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Mondeo Fault P0135 O2 Sensor Heater Circuit Malfunction (Bank 1 Sensor 1)


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Good afternoon all,

I am a newbie looking for some advice on Mondeo 2.0L petrol; year 2001 (Y registration)

The MIL light came on; garage diagnosed "lambda sensor". I spotted the post cat one and replaced it with a universal (4 lead) one, then disconnected battery but after 2 engine run cycles the MIL light came on. Found the lamba sensor on the exhaust manifold and replaced that, repeated baterry disconnect but after 2 engine run cycles the MIL light came on again.

I then bought a DTC code reader; this displays "P0135 O2 Sensor Heater Circuit Malfunction (Bank 1 Sensor 1)"

Is this the exhaust manifold sensor? I know that the two white leads are sensor heating. Has anyone had a similar experience and what was the solution? Does anyone know how the heater is switched; I imagine it must be controlled by the ECU but is there a seperate timed relay involved or is all control within the ECU. Of course a broken wire or dodgy connector block are also candidates. Any guidance appreciated.


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It could be the fact that you are using a universal sensor. I have found to my cost in the past that pattern made parts do not always communicate with the EMU correctly or are not set up correctly for that particular car.

Ive just had a problem with a number of coil packs on my wifes SportKA. 4th time lucky with a genuine Ford Part and its OK now.

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  • 8 years later...

Hi All,

I have an oxygen sensor fault PO135 with my Mk4 mondeo 2.0 Tdci. Ive replaced the sensor (which when tested with a multimeter had no continuity in the heater circuit) with a new one and the fault keeps coming back when cleared. There dosent seem to be a 12V supply in any of the pins on the plug when the ignition is switched on. Anyone any ideas?

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Sent from my SM-G965F using Tapatalk

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