Code P0135 will not go away

Tiny
SICKPETE
  • MEMBER
  • 2003 HYUNDAI SONATA
  • 2.4L
  • 4 CYL
  • 2WD
  • AUTOMATIC
  • 196,044 MILES
I recently took my car into a mechanic to get the emissions diagnosed and p0135 was one of the codes that came up. I asked the mechanic if this was something that I could fix and was told that the upstream and downstream oxygen sensors needed to be changed. I was able to change the downstream sensor and get rid of that code but after changing the upstream one I drove the car to reset everything, and the check engine light stayed off the whole seventy-five miles that I drove. I stopped the car and turned it off then about an hour later I started the car up and the check engine light kicked on. I tested the car with an odb2 and got p0135. I inspected the wiring harness that the sensor connects into and didn't see anything damaged or broken. I returned the sensor back to the store I got it from because when I took it out of the box there was grease on the tip, so they gave me a new one and I installed it. Same thing happened after I drove it for seventy-five miles, turned the car off and then when I went to drive again the check engine light came on, so I tested the car again with the odb2 and the code is p0135. I used a multimeter and got a good voltage reading on the connector going into the car I then tested the sensor and got a good ohm reading. I have checked the fuses to the ECU and none of them are bad. The sensor I replaced the old one with is a Bosch Exact Fit Oxygen Sensor 13753. I noticed when I pulled the old one out that the tip was way different, and the wire was a little bit longer. The Bosch also doesn't reach to clip on the little metal piece the old one clipped onto so the wires aren't just free hanging. Any advice would help I don't want to have to bring it back to the mechanic but will if that's my only option.
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Wednesday, June 15th, 2022 AT 10:08 PM

3 Replies

Tiny
STEVE W.
  • EXPERT
That is a code for the upstream heater on the O2 sensor. Usually indicates either a short or excessive resistance in the heater circuit. I would look over the harness real close for an area where it is rubbed through or damaged. It's possible it's an intermittent issue from a short. The heater wires are the two pink ones. One runs to the fuse for constant power. The other goes to the PCM 1 Connector on pin 3.
What the system does is monitor the battery voltage, then it compares that voltage to the voltage it sees from the O2 heater circuit. It knows the resistance of the heater in the sensor and the voltage drop it should cause. It sets the code if that voltage goes lower than expected through the higher resistance or a short.
To find the problem will take some testing. I would start the process with the meter. Go to the sensor and test the voltage feed to it from the fuse, make sure you have battery voltage.
Now go to the PCM and find pin 3 and the Pink wire. Use a T pin and back probe that connection. Connect the meter and look at the voltage returning from the sensor. Now go and wiggle and move the harness gently, look for the meter voltage to change or drop. Watch the harness as you do that and see if you can find the area that is causing the problem.
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Thursday, June 16th, 2022 AT 12:41 AM
Tiny
SICKPETE
  • MEMBER
I already did this testing.
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Thursday, June 23rd, 2022 AT 12:44 PM
Tiny
STEVE W.
  • EXPERT
If you didn't find a problem, then it's likely a bad PCM. It happens but it's rare, the common problems are in the harness.
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Thursday, June 23rd, 2022 AT 3:18 PM

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