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Fiat Sedici o2 sensor

posted Apr 22, 2016 11:45:48 by barry
Sorry if this is something basic i am missing, but I'm new trying to learn from here.

My daughter bought a 2006 Fiat Sedici petrol 1.6. Was working fine however the engine management light came on with what i remember was a no. 2 o2 sensor fault. I checked the o2 sensors and with the scan tool it appeared the post cat o2 sensor wasn't moving.

When i physically looked, the O2 sensor was removed from the exhaust and actually cable tied up beside the connector (covered in black tape), and a bolt in the sensor hole.

I fitted a new O2 sensor and everything appeared to be working as normal, but the engine management light came on again with a P0420 code.

I have looked at the videos and tried to follow everything through, and it does maybe appear that the CAT needs replacing, however the car will run fine and the engine management light will stay off for hundreds of miles, then can come on suddenly, not dependent on amount of driving etc.

The main thing that is puzzling me is that if i connect the scanner to the car with the ignition on and with or without the o2 sensor connected i get a 1.37v reading on the scanner. O2 sensor 1 shows 0.00v, i tried the bias testing with a test light as per videos and nothing changes, it stays at 1.37v. With the engine running the PID follows closely pre cat o2 sensor, and has quote a bit of movement.

Before i get a replacement CAT , i want to be sure this is correct.
(it has 4 wires , 2 giving a voltage with only ignition on and sensor unplugged, 12v (battery voltage)and other 1.37v. With the sensor connected it still shows 1.37v

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2 replies
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AndyMacFadyen said Apr 22, 2016 16:52:45
P0420 is a code flagged by test that checks if the catalyst is working. A cat may not work because of an engine or exhaust fault but if when fully hot the upstream sensor is oscillating normaly and the downstream sensor (B2S2) is closely following the upstream (B1S1)then it points to the cat.


"Rust never sleeps"
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Tyler said Apr 23, 2016 15:27:04
The main thing that is puzzling me is that if i connect the scanner to the car with the ignition on and with or without the o2 sensor connected i get a 1.37v reading on the scanner. O2 sensor 1 shows 0.00v, i tried the bias testing with a test light as per videos and nothing changes, it stays at 1.37v. With the engine running the PID follows closely pre cat o2 sensor, and has quote a bit of movement.

(it has 4 wires , 2 giving a voltage with only ignition on and sensor unplugged, 12v (battery voltage)and other 1.37v. With the sensor connected it still shows 1.37v


Very odd that the bias voltage doesn't respond correctly on the scanner, I think you were right to expect it to! I'm wondering what would happen if you had your multimeter on the wire with 1.37V, and used the test light to pull it to ground.

This Fiat bias voltage is a new one to me, especially with it's weird behavior. Can't say I've seen similar behavior on anything we get over here.

Obviously, the car is happy enough with the O2 sensor performance, otherwise it'd flag O2 codes before a catalyst code. As much as it sucks, a P0420 code is almost always correct (in my experience, anyway).

Check for exhaust leaks, verify good fuel trims, make sure the engine isn't burning oil/coolant or misfiring, then it's catalyst time :-(
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