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02 Nissan Sentra DTC problem

4cyl 1.8l sentra gxe…I have a P0031 code (Ho2s1 bank 1). Replaced the O2 sensor but check engine light came back on. I checked everything that i was able to check with the information and pin points that i had. Everything seems to be fine, has power, ground, fuse is good. Even checked between ECM and O2 for any open circuits, NOTHING. Any ideas? Can it possibly be an ECM, maybe needs to be reprogram? Any help would be very much appreciated. Thanks

Did you replace the correct O2 sensor? There are two on this vehicle.


Yes im pretty sure that i did. There’s actually four O2 sensors, two which are before the cat and two are after the cat. Since i still had the old O2 sensor that i replaced i decided to switch it with the other sensor(sensor 1 bank 2) which is also before the cat. No luck after a decent test drive and there was actually no new codes that came up, meaning the sensor that i replaced might have been good and there could be a different problem.

The engine trouble code NEVER says to change a sensor (nor, other part). Instead, it alerts you to a problem in the CIRCUIT that the sensor (or, other part) is a part of. There is a BIG difference.
Your check engine light came on. You scanned the engine computer for the DTC (Diagnostic Trouble Codes) [often, simply called: codes]. Your scan showed DTC P0031, which is defined as: H2OS Heater Control Circuit Low (Bank 1, Sensor 1). This means that the front oxygen sensor heater element circuit has (too) low voltage.
The ECM compares voltages from sensors (and other parts) to what it knows is the expected voltage values. When it detects a mismatch to those expected values, it turns on the check engine light and sets a DTC (diagnostic trouble code) in its memory.
Low voltage can come from a low battery, or caused by resistance dropping the voltage. Fuse #8 supplies voltage (12+ volts) to the oxygen sensor heaters on the blue/red wire. The black wire provides the heater circuits ground.
Use an accurate digital multimeter to measure the voltage on the blue/red wire, with ignition switch ON. It should be the same as battery voltage (12 plus volts). The resistance on the black wire to ground should be zero ohms.
The green wire is the front oxygen sensor signal wire. It goes to the ECM (Engine Control Module, aka the engine computer) terminal 94 (HO2S1B1). It’s signal (which looks like a seismograph earthquake tracing) is generated by the oxygen sensor, itself.
The gray/blue wire from the ECM terminal 49 (HO2S1HB1) should be the wire that carries the signal to the oxygen sensor to close the “switch” inside the oxygen sensor for the heater circuit. I’m guessing. Check your manual.

Thanks for the reply. Yeah i do know that the trouble code isn’t going to tell me to replace a sensor that’s why i tried to diagnose everything before replacing it. Everything that you mentioned i did, which i mentioned on my first forum. Thanks for the help though, it seems like you really know what your talking about. I was just hoping that this could possibly be a common problem with this vehicle and that someone out there went through the same situation and finally figured out what the problem was. I did see someone else which had the same problem as i, but his car was a 2003, but no one actually wrote back. Thanks again

You said you checked for power, ground, fuse. I DIDN’T say that. I said to check for specific voltages, and ohm values, at specific places, in specific conditions.
You probably need to take it to a repair shop.

I guess i wasn’t specific enough huh? YES i did every single thing that you mentioned! Everything was within specs.

then it’s fixed, thats GREAT,

there seems to be a common problem with codes coming on relating to O2 sensors on nissan vehicles. i was just wondering if anyone could tell me if they had or knew someone with the same problem and actually fixed it. Oh yeah, No its not fixed yet.

you better check with the dealer,and I say this because ,I have replaced O2 sensors,and got burned due to PCM programing issues,the code may say one thing,but the PCM sees it as another ,eg; a programmer wrote the wrong lines of code, and the puter only knows what it is told.
B1/S1 could be B2/S1(due to programming) FlASH my be in order .dont know.

hope that helps

Thanks so much…i was thinking it was gonna possibly be a PCM either having to be replaced or having to be reprogrammed. What you said sounds right, things like that could happen. I guess i never really thought about it that way, but it’s true. Thanks again

I see what mmsamma is saying about the computer misidentifying a circuit. That could complicate things considerably. For now, lets assume the computer is ok.
What does the computer say is wrong? It says the voltage in the heater circuit of sensor 1, bank 1, is low. Resistance makes voltage lower. Excessive resistanc makes voltage too low. This is a simple resistive circuit. Voltage comes from the fuse, through a wire, goes through the oxygen sensor, goes through a wire to the engine computer, and through the engine computer when the engine computer makes a ground path. The computer keeps that ground made for a few seconds for current to flow through the oxygen sensor, heating it. Then, it opens that ground, and current ceases to flow in the heater circuit.

The engine computer wiring terminal numbers, and the wire colors, I listed are from the wiring diagrams I found made available by Point 5 Technologies. Compare my terminal numbers and wire colors with those on your wiring diagrams. They may, or may not, differ.

To check the heater circuit up to the engine computer, turn ignition key OFF, disconnect electrical connector from the ECM, turn the ignition key ON, and check for 12+ volts on the gray/blue wire that goes to ECM terminal 49. You could compare to the bank 2 oxygen sensor heater circuit at red/black wire going to ECM terminal 4. Turn ignition switch OFF before reconnecting electrical connector.
You want to check that the ECM makes a ground for the oxygen sensor heater circuit when the oxygen sensor (and engine) are cold. With the electrical connector connected, put a multimeter back-probe to the ECM oxygen sensor bank 1 heater terminal 49 to read voltage. Turn the ignition key ON, and observe the meter. The voltage should drop towards zero. Compare the voltage to the oxygen sensor bank 2 heater circuit terminal 4 voltage. If the voltage didn’t drop, the ECM didn’t make the necessary ground.

Hey thanks, im gonna go ahead and try out what you recommended.

To backprobe the ECM electrical connector without piercing the wire (which is bad), to read voltages, you need backprobe pins (or, test leads) like these from FLUKE: …Alternatively, you could file/sand your own electrical test meter probe tips down to 0.040" (1 mm).

I have the exact same issue on an 03 Altima. Sensors checked good, voltages, and ohm values checked good, swapped O2 senors and code didn’t follow sensor. Wire continuity checked good and wiggle check didn’t show and discontinuity… hadn’t thought to check voltage at ECU or backprobe ECU… when I did voltage checked good on both bank 1 and bank 2 #1 sensors. Bank 2 sensor pulled low on startup but had 13 plus volts on bank 1 sensor all the time. I am now suspecting a faulty ECU. I believe that federal law requires Manufacturers to warranty vehicle ECU’s for 8yrs/80K … so what do you think my chances would be if I tried to get Nissan to repair under warranty???

I took my Altima to Perry Nissan In Columbia Mo. today and they confirmed the diagnosis of a bad heater driver circuit in the ECU and have ordered one under the federally mandated 8 year 80k mile warranty on major emissions components. They were happy to warrany the issue. The ECU will be in next week…

I’m having the exact same problem with my 2002 1.8L sentra GXE. My code is 0037 which is the bank 1 sensor 2. I had the o2 sensor replaced, but the light keeps coming back on. Did you have any luck with getting this fixed/the light off? Also, do you know if what the other person said about the 80k 8 year warranty is true? Any help…

I do not have an answer for you, but I do own the exact same car, 2001 Nissan Sentra GXE 1.8 and it started on a dark and stormy night. My Check Engine Light came on and I got two different diagnosis. A mechanic said it was my Massive Air Flow (which was still under warranty at the time) and the Nissan dealership said it was my O2 sensor. So, I went with the Nissan dealer and they replaced it and within a week my light was back on. I took it back to the dealer and insisted they replace the Massive Air Flow (under warranty) and they did and refunded ALL my money for the replacing of the O2 sensor. It was fixed; now three years later, my light has come back on again with code 0420. My mechanic checked and replaced the “problem” O2 sensor and within a week my light is back on again. He has checked everything and now says it is the cat which is under warranty for 80,000 miles and I have 87,000 on my car. This may be an issue with this particular car.

And I thought that I wasted my time and effort going into that much detail! I wonder, if your mention of the oxygen heater circuit through the engine computer helped to shorten the diagnostic time for your car, flyguy?

bigpapies, you might get responses if you open a new post. Ask the dealer about the warranty period.

I am having the exact same problem as gosox27. Have you found out what the problem is? I traced it back to the ECU/PCM (same thing) and it getting 12+ volts to it but nothing on the grounding side. I had a mechanic look at it and he told me the same thing I thought a bad ECU so I just replaced the ECU and it’s still throwing the same codes as it was before mine is a p0057 which is bank 2 sensor 2. I think it has something to do with a bad ground somewhere but I’m not sure where to start looking…