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Front oxygen sensor replacement

6130 Views 21 Replies 3 Participants Last post by  billymade
I put the hood up and before it cooled enough to touch anything it got nice and dark out of course. I figured I would at least be able to see the oxygen sensor, before it got dark out. I cant even see it or its wires.
Then I went to go get my down pipe and converter from a 2003 bug. It was in the back yard last time I saw it, figured I could use that as a cheater to get a real good idea where the sensor was how it was pointed and so on. It was gone, some crack head must have jumped my fence and stole it. If I remember correctly the sensor was pretty close to the turbo.

Whats the best way to get at that sensor?
Do I need to pull the intake?

I am replacing the sensor because I have been getting a lean code and I was able to find a brand new 4 wire oxygen sensor at the junk yard and it was free. So I would like to try and use the free sensor first. If I remember correctly the 2001 had a 4 wire.
I think the next step is to start replacing fuel system related vacuum lines.
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Please post any and all trouble codes. :cool: what are the fuel trim levels?
I got a crank position sensor code once about 6 months ago, I cleared it and it has not been back. This latest code is bank 1 lean.
Its P0171. It started real out throwing that code once every few months to every few weeks, I would clear it, then it started coming back every few days.
Fuel mileage hasn't really changed and it doesn't seem to be running any different.
I cant get fuel trim levels with my little code reader. I would have to transplant my 5 wire oxygen sensor/meter from my Camaro I use to tune my Camaros carb to the bug.
16555/P0171/000369 - Ross-Tech Wiki

Before you start throwing parts at the problem: read the above link and start troubleshooting the list of possibilities; step by step, until you find the problem. Keep us posted; as your troubleshooting and repairs progress.
I was only going to throw the free new oxygen sensor on there, but the car has a 5 wire sensor, my free sensor is a 4 wire so that wont work.
Looking around yesterday evening waiting for it to cool off I did fix an air intake leak but the leak was forward of the air filter so I don't really think that could be it.
It looked like split plastic wire conduit that went to the air cleaner and didn't have a split or any wires in it. Just went to the bottom of the air cleaner from the large black thing under the intake manifold. What ever that black thing is, it seems to function on startup for a minute or so and what ever its doing makes the intake noise a little louder.

And the mass air flow has been messed with before, there are tool marks all over the clamp, like someone tired to use pliers that were too small to pinch the 2 tabs together to remove it. I know it wasn't me.

The bolt that holds the turbo inlet snorkel to the turbo is gone but it seems like the snorkel is firmly seated against the turbo and the bolt holes still look like they are lined up.
Now I have a P0300, 301 and 302 after taking it apart looking for a loose connection.
16684/P0300/000768 - Ross-Tech Wiki

16685/P0301/000769 - Ross-Tech Wiki

16686/P0302/000770 - Ross-Tech Wiki

Well, based on the above; you are getting random/multiple misfires and misfires in cylinder(s) #2 and #1.
The large "black thing"; is the secondary air injection pump, this turns on for a little while on initial startup; if there are any leaky or broken hoses, fix them (cracked hoses; can be taped up with electrical tape).

If there are ANY boost related hose leaks; you definitely, want to fix those, for sure.

For the 16684/P0300/000768 - Random/Multiple Cylinder Misfire Detected

Possible Causes:

#1 Possibility listed is: Air Intake System leaking

So, check and fix all leaking and unsecured boost related hoses.
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If the boost was leaking out of the piping between the turbo and the throttle body wouldn't that cause a rich condition?

All the 300 codes went away after I drove it, turn it off and restarted. I think when I hit the MAF with compressed air it may have knocked some dust or something off it.
Can I pull the MAF apart and hit it with MAF cleaner or electronics cleaner with out causing any problems?

I cant find any inward air leaks between the MAF and turbo inlet.

The crank case vent is between the MAF and turbo inlet, is there any possibility a broken dip stick tube or cracked rubber crank case vent line would allow more air to be drawn through the engine and into the intake between the MAF and turbo?
I have cleaned my maf with crc maf cleaner; with good results, undo the screws and remove the maf insert to clean (requires a torx plus 5 point security bit; got mine @ PepBoys CTA Tools Part # 9635 (5 Point Tamper Proof TORX PLUS 1/4 socket/bit: T25 size) for $5, it is Taiwanese but seems to be decent quality, not Snap On quality but for $5, what do you expect? This is a low torque situation; so, I think it will be ok for this situation. Some people; just use vise grips and then replace the screws with sheet metal screws and normal hex or phillips heads, that way you don't need a special tool.

Or just clean it; without removing the insert and just spray inside the tube, I remove mine (I figured it would be better to do it that way) but I'm kinda overkill! :)

Lots of diy's/videos for the 1.8T maf cleaning;

https://www.google.com/webhp?source...=1&espv=2&ie=UTF-8#q=vw 1.8T maf cleaning diy


This is a nice little maf testing article:

Part 1 -MAF Sensor Test: Jetta, New Beetle (5 wire type).

for the questions about rich/lean conditions: read this fuel trim info:

(reading/knowing trim levels, can help you figure out what is wrong: leaks or maf)


If you are new to the 1.8T; here is a great "faq" resource:


I would really recommend; you get a basic vw scan tool/code reader like a vgate vs450 on amazon.com or go all the way, with factory level scan tool vcds from ross tech.
I have the T25 security bit. I will try some MAF cleaner.
My replacement dip stick tube will be delivered tomorrow and I will tape up the crack in the crank case vent line.
Lean codes are about 90% of the time intake and/or crankcase air leaks.

OBDII cars RARELY run rich and if they do give a rich indicator, this is usually due to bad data from a crappy/counterfeit sensor.

O2 sensors rarely trigger DCT's/SES/CEL/MIL unless the heater fails. I have been finding all sorts of bad O2 sensors and non of them ever trigger DTC's.

Your misfires are probably Lean misfires at idle.

Get the Fuel Trims at warm idle and steady 50-65 MPH cruise, as verify the engine cooling temp, should be 205F at idle and not below 200F while cruise.

See graphs of O2 sensors, not from a I4 VW but an I6 BMW.

Graph of engine temp is from my 2003 1.8t NBC


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I replaced the cracked dip stick tube. Then found a tear in the rubber PCV y tube and cleaned it up and sloped RTV.
When I started it up there was a change to how it ran. I drove and the engine was running real rough flashing the CEL and giving a p0302 code. Which went away. So there was air being drawn through those 2 openings for sure.
But then the P0171 code came back a little while later.

Looks like you were having a issue specifically; with cylinder number two. If you continue to have a rough running engine and the flashing light;
try swapping the coil pack, injector, spark plug, etc to another cylinder and see if the code follows the changes made; then, you know that component is the culprit. Replace the offending part.


Again I would start looking at the list of possibilities in the above link and start a process of elimination, until you hopefully find the problem.

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The P0302 code only happens when I change something on the engine, like seal up an inward air leak. Other wise I never see it and it goes away completely after I restart the engine.
I changed the plugs last year about this time, because they likely had almost 70,000 miles on them.

It looks like I am running out of air leaks to seal up.
You have a whole list of possibilities listed below: you have your work cut out for you, start testing and checking off each item as you go.

Contents [hide]
1 16555/P0171/000369 - Fuel Trim; Bank 1: System Too Lean
1.1 Possible Causes
1.2 Possible Solutions
1.3 Special Notes
16555/P0171/000369 - Fuel Trim; Bank 1: System Too Lean

Possible Causes
Mass Air Flow Sensor (G70) defective
Intake System Leak(s)
Possible Solutions
Check Mass Air Flow Sensor (G70)
Check Fuel Pressure Regulation
Check Fuel Pump
Check Injectors
Check Intake System for Leaks
Check Exhaust System for Leaks
Check Secondary Air System for Leaks
Check Vacuum Pipes for Leaks
Special Notes
Fuel Trim Info
Category: Fault Codes

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I found out what I had after I dragged it to the dealer ship by trailer.
I had the bad O2 sensor code for both sensors, lean code, misfire on cylinder 1 2 3 4 (a code for each cylinder), miss fire on all cylinders.
Maybe a few more. The car would barely move.

The dealer recommended a new MAF, coolant temperature sensor and bypass valve and rear O2.
The bypass valve was bad, if you can blow air through the vacuum line through the valve, its bad.
I replaced that MAF, that cured most of the problems instantly.
I replaced the coolant temperature sensor with one form napa, the new one from napa was bad, I got one from the dealer, that helped a little more.
Then I replaced the bypass valve and that made the car even more peppy.

Fuel economy and power were improved over it running ok before all this happened. Fuel economy has been up to 25 to 27mpg and I do not drive it slow, I am in texas, the speed limit is 75 and there are huge expanses of ranch land between 2 horse towns.
Before I was getting between 22 and 25 mpg.
If I did not drive it like I stole it I am sure it could get 30.
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Glad you got it handled; what did the dealer charge you to diagnose the car? Did you then; do all the repairs yourself?

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