2002 ford focus 1.6L zetec se engine - oxygen sensor malfunctioning

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Fri 31 Oct 2014 08:25
2002 ford focus 1.6L zetec se engine - oxygen sensor malfunctioning - wellafella

Went with the car for vehicle diagnostics and the following codes PO136, PO141 and PO135 were detected. they indicate that there is malfunctioning of the oxygen sensor or sensors. May you please assist 1. am i supposed to change two or just one sensor 2. where on the engine do i locate these sensors.

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Fri 31 Oct 2014 10:21
2002 ford focus 1.6L zetec se engine - oxygen sensor malfunctioning - elekie&a/c doctor
A diagnostic test may have detected a problem with the oxygen sensors,but it does not necessarily mean they are faulty.It would be a good idea to get the air intake and breather system checked for leaks first.Common problem is the rubber flexi hose split under inlet manifold.
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Fri 31 Oct 2014 11:45
2002 ford focus 1.6L zetec se engine - oxygen sensor malfunctioning - Railroad.
The sensors themselves are actually fairly unlikely to be the cause of the problem. The ECU cannot directly tell if there's a mechanical fault which is causing inefficient combustion, it only knows what the oxygen sensors tell it. The sensors could be faulty or they could be telling the truth. The ECU can't tell the difference. One way to check to see if the sensor is faulty is to measure it's output voltage range which should be switching between 0.1v and 0.9v. Then gently crimp the fuel return pipe to force the engine to run rich. You should see a momentary rise in the output voltage. If you do it's probably safe to say the sensor is working normally and that your fault is something mechanical such as that listed in the above post.

Edited by Railroad. on 31/10/2014 at 11:46

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Tue 4 Nov 2014 08:30
2002 ford focus 1.6L zetec se engine - oxygen sensor malfunctioning - wellafella

thanks a lot for the feedback. any idea where i locate these sensors (that is if the sensors are faulty)?

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Tue 4 Nov 2014 08:44
2002 ford focus 1.6L zetec se engine - oxygen sensor malfunctioning - elekie&a/c doctor

The sensors are located in the exhaust system.Unless you have the correct tools to remove them,as they can be a a pain to get out ,I would leave the job to somebody that is familiar with the system.

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Tue 4 Nov 2014 09:47
2002 ford focus 1.6L zetec se engine - oxygen sensor malfunctioning - Railroad.
P0135 and P0141 refer to the sensor heater circuits on B1S1 and B1S2 respectively. Although of course it's possible it does seem a bit coincidental that both sensors have failed at the same time. Both O2 sensors are heated when they're cold because the sensors cannot function properly until they're warmed up. The heaters are switched on by the ECM. Locate the connector on both sensors and disconnect them. On the sensor side of the connector use a ohmmeter to check for resistance on the two white wires. If you have a few ohms resistance then then the sensors are ok. Check the fuses and wiring back to the ECM. If both white wires on one or both of the sensors show to be open circuit then the heater coils have failed, in which case replace the sensor/s. Make sure you do this check before you buy new sensors otherwise you're wasting your money. Often new sensors are supplied with a sachet of high temperature anti-seize paste. Make sure you apply it to the threads of the new sensor before fitting. This will make it much easier to remove it next time.
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Mon 10 Nov 2014 07:10
2002 ford focus 1.6L zetec se engine - oxygen sensor malfunctioning - wellafella

I realised the rear sensor connector (blue connector) is faulty. in the meanitime is it possible/advisable to bypass the connectors and connect the wires directly.? My problem though is with the two white wires (heaters) , How do i know which is which?

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Mon 10 Nov 2014 08:06
2002 ford focus 1.6L zetec se engine - oxygen sensor malfunctioning - elekie&a/c doctor

How is the connector faulty?You can by pass the connector,but it will make any future replacement awkward.The 2 white wires can be connected either way round.

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Mon 10 Nov 2014 08:59
2002 ford focus 1.6L zetec se engine - oxygen sensor malfunctioning - Railroad.
The two white wires are the heater circuit. The black wire is the voltage return to the ECM, and the grey wire is earth.
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Mon 10 Nov 2014 09:37
2002 ford focus 1.6L zetec se engine - oxygen sensor malfunctioning - wellafella

The connector on the sensor side is missing actually. Its as if it was cut but I am hoping fixing that will solve my engine check light

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Tue 11 Nov 2014 13:50
2002 ford focus 1.6L zetec se engine - oxygen sensor malfunctioning - wellafella

will it cause any harm in replacing the rear sensor with the front sensor. i have a spare front sensor?

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Tue 11 Nov 2014 17:25
2002 ford focus 1.6L zetec se engine - oxygen sensor malfunctioning - elekie&a/c doctor

Both sensors are the same,so yes,they can be swapped.Just make sure the cable length is adequate.

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