O2 Heater Fuse Blows

[greasemanicure]

After chasing a cylinder 4 misfire (new plug was bad?) our 96 Ex 4.0 OHV threw four new codes P0135, P0141, P0155 and P0153. The first three relate to the heater circuit for the three O2 sensors. Found a 15A fuse blown. Circuit calls that fuse 11 to power the heaters. Sounds like I have a heater shorted or a wiring problem to trace. I can see how a heater might fail open. Do they also fail short?

Thanks!

Joe

 

[2000StreetRod]

Check fuse 24

P0135 02 Sensor Heater Circuit Malfunction (Bank 1 Sensor 1)
P0141 02 Sensor Heater Circuit Malfunction (Bank 1 Sensor 2)
P0153 02 Sensor Circuit Slow Response (Bank 2 Sensor 1)
P0155 02 Sensor Heater Circuit Malfunction (Bank 2 Sensor 1)

The PCM has a connection for each sensor heater circuit to monitor the heater operation. However, all three sensors have a common supply according to the 1996 V6 wiring diagram (20 amp fuse 24 in the interior fuse panel).

According to the wiring diagram interior fuse panel fuse 11 is a 7.5 amp fuse that powers the Anti-Lock Brake indicator in the instrument cluster.

I suggest that you check fuse 24.

 

[greasemanicure]

The 15 amp fuse that blew is in the panel under the hood. I was confused by the diagram that I have that said that the fuse for the heater is #11. It is 7.5A as you said. The fuse in slot 24 is a 10A and is good. It is time to crawl under for a look-see. I can check for battery at the O2 heater terminals and verify that the fuse under the hood that is blowing is wired to them, or not. Why don't these things crop up during daylight saving times? It's dark by the time I get home from work.
Thanks!

Joe

 

[greasemanicure]

Quick Fix

I rolled the Ex up on the ramps and checked the wiring to the O2 sensors. The wires to the right bank front sensor were melted to the exhaust pipe. One tie wrap later the wires were secured away from the pipe and the fuse did not blow. It must have been leaning on the pipe for some time as when I cleared the grounding of the wires, not only did the codes related to the fuse blowing clear but the ones related to catalyst efficiency went away and have stayed away. It may be that the sense leads had a short to ground for some time before the heater circuit grounded out. When things dry out I may tidy things up with some added insulation, but I don't see a reason to replace the O2 sensor given that it is working.
Joe

 

[2000StreetRod]

good news!

I'm glad that you found the source of your problem. Thanks for the update!