My 1997 Ford Ranger 2.3L 4x2 has 12 Volts (light blue/Orange) and good grounds through PCM to both O2 sensors and each sensor shows 4 ohms for the heater. Wires appear unscathed.

It then throws P0135 & P0141 simultaneously while driving to clear the monitors for test. Usually fine for 3-12 miles (slightly rough idle), then boom.. both codes. I clear the codes, check all the above again. then hit the road. Same thing. Over and over.

So we have two new O2 sensors, 4.n ohms for each heater, and confirmed 12v through fuse 20, confirmed PCM switched ground at both sensors. I clear the codes, drive 3-13 to clear readiness monitors.. and within a mile of the Smog Check station.. it then throws P0135 & P0141 simultaneously

What would you check next?

Thanks,

-Michael

P.S. If it matters, this is a manual five speed with about 150K miles

 

Welcome to the forum

Good testing
P0135 02 Sensor Heater Circuit Malfunction (Bank 1 Sensor 1)
P0141 02 Sensor Heater Circuit Malfunction (Bank 1 Sensor 2)

I answered in another forum but reposted here

In 1997 2.3l Ranger Fuse 20 in engine bay, 15amp, gets 12v with key on, from PCM relay, red wire
That's the O2 heater 12v power
Heater is just a coil of wire, so the 12v travels thru the coil and to the computer's ground wire for each O2 heater
If computer doesn't see the 12v with key on it will set a code for that one O2 sensor

So you have the 12v with key on or codes would be set almost instantly

Computer then randomly ungrounds each wire to test if 12v is still present, if not it will set a heater code

Only after a few minutes do you get the codes...........and you get the codes for BOTH O2s on your 4cyl engine

To me that would mean the 12v side is the issue, if it was an O2 ground wire to computer then you would get only the one code for that one O2 heater

There could be corrosion on the underside of fuse box on that fuse holder, when it heats up voltage drops
Or at the splice where the 12v wire splits and goes to the two O2 heaters
In 1997 O2 heaters use a light blue/orange stripe wire for 12v out from fuse and to each O2

Test battery volts with key on, say it's 12.5volts
With both O2s unplugged, test voltage at each blue/orange wire, should be exactly 12.5v in this example, no drop
Then plug in either O2 and retest the unhooked one again, might be a slight 0.1drop but shouldn't be
Then reverse it, and test unhooked O2 wire on other sensor


Longshot would be computer issue, or one of the computers ground wires, pins 76, 77 or 103, theses are not "shared" grounds inside the computer, each is for specific circuits
Pins 25 and 33 are computer's case/chassis grounds, I doubt these would be used for O2 heater grounds but easy enough to test them
But you would most likely have other codes if it was a computer ground wire issue, but there are no internal schematics for these

 

Ron, That is one of the most thorough responses I've ever seen in a forum. But I can see you help a lot of people here. Thank you.

And I've done just what you suggested, with interesting results. I tested at the sensor (disconnected) and then added a wired a ground direct from the battery, then a positive line direct from the battery and noted the differences Essentially a 0.03VDC drop at max. So that's probably not material. Visual inspection under the engine fuse box showed 'new' looking wires and connections, so I suspect we're good there. No signs squirrel chewing or corrosion present.

I cleaned and tightened all the ground connections to the body in the engine compartment. They looked very clean, but... why not? Interesting that the rough idle seemed to go away. Ever the optimist, I took a drive and miles later, both codes triggered - this time from 'pending' to switch on the CEL and reset all monitors. Darn it.

So I go back to the essential, "What changed?" and ... not much. I was seeing the typical evap cannister purge valve/sensor issues occasionally over the last few years and replaced one or the other. But those haven't been up for a long time. The simultaneous O2 Heater Circuit codes started about eight months ago, with no work done on the truck.

So what's left is a checklist of three things, I'm thinking:

1. Why not replace both sensors again? Well, the simultaneous trigger of both O2 heater circuit codes happened before the new O2 sensors, and after as well. So this is inexpensive, but unlikely.
2. Check other sensors, like water temp, that could tell the PCM to do something with both O2 sensors? Also unlikely since I'm seeing codes trigger at 160 to 183 degrees f.
3. Replace the PCM. Ugh. OK. I think that's next. I may drive it a few more cycles beforehand just to see if it 'learns' and stops triggering the codes!

Thanks again for your help, and I hope this saga helps others.

-Michael

 

UPDATE: So the short answer is yes, A bad PCM can and will trigger P0135 & P0141 (Heater circuit) for both O2 sensors simultaneously and in error. I managed to clear the readiness codes, and pass smog today.

The steps:

1. Checked all fuses
2. Replaced both O2 Sensors
3. Confirmed voltage and switched grounds at each O2 sensor
4. Confirmed ohms and load for each heater circuit
5. Checked grounds at engine
6. Did thorough voltage drop tests across all ground points and key fuse points in the engine fuse box
7. Ran secondary +12v wires to rear o2 heater to supplement in case of occasional gap
8. Replaced both O2 sensors again.
9. Over 18 test drives - each triggered the errors after 6-8 miles
10. Replaced the PCM
11. Ran readiness drive cycle, clearing all codes, and .. passed smog.
    

 

I was wondering. When you were having pcm issues and simultaneous codes p0135 and p0141 did your truck immediately start and die on cold starts?

 

Hi. No it ran fairly well, with the only problem being some slight stumbling when coming to idle from driving.