can find, had no codes before installing headers. The first few trips after installing them (about 50 miles) I generated a p0131 code and a p0140 code. My rear o2 sensor was not plugged in and the headers were plugged, so no surprise there. My front o2 was installed with a ZZP extension harness (no way the connectors would reach with stock wiring). After a few more trips, however, the p0131 code went away, and now I'm getting a p0171 code. I'm guessing that the front o2 died. Opinions?

O2 could be bad but I'd double check vacuum lines.

O2 is not bad. If you replace it..you are throwing away money. It's the O2 that is reporting the problem.

171 can be a leak in the exhaust before the sensor. Can be a plug wire sitting on the O2 extension (touching) etc etc.

Originally Posted by BillBoost37

O2 is not bad. If you replace it..you are throwing away money. It's the O2 that is reporting the problem.

171 can be a leak in the exhaust before the sensor. Can be a plug wire sitting on the O2 extension (touching) etc etc.

Thanks Bill, I didn't see any leaks/cracks in the vacuum lines (replaced somewhat recently with tubing/fittings from advance auto). What level of vacuum should I be running at idle? Also, a leak in the pre-O2 exhaust is a very real possibility. I used the U-clamps that came with the headers, and while the downpipe looks great, the crossover seems to fit poorly at the slip fits. I'm pretty sure I would hear an exhaust leak, but even at WOT I don't hear anything.

I'll check to make sure the o2 extension isn't hitting the plug wires, but why is this a concern? Interference?

did you pull the battery cables when you did this work?

Stock valvetrain'd car should be around 19-21~ in.hg @ warmed up idle...about 30~ seconds in.

Originally Posted by BillBoost37

did you pull the battery cables when you did this work?

The battery cables were disconnected from the battery for the entire header install, though they were not removed from the vehicle. I only reconnected the cables when I was ready to start the car. The reason I suspected the O2 is because it was throwing a low voltage code prior to this code. What is your line of thinking in terms of the battery cables?

Also, thank you blueguy. Is it worth picking up a gauge to find the level of vacuum? Almost every post I've read about the p0171 indicated a vacuum leak, and a few had electrical issues.

I'm thinking my exhaust crossover connections may be my issue. Thoughts?

Update: The car seems to be running more rough now and occasionally has a misfire. Is this something that could be caused by an exhaust leak upstream of the 1st o2? I have cleaned the MAF with MAF cleaner, but it's very clean and also relatively new. Logic would say that the issue was caused by something I did wrong in my header install, as I did not have the issue before.

Slap some band clamps on the crossover and fix your exhaust leaks.

Post pics, that's always the best bet.

Alright, installed band clamps, cleared the code, came back when I started the car. (The code comes back consistently within 5 minutes of starting the car/clearing the code.) Replaced a vacuum fitting on the FPR that wasn't leaking, but seemed a bit pinched. Again, code came back after clearing. Cleaned my MAF and checked for more vacuum leaks, nothing. Then, on a whim based on a few of bill's old posts, I took the battery terminals off and cleaned them with a dremel tool. Rescanned... no P0171! (The p0140 came back because I had no rear o2 sensor, but this code didn't come up when the p0171 was active). After a very short trip, still no p0171. I'll keep you guys posted.

I was getting back to the question of "bill...why are you asking about the battery cables" LOL. It happens. If you continue to see it at times, you may want to replace them entirely.

Originally Posted by BillBoost37

I was getting back to the question of "bill...why are you asking about the battery cables" LOL. It happens. If you continue to see it at times, you may want to replace them entirely.

Kind of crazy stuff. I'll keep an eye on it, though. Thanks Bill.

What I sometimes think crazy is that I can pull that you may have pulled a battery cable out of the air.

Originally Posted by BillBoost37

What I sometimes think crazy is that I can pull that you may have pulled a battery cable out of the air.

Maybe you were sleepy when you typed this? It doesn't make grammatical sense. Anyway, the p0171 is still completely gone, and I am left with the p131 and p140 that I started with. As previously stated, the p0140 is not an issue because I have the rear 02 disconnected. But how do I go about getting rid of the p0131? The wiring between the sensor and cables, as well as the extension, all look good. Suggestions?

I don't know what a 131 is off the top of my head. Sorry, would you mind at least listing the name of the code. Maybe a link to GearChatter or something with the FSM info to read up on.

As for the gramatics, it was early and yeah..it made sense in my head. What I tried saying is. It is funny to me, that I pulled the cable possibility out of thin air. You hadn't mentioned removing the cables or anything of that nature. Somehow, I knew.

Originally Posted by BillBoost37

I don't know what a 131 is off the top of my head. Sorry, would you mind at least listing the name of the code. Maybe a link to GearChatter or something with the FSM info to read up on.

As for the gramatics, it was early and yeah..it made sense in my head. What I tried saying is. It is funny to me, that I pulled the cable possibility out of thin air. You hadn't mentioned removing the cables or anything of that nature. Somehow, I knew.

http://www.obd-codes.com/p0131

"P0131 OBD-II Trouble Code

Technical Description

Oxygen O2 Sensor Circuit Low Voltage (Bank 1, Sensor 1)
What does that mean?

Essentially the same as P0136 and P0137, a P0131 code refers to the first oxygen sensor on Bank 1. P0131 means the O2 oxygen sensor's voltage remained low for longer than 2 minutes. This, is interpreted by the ECM as a low voltage condition and sets the MIL. Bank 1 Sensor 1 is located in front of the catalytic converter."

And check to make sure your 02 isn't smashed against the firewall depending on which kind you have.

Alright, P0171 is back after about a day. I have done the following:

Checked all vacuum connections on the engine. All are good.
Replaced the crossover clamps with RTV'd band clamps. Look perfect, all set.
Relatively new MAF has been cleaned with MAF cleaner.
All plug wires are away from o2 sensor wires. All are practically new.
o2 sensor wire is not touching headers or other electrical wires. The wires gently touch the firewall (still inside their insulation).
Battery terminals are cleaned and performing well otherwise.
All fuses in underhood box look good, although I'm not sure which specific fuse would be o2 related based on the manual.
Checked vacuum pressure, held a steady 22" of mercury at idle.
Checked exhaust manifold bolts, plenty tight and no signs of leaking.
Checked EGR connections and hoses, no signs of leaking.

Sometimes, I get a p0131 code instead of the p0171. These two codes never show up at the same time. Input please?

Fault Code Definition

The Power Train Computer or PCM has determined that the Oxygen Sensor voltage remained below 400 millivolts for more than two minutes (varies with vehicle make and model) or that the Air Fuel Ratio Sensor remained in a lean-biased mode for too long (varies with vehicle make and model).

• OBD II P0131 Oxygen Sensor Circuit Low Voltage (Bank 1, Sensor 1)

Common Problems that Trigger the P0131 and P0137 Code

• Defective Oxygen Sensor/Air Fuel Ratio Sensor
• Defective Oxygen Sensor/Air Fuel Ratio Sensor Heater circuit
• High Fuel Pressure
• Defective Engine Coolant Temperature Sensor
• Defective sensor wiring and/or circuit problem
• PCM software needs to be updated
• Defective PCM

There are numerous, complex tests for an Air Fuel Ratio Sensor, but these are the simplest and most time-efficient tests:

• Air Fuel Ratio Senors may have several wires, but there are two key wires. Using a DVOM with the key on and the engine off, disconnect the sensor and probe the harness going to the PCM. Make sure one wire has 3.0 volts and another wire has 3.3 volts. The other wires are the 12-volt power(s) and ground(s) for the heater circuits. In some cases, you may have to start the engine and let it idle to find the proper voltages on all the wires.
• Use jumper wires to connect the sensor to the harness. Connect your DVOM in series with the 3.3 volt wire. Turn your DVOM to the milliamp scale and start the engine, letting it idle. The 3.3 volt wire should cross-count between +/- 10 milliamps. Vary the RPM and as you add and decrease throttle, you should see the signal respond to subtle changes in mixture. If you don't consistently see the +/- 10 milliamp variation in this wire, then the Air Fuel Ratio Sensor is defective.
• If all the above tests and inspections do not produce verifiable results, then physically remove the Air Fuel Ratio Sensor. If the Sensor Probe has a white and chalky appearance, the sensor has been lagging between switching phases and needs to be replaced. It should have the light tannish coloration of a healthy spark plug.