xeleris
New member
- Joined
- Sep 5, 2025
- Messages
- 1
- Reaction score
- 0
- Points
- 1
- Buick Ownership
- 2003 Buick Century
Hey all, I’m having trouble resolving a P0135 code (heater circuit B1S1) and could use some help from anyone who may have some experience with this.
I’ll start by saying that the PCM on this car was bad so I had to replace it. When I replaced it, a P0134 and P0135 code came up. So I then replaced the sensor with a Bosch one. Didn’t solve my issue, so I replaced with an ACDelco one. P0134 went away, but P0135 remained. Their techs didn’t seem to find any issues with the one I sent in, but they sent me another rebuilt one. This is where I began to do some digging into the sensor and wiring harness.
Checked resistance from PCM pin to o2 ground wire (.4ohms) and checked PCM wires going to the two main ground lugs on transaxle and thoroughly cleaned them just to be sure (they were a tiny bit dirty/corroded). Inspected wire paths, and there are no visible breaks in the harness or wires that I could see. I believe these are the only two points that this PCM grounds to.
Hooked up my test light and probed light to the o2 heater ground wire with light clipped to battery positive to see the grounding behavior. Key out, car off, light is on. I’m assuming because the circuit is being completed through the pink 12V feed wire. Key on, car off, light stays on. When I turn the car on, light turns off almost instantly. Live o2 data on my scanner shows the sensor reading start to fluctuate after about 30-40 seconds. So the sensor seems to not have any trouble with grounding or heating up to operating temperature. The car runs just fine now compared to the other Bosch sensor which didn’t heat at all when cold and took several minutes to heat up and read correctly. And the wire on the harness to the heater ground has low resistance and no breaks. Despite all of this though, the car still continues to throw a P0135.
My PCM rebuild has a lifetime warranty from FlagshipOne, so I sent it back to them and they sent me another one. Plugged it in, same issue appeared.
So I’m thinking it’s one of two things: the sensor itself or the PCM, but I’m not sure and need some expertise to weigh in. From what I can gather online, the resistance of the heater wires on a healthy o2 sensor should be between 2 and 10 ohms. And from a cold start, I gathered that the PCM would expect a 1.5-2A draw. I’ve gone through multiple brands of sensors and tested the few sensors that each of the auto parts stores had in stock and the lowest I could find (and the one installed right now) has a heater wire resistance of 12.4 ohms. Which I believe would mean that the draw is ~1amp cold.
___
So from anyone who might know, would a lower draw like that be enough to throw a P0135 code? There is little info I can find online so I have no clue. I’ve checked 6 sensors at the auto parts stores (ACDelco, Bosch, Delphi), and tested three ACDelco ones that I've purchased, all with resistances between ~12-14 ohms. I will also mention that every one of these sensors was manufactured between 2020 and 2023 (new parts bad? idk). If that is the issue, then I’m not sure where I’m going to find a sensor with a lower heater wire resistance... but surely I'm not the only person who's replaced an upstream sensor on a century and couldn't get rid of the P0135.
Or alternatively, is it more likely that it could be the PCM programming parameters on FlagshipOne’s side are bad? The old PCM didn’t throw P0134 or P0135, but threw a P1639 (bad 5V ref to circuit). I’m assuming that the bad PCM wouldn’t be able to throw codes for other systems like o2 sensors because of that. I put it in the car to see if it would throw the o2 codes after replacing my o2 sensor and it didn’t. But the car also stalled and ran terribly too. If anyone knows what the computer expects to see (amp draw, for example) that would be useful info.
Or am I misinformed and missing something completely? I’m all ears for any other suggestions if so.
Thanks! 🙂
I’ll start by saying that the PCM on this car was bad so I had to replace it. When I replaced it, a P0134 and P0135 code came up. So I then replaced the sensor with a Bosch one. Didn’t solve my issue, so I replaced with an ACDelco one. P0134 went away, but P0135 remained. Their techs didn’t seem to find any issues with the one I sent in, but they sent me another rebuilt one. This is where I began to do some digging into the sensor and wiring harness.
Checked resistance from PCM pin to o2 ground wire (.4ohms) and checked PCM wires going to the two main ground lugs on transaxle and thoroughly cleaned them just to be sure (they were a tiny bit dirty/corroded). Inspected wire paths, and there are no visible breaks in the harness or wires that I could see. I believe these are the only two points that this PCM grounds to.
Hooked up my test light and probed light to the o2 heater ground wire with light clipped to battery positive to see the grounding behavior. Key out, car off, light is on. I’m assuming because the circuit is being completed through the pink 12V feed wire. Key on, car off, light stays on. When I turn the car on, light turns off almost instantly. Live o2 data on my scanner shows the sensor reading start to fluctuate after about 30-40 seconds. So the sensor seems to not have any trouble with grounding or heating up to operating temperature. The car runs just fine now compared to the other Bosch sensor which didn’t heat at all when cold and took several minutes to heat up and read correctly. And the wire on the harness to the heater ground has low resistance and no breaks. Despite all of this though, the car still continues to throw a P0135.
My PCM rebuild has a lifetime warranty from FlagshipOne, so I sent it back to them and they sent me another one. Plugged it in, same issue appeared.
So I’m thinking it’s one of two things: the sensor itself or the PCM, but I’m not sure and need some expertise to weigh in. From what I can gather online, the resistance of the heater wires on a healthy o2 sensor should be between 2 and 10 ohms. And from a cold start, I gathered that the PCM would expect a 1.5-2A draw. I’ve gone through multiple brands of sensors and tested the few sensors that each of the auto parts stores had in stock and the lowest I could find (and the one installed right now) has a heater wire resistance of 12.4 ohms. Which I believe would mean that the draw is ~1amp cold.
___
So from anyone who might know, would a lower draw like that be enough to throw a P0135 code? There is little info I can find online so I have no clue. I’ve checked 6 sensors at the auto parts stores (ACDelco, Bosch, Delphi), and tested three ACDelco ones that I've purchased, all with resistances between ~12-14 ohms. I will also mention that every one of these sensors was manufactured between 2020 and 2023 (new parts bad? idk). If that is the issue, then I’m not sure where I’m going to find a sensor with a lower heater wire resistance... but surely I'm not the only person who's replaced an upstream sensor on a century and couldn't get rid of the P0135.
Or alternatively, is it more likely that it could be the PCM programming parameters on FlagshipOne’s side are bad? The old PCM didn’t throw P0134 or P0135, but threw a P1639 (bad 5V ref to circuit). I’m assuming that the bad PCM wouldn’t be able to throw codes for other systems like o2 sensors because of that. I put it in the car to see if it would throw the o2 codes after replacing my o2 sensor and it didn’t. But the car also stalled and ran terribly too. If anyone knows what the computer expects to see (amp draw, for example) that would be useful info.
Or am I misinformed and missing something completely? I’m all ears for any other suggestions if so.
Thanks! 🙂
