O2 sensor heater circuit - fuse location?

[vtraudt]

Getting heater circuit malfunction code. Tried 3 sensors, always same code. Suspect fuse?
If there is a fuse, where is it located?
Car: 1998 Passat 1.8T AEB, 10/98

[john prusinski]

oh man, this happened too me and it turned out to be my ecu that wasnt putting enough volts out to heat my o2 sensor.

i think its fuse 34 but if thats good try this

#1 o2 sensor should be wired like this...

from fuse 34 through the E30 power wire in the engine wiring harness (this wire powers several things) to green/yellow wire to 4 pin connector pin 1
from ecm connector T121 pin 5, red/white wire, to 4 pin connector pin 2
from ecm connector T121 pin 51, brown wire, to 4 pin connector pin 3
from ecm connector T121 pin 70, white wire, to 4 pin connector pin 4

Pin 1 in the 4 pin connector should be full battery voltage (measured against a good frame ground) with the engine running. This is the pin powering the heater in the sensor.

Pin 2 in the 4 pin connector is the other side of the heater circuit. It leads back to the ecm. The ecm grounds this to allow current to flow through the heater and un-grounds (opens) the circuit to stop current flow. During operation, this conductor will switch back and forth between being grounded and not being grounded. This is how the ecm controls the heating of that sensor.

Pins 3 & 4 are the lamda sensor circuit. You can check the two wires from the ecm connector to the 4 pin connector with an ohm meter. You should see no more than 1.5 ohms resistance in either wire.


#2 o2 sensor should be wired like this...

from fuse 34 through the E30 power wire in the engine wiring harness to green/yellow wire to 4 pin connector pin 1
from ecm connector T121 pin 63, white/gray wire, to 4 pin connector pin 2
from ecm connector T121 pin 68, violet wire, to 4 pin connector pin 3
from ecm connector T121 pin 69, red wire, to 4 pin connector pin 4

Pin 1 in the 4 pin connector should again be full battery voltage (measured against a good frame ground) with the engine running. This is the pin powering the heater in the sensor.

Pin 2 in the 4 pin connector is again the other side of the heater circuit. It leads back to the ecm. The ecm grounds this to allow current to flow through the heater and un-grounds (opens) the circuit to stop current flow. During operation, this conductor will switch back and forth between being grounded and not being grounded.

Pins 3 & 4 are the lamda sensor circuit. You can check the two wires from the ecm connector to the 4 pin connector with an ohm meter. You should see no more than 1.5 ohms resistance in either wire.


i copied this from passatworld when steve helped me out with this. i hope this helps.

[vtraudt]

Put another O2 sensor in, same code.
Heater resistance with Voltmeter: 9.5 Ohm.
Logged: resistance in VAGcom showed 20 Ohm or so.
Signal voltage switching nice: 0.7 or so to under 0.1 Volt.

Checked voltage on plug: 14 Volt

Installed new 4 wire I had on hand (Wagner). Resistance across heater: around 5 Ohm.
No code.
Vagcom resistance: 15 Ohm

Probably have to trash all my 'tested good' (torch heating creates higher than 0.7V signal, switching, all in the 9.5 Ohm resistance range of the heater).

==> bench test is NOT conclusive. Either the ECU for some reason 'kicks out' (by showing higher resistance than measured at the sensor itself), or something else is fishy.

[john prusinski]

i cant really remember all of the details but when this happened to me my ecu that was suppose to put out the battery voltage to actually heat the resistor would only put out around 6 volts instead of the normal. i messed with this for months before i actually found the problem. my ecu is doing something fishy.

[vtraudt]

I couldn't find a fuse that made sense.
But I had 14V on the B1S1 connector.
I logged the O2 sensor signal and they were switching voltage fine.
Also showed resistance in VAGcom (but more than what I measured with the voltmeter).

Finally cut up a new Wagner O2 sensor and soldered it onto the brown plug.

==> no code since then.

The wagner had only about 5 Ohm with the multimeter (vs. 9.5 Ohm I had on all the bench tested Bosch O2). In VAGcom, it also showed lower resistance than the Bosch; maybe the ECU has a ohm threshold in the heater circuit, and all my 9.5 ohm bench were just too high?