Hey folks so we've got a 96 1500 with the 318 at 184k miles and it's been throwing codes for the Downstream (rear) o2 sensor, only two sensors on this truck the Upstream and Downstream. I've got two different codes stating the same thing; failure in the rear O2 heater circuit. It's been hard to start, runs kinda choppy until it's warmed up and been consuming more gas than usual but I would think the rear O2 shouldn't have any affect on that and should only read the exhaust flow after the cat.
I found the PCM wiring diagram and everything adds up except that there are no wires going into the pins for the Upstream sensor A24 and A26 . A25 however does have the Orange/Black wire which I have verified is the correct color for one of the four wires going into the plug/harness underneath the truck for the Downstream O2.
I haven't followed the wires all the way up because they go into a thicker loom that is running along the top of the trans and is quite difficult to get to. And I was thinking of just running new wires and loom from the PCM down to the sensor to avoid whatever brittle mess i might be getting into with the loom.
Both sensors appear fairly new and the only numbers I've found on the sensors don't bring up anything in Google so I have no idea what brand they are.
So my questions are how should I be testing this heater circuit? Is there a fuse or relay for this that could be bad? And also shouldn't there be a Tan/White wire in pin A24 going from the PCM to the pre-cat sensor?
My mom bought the truck off a friend who got it from a used car dealership about 7 months ago. I'm thinking maybe they jerry rigged it or something.
Kinda tough to say much without looking at the wiring diagram along with you. I don't have one...so.
But, the ASD relay powers the PCM and the O2 heater circuits. So a short in the heater circuit can affect performance.
Also you are correct in thinking the downstream sensor doesn't affect performance. Many guys think the do, but only the upstream is used for stoich feedback.
What code exactly is it setting? Also code description is important.
O2 sensor problem is too generic to evolve a Diag routine from.
The verbal description of the code will help a lot.
So there's three common wires coming out of the two sensors, Black with Blue which seems to be the ground, Black and white and then Green and Orange. I'm assuming the last two are the heater wires?
Can I run a resistor for these to make the ECU think it's working properly?
I wouldn't run a resistor ,but you can use a test light on the o2 heater+ and the ground to verify the PCM is powering the circuit.
After connecting start the motor and see if it is turning the light on.
The sensor is probably bad, but the test light will verify the PCM power up.
I have had lots of Bosch sensors that were bad recently.
I have switched to denso. They are more money, but they work.
You can also OHM the heater circuit of the sensor. BUT be absolutely sure you have the correct 2 wires.
I cant tell much from the wire color list you gave. No where does it list the heater circuit ground color for the O2 sensor. I would assume that the black/Bl wires are the grounds. Use an Ohmeter to verify that before you do the test light procedure.
Some carmakers power the heater when the ignition is on, and the ground side is PCM controlled. IIRC on a dodge the ground is always applied and the PCM controls the power side of the heater circuit.
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