DIY: O2 Sensor Removal & Replacement. 2003 WRX.

Dyjital · 01-26-2010, 09:30 PM

I recently encountered a Check Engine Light (CEL)

Code read: P0031
http://www.obd-codes.com/p0031

Quote:

Oxygen (A/F) Sensor Heater Control Circuit Low (Bank 1 Sensor 1)
What does that mean?

A P0031 DTC (diagnostic trouble code) refers to the O2 sensor (oxygen sensor) located on Bank 1 in front of the catalytic converter. There is also an oxygen sensor behind the converter which is Sensor #2.
This O2 sensor #1 may also be refered to as an air/fuel ratio sensor since on some vehicles it is. It detects the amount of oxygen in the exhaust gas compared to the outside air and then vehicle's computer adjusts the air/fuel ratio going into the engine. The sensor is less effective when the exhaust gas temperature is low, so it includes a heater which is activated to help get better readings from the A/F O2 sensor. Essentially this P0031 code means that the resistance of the heater circuit is lower than normal. In most cases, that resistance level must fall below 0.8 A to trigger the DTC code.
Note, this code is very similar in nature to P0032, P0051, and P0052
OBD-Codes.com on Facebook

Potential Symptoms

Most likely you'll not notice any symptoms other than the MIL (malfunction indicator lamp, a.k.a. the check engine light) will illuminate.
Causes

A P0031 DTC trouble code may be caused by one or more of the following:

A short or open in the heater circuit in the sensor
A failed O2 sensor heater
Wiring/connectors broken/frayed leading to sensor and/or relay
Failed PCM/ECM

Possible Solutions

Need an O2 Sensor? Visit our O2 Sensor shopping area for the lowest price on your replacement sensor!
To fix a P0031 DTC code, you'll need to do a proper diagnosis. To do that, you'll want to inspect the wiring and connectors leading to the sensor. Also if equipped with a heater relay and fuse, you'll want to check those as well. Use a digital volt ohm meter to:

check for 12 volts at the heater circuit feed (hint: unplug the sensor and check at the wiring connector to do this measurement)
check the ground circuit for continuity
measure the resistance of the heater circuit (done on the sensor itself)
measure the resistance and voltage of the wiring

Refer to a service manual for the correct specifications (volts, ohms) for your vehicle. On some Toyota vehicles this code is triggered when the resistance of the heater circuit is below 0.8 A.
With that said, a common fix for this DTC is to replace the air/fuel (O2 oxygen) sensor #1 on bank 1.
Please keep in mind that OEM (original equipment) replacement sensors are recommended (from the dealer). Aftermarket sensors can be less reliable and of poorer quality (not always, but more often). There's also a chance that replacement parts for the P0031 code may also fall under a federal emissions warranty (check with your dealer to see if it's covered).

A couple points in there. I set them as BOLD so you can easily identify them.

In my quest to resolve the issue I came across this information.

Quote:

2)Measure the resistance between front oxygen (AF) sensor connector terminals. Terminals NO. 2 -NO. 1:

Is the resistance less than 10 ohms

Quote:

I'm posting this in hopes that it might save someone hours of troubleshooting like I endured over the past few weeks.

I recently had a P0031 code on my 02' WRX w/ 130,xxx miles. I had replaced the sensor about 40,000 miles ago, so I was surprised to see the CEL on. Naturally, I figured the sensor was bad, so I ordered a new one only to find out that it didn't fix the problem.

Everything checked out until the last step of troubleshooting P0031 in the FSM (Check ohms on terminals 1&2 of sensor connector) and I was getting no reading. There is a Yellow/Red wire that goes from the A/F connector to the main relay (inside the car on the passenger side around the glovebox - the connector/relay is brown). This was the culprit of my problem. Apparently the connector had come loose enough that it wasn't making a solid contact. After re-seating the connector - the CEL went off!

This is probably not the case for most cases of P0031, but it's worth a shot and it's easy to check.

Things I learned in the process:
-According to Denso: you can check the ohms on the two black wires of the front sensor and it should read in the 1-2 ohm range. If it doesn't, the heater is probably bad on the sensor and you need a new sensor.

From Nasioc:
http://forums.nasioc.com/forums/show...ighlight=P0031

Some of that was confusing to me at first, checking ohms of the O2 sensor. Was it good if it was below 10 ohms? Was it bad if it was below 10 ohms?

I went ahead and ordered one from Napa. I was able to get a STEAL on it $135.00. Otherwise I would have went with this:

More Info:
https://www.chaplinssubarugenuineparts.com
sells these O2 sensors for $168.xx bonez.
PN: 22641AA042

I saved $30 + about $9 shipping.

Dyjital · 01-26-2010, 09:30 PM

I have a PDF manual for the 2002/2003 Impreza WRX.

In the manual states this for the P0031 code.

This goes through a rather nice diagnostics.

I decided to skip to the good parts.

I went straight for measuring the ohms of the two black wires on the O2 sensor when it was in the car.

Dyjital · 01-26-2010, 09:31 PM

Since I jumped to the good stuff.

Here is the ohms measurement on the O2 sensor when it was out of the car. (this was post install)
I did check it while it was still in the car. Though I didn't see any point in yanking it just to test.

You can see that nothing registers on the meter.

Dyjital · 01-26-2010, 09:34 PM

On to the install.

After the test I realized it was bad. Ordered one through Napa and a shop that I know.
I was able to get a deal as stated above.

Onward to photos.

Dyjital · 01-26-2010, 09:36 PM

I decided to test the new O2 sensor because I just don't trust myself.

Measure twice, cut once is a good motto. Why replace a part if it isn't broken.

Dyjital · 01-26-2010, 09:37 PM

Since the new one tested out fine, AND it showed different than the original one I was set for the install.

Here's the details.

Time it took: 30 minutes from time I pulled into garage to time I backed out.

Dyjital · 01-26-2010, 09:38 PM

Photos say a thousand words.

Dyjital · 01-26-2010, 09:40 PM

I show that I did not remove tire like some say you should.

I lifted with enough clearance so that I could fit my fat butt under the car. With the car turned all the way to the right I had plenty of room to work in that space.

I didn't see a need to do extra work when it wasn't necessary to do so.

Dyjital · 01-26-2010, 09:41 PM

I added the photos of the old O2 readings again.

Once you have the new O2 sensor in, reverse the instructions and you are good to go.

Time spent: 30 minutes from start to finish. (with taking photos)

Time on tutorial: 60 minutes.

Money spent: $135.00
Money saved vs going to dealership: A LOT

c0mpl3x · 01-27-2010, 04:34 AM

dyj i'm having trouble locating the plug

it has 3 wires right??

narcispy · 01-27-2010, 05:07 AM

just a question, do you drive through lots of mud/dirt on a regular basis?

Dyjital · 01-27-2010, 10:10 AM

Quote:

Originally Posted by c0mpl3x

dyj i'm having trouble locating the plug

it has 3 wires right??

I can't help you then... i'm sorry.
(that image was my attempt at humorous sarcasm)

Quote:

Originally Posted by narcispy

just a question, do you drive through lots of mud/dirt on a regular basis?

Never... why?

c0mpl3x · 01-27-2010, 08:42 PM

Quote:

Originally Posted by Dyjital

I can't help you then... i'm sorry.
(that image was my attempt at humorous sarcasm)

Never... why?

that was my attempt at humorous sarcasm

Dyjital · 01-27-2010, 09:35 PM

I know... gotcha!

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