P0031 - Heater Control Circuit Low

[Slashzero]

2003 LC. My CEL light came on today during a short drive. Scanned and it’s showing P0031, Heater Control Circuit Low. I haven’t reset it yet to see if it comes back.

I searched the forums and saw a couple of posts that stated replacing the faulty sensor fixed the issue for most, but...

1. How do I determine which sensor is bad? The error code wasn’t specific.
   
2. My truck occasionally has the exhaust “tick-tick-tick” so I know it has a leak somewhere in the manifold. The ticking noise isn’t that bad at all though, and it’s not always audible. Could a failing sensor mean the leak is getting worse? We don’t smell any exhaust fumes while driving around.
3. Somewhat related to this post... I think I might try this in the next few months:
   Note: This video goes over repairing exhaust manifold leaks/small cracks using Extreme Heat High Temp J-B Weld. That's what I am referring to trying in the next few months. The Youtuber stated he did it about 8 years ago to his truck and hasn't had issues. My exhaust system does not have any huge holes in it.

Thanks!

 

[Slashzero]

To answer my own question (how do I determine which one is bad?), I checked the service manual. Yep, I should have done that first.

P0031 - Oxygen Sensor Heater Control Circuit Low (Bank 1 Sensor 1)​

1. For those who don't have a service manual (and you are just unlucky and can't find an answer using google): You will have to pull each one and use an ohmmeter to measure the resistance between terminal +B and HT.
   Resistance:
   11 - 16 Ω at 20 ° C (68 ° F)
   
   
2. If resistance is not as specified, replace the sensor
   Torque:
   44 N-m (450 kgf-cm, 32 ft-lbf) for sensor 1
   20 N-m (200 kgf-cm, 14 ft-lbf) for sensor 2

According to the service manual I will need to take the Engine Under Cover (front skid plate) off to access this sensor and replace it (if it is faulty). I was hoping I could access it from above. Anyway, I've cleared the faults, and it came back after driving for 15 - 20 minutes, so it is more than likely faulty. I will try to get at it and measure the resistance. I am also going to order a new sensor. $66 for a Bosch (exact fit) sensor... with the $15 off coupon I found... and that's with next day free shipping!

Bosch Exact Fit Oxygen Sensor
Part Number: 13441
Application: Upstream sensor (located before catalytic converter)

 

[jerryb]

check fuse and relay?

 

[Slashzero]

check fuse and relay?

I scanned via OBD-II to get the code once the CEL came on. Do you think this could really be because of a blown fuse or bad relay? If so I will check. Which fuse(s) could be potentially cause this code (P0031)?

 

[jerryb]

couldn't say, make sure it's a clean fuse though. Look in manual or your FSM.

I don't really know if that's it, but I would at least check that first. The light comes with under/over voltage amp in the ecm. I don't know what code would come first though as far as the four sensors, working off one fuse/relay

edit, yours is low.

 

[Spike555]

Your O2 is bad. It has a electric heater in it to speed up it's warm up time, the o2 sensor needs to be warm to work properly.
You only need to remove the middle skid plate.
Replace both sensors at the same time.
Run the singe and get the exhaust hot, this helps you break the rust free.
Make sure you use the proper tools, rent or buy a O2 tool.

 

[Slashzero]

Your O2 is bad. It has a electric heater in it to speed up it's warm up time, the o2 sensor needs to be warm to work properly.
You only need to remove the middle skid plate.
Replace both sensors at the same time.
Run the singe and get the exhaust hot, this helps you break the rust free.
Make sure you use the proper tools, rent or buy a O2 tool.

When you say replace both, you mean both the upstream and downstream on bank 1, right? My bank 1 upstream is bad according to the P0031 code. I suppose it would be prudent to replace both to keep them a "matching" set. I went ahead and also ordered the downstream sensor. I don't think I need to replace all 4 at this point, right?

I ordered an O2 tool from amazon. Hopefully it isn't junk.

 

[Slashzero]

couldn't say, make sure it's a clean fuse though. Look in manual or your FSM.

I don't really know if that's it, but I would at least check that first. The light comes with under/over voltage amp in the ecm. I don't know what code would come first though as far as the four sensors, working off one fuse/relay

edit, yours is low.

OK, that's a good point! I will check in the manual to see if I can narrow it down to a couple of fuses and check those. It would be frustrating if I found a bad fuse at this point, since I already ordered both upstream and downstream O2 replacement sensors.

 

[Spike555]

When you say replace both, you mean both the upstream and downstream on bank 1, right? My bank 1 upstream is bad according to the P0031 code. I suppose it would be prudent to replace both to keep them a "matching" set. I went ahead and also ordered the downstream sensor. I don't think I need to replace all 4 at this point, right?

I ordered an O2 tool from amazon. Hopefully it isn't junk.

No. Both fronts. Replace both fronts. Then when the rears go bad replace both of them at the same time.

The front O2's do all the work, the rears only monitor the catylictic converters and tell you if those are working or not. That is all they do. The fronts monitor engine fueling etc.

 

[Slashzero]

No. Both fronts. Replace both fronts. Then when the rears go bad replace both of them at the same time.

The front O2's do all the work, the rears only monitor the catylictic converters and tell you if those are working or not. That is all they do. The fronts monitor engine fueling etc.

Thanks for the clarification!

 

[Spike555]

You're welcome.

 

[pstevens3307]

Just had the same issue with my '04 (164k mi). Both VSC lights came on as well, which I see from a separate thread probably doesn't actually indicate a problem with the VSC. I'm going to hope for the best, anyway, and assume it's just the O2 sensor.

Question: How urgent is this? I rely on my truck as my DD and am going to be using it to move over the next few days (I know, perfect timing). Is it likely I'll fry something if I run it for a few hundred miles before fixing it?

 

[hortandr]

Just had the same issue with my '04 (164k mi). Both VSC lights came on as well, which I see from a separate thread probably doesn't actually indicate a problem with the VSC. I'm going to hope for the best, anyway, and assume it's just the O2 sensor.

Question: How urgent is this? I rely on my truck as my DD and am going to be using it to move over the next few days (I know, perfect timing). Is it likely I'll fry something if I run it for a few hundred miles before fixing it?

I am curious what ended up happening. I just had this happen with my lx and I was not planning on replacing o2 sensor any time soon. Is it ok to run in that condition. I will probably run fuel heavy but I don't think it will hurt anything else.

 

[hortandr]

2003 LC. My CEL light came on today during a short drive. Scanned and it’s showing P0031, Heater Control Circuit Low. I haven’t reset it yet to see if it comes back.

I searched the forums and saw a couple of posts that stated replacing the faulty sensor fixed the issue for most, but...

1. How do I determine which sensor is bad? The error code wasn’t specific. View attachment 1759135
2. My truck occasionally has the exhaust “tick-tick-tick” so I know it has a leak somewhere in the manifold. The ticking noise isn’t that bad at all though, and it’s not always audible. Could a failing sensor mean the leak is getting worse? We don’t smell any exhaust fumes while driving around.
3. Somewhat related to this post... I think I might try this in the next few months:
   Note: This video goes over repairing exhaust manifold leaks/small cracks using Extreme Heat High Temp J-B Weld. That's what I am referring to trying in the next few months. The Youtuber stated he did it about 8 years ago to his truck and hasn't had issues. My exhaust system does not have any huge holes in it.

Thanks!

Hey Partner. Did replacing those front Sensors fix your issue?

 

[Slashzero]

Hey Partner. Did replacing those front Sensors fix your issue?

Yup.

 

[Mattiboy]

I know this is an old thread but I am experiencing the same issue on my 2006 GX470, and figured maybe this will still be monitored by some of the posters in this great community. I am happy to start a new thread in The GX section but figured maybe my issue would add to the discussion given that the engine and sensors are (mostly) the same I believe.

I too got a P0031 Code (HO2S Heater Control Circuit Low Bank1 Sensor1)

From the research I did this seems to indicate a faulty heater element in the O2 sensor. By the official GX470 service manual the test is just like the one for the LC, check resistance across the two same colored wires (black) as these are the heating wires (the other two are the data transmitting wires). Acceptable range is 11-16 Ohms.

I pulled the Driver side (Bank1) O2 Sensor upstream of the CAT (Sensor1) and when I took an Ohm reading it was 2.9. Ok so bad O2 sensor, well below the proper operating range. I then purchased a new OEM Denso O2 sensor, same part number, and when I tested its resistance, I got the same 2.9. So, I disconnected the harness on the passenger side O2 Sensor (Bank2 Sensor1) and again got 2.9. I tried this test with two different multimeters.

I am throughly confused why I got these results. Any advice would be helpful

Thanks

 

[hortandr]

I know this is an old thread but I am experiencing the same issue on my 2006 GX470, and figured maybe this will still be monitored by some of the posters in this great community. I am happy to start a new thread in The GX section but figured maybe my issue would add to the discussion given that the engine and sensors are (mostly) the same I believe.

I too got a P0031 Code (HO2S Heater Control Circuit Low Bank1 Sensor1)

From the research I did this seems to indicate a faulty heater element in the O2 sensor. By the official GX470 service manual the test is just like the one for the LC, check resistance across the two same colored wires (black) as these are the heating wires (the other two are the data transmitting wires). Acceptable range is 11-16 Ohms.

I pulled the Driver side (Bank1) O2 Sensor upstream of the CAT (Sensor1) and when I took an Ohm reading it was 2.9. Ok so bad O2 sensor, well below the proper operating range. I then purchased a new OEM Denso O2 sensor, same part number, and when I tested its resistance, I got the same 2.9. So, I disconnected the harness on the passenger side O2 Sensor (Bank2 Sensor1) and again got 2.9. I tried this test with two different multimeters.

I am thoroughly confused why I got these results. Any advice would be helpful

Thanks

Follow the FSM procedure. First step is test across A/F sensor connector. 2.9 OHMs is right in the acceptable range (1.8-3.4 OHMS @20C/68F) across HT (1) - +B (2). Across HT(1) - AF (4) you should read 10 kOHMS or Higher. Also doesn't hurt to test voltage to your vehicle side connector with Ignition on while down there.

If not that now time to test relay. I would argue the FSM is incorrect for how to properly test the Relay as across the 3-5 pin should always be open unless the connector is on 12V power.

If not that then welcome to the club because i have P0031 and P0051 at the same time and not its time to test across ECU/ECM behind glove box.
this test is also inaccurate on the FSM as you would need an oscilloscope to test across this as these are signal wires and generate a pulse not constant voltage.
I am attaching ECU Pin Layout and Test procedures for ECU testing. You will need Back probe pins to test the ECU connectors.

Hopefully you do not end up where I am by chasing down wiring in the harness.

 

[Mattiboy]

Thank you for this hortandr. This will be great for future reference. I think I was getting hung up on the resistance numbers. My failed O2 sensor (Bank 1) was only reading 2.9 ohms so when the new sensor was also only reading 2.9 ohms, I assumed I was doing something wrong. When I tested the the Bank 2 sensor (passenger side), it also tested out at 2.9 ohms. When I ran the engine with the Bank 2 sensor disconnected from the wiring harness the car threw code P0051 as wellI. This led me to believe that the Bank 2 sensor with a 2.9 ohm reading was functioning properly. I figured that I should just install the new O2 sensor on Bank1, and see if the code would return, So far after 3 shortish dives (5+ miles) no codes are being registered.