Hey Guys, I am the FNG today! Need some expert opinions...

The Truck (my baby):
2004 Dodge RAM 1500 SLT Quad Cab 4x4 4.7l vin 'N' (NGC) (California).

Repair:
*Replaced all 4 O2 sensors with new NGK OEM units.

Reason: the old ones were lazy and not working properly, fouled, and the fuel economy was toast at 6 MPG. Verified with scan tool diagnostics. It is now 13.8 MPG on the overhead console.

Problem:
I have an odd problem I want to bring to the experts here.

They worked perfectly for the first 2 weeks. Fuel economy was a little over 2x what it was. The guy I bought the truck from with 212,000 miles had factory installed shocks still. Yes, and he worked on the RAM line when the truck was built. I have a Launch X431 V+, and an Autel MS906BT dealer level scanners. I am actually a certified electronic tech, so I am better with the computers and electronics than something like rebuilding a transmission. I have rebuilt many engines in my years.

Note:
I have a Hantek 8 Channel scope and a PICO 2 channel lab scope with all the probes, back probes, etc. etc. I did check the new sensors just to make sure they didn't go bad for some reason. They are all perfect.

First Issue:
All of a sudden I get all 4 O2 sensor codes P0135, etc saying the heater circuit performance was bad. Checked grounds and found lost grounds. I repaired the grounds at their termination point G107 right passenger fender. That fixed both upstream sensors and no codes on them for several trips. I also verified good ground at the O2 harness to sensor connector for all 4.

However, both downstream O2 sensors popped the same codes up again after one trip. I was monitoring them on the Launch and the heater circuit was pegged at 100% all the time. I checked the voltage to the heater. Being it is Pulse Width Modulated, I had to activate the heaters independently through the scan tool.

***The voltage I get is 7.74vdc. Not battery voltage.*** Unfortunately, the factory manual didn't indicate a voltage, and neither AllData nor Mitchell's indicated it.

***Is that voltage correct due to it being PWM. I am pretty sure I should be getting 'battery voltage when I activate the heater for testing the voltage on the heater circuit. Am I missing the plot here?***

Second sort of issue:
In the TIPM, there is a fuse and relay socket for "Downstream O2 Sensors". Under the cover, it says O2 Sens. (8.0L Only) for the relay and fuse. There is a fuse in there and the relay socket has wires and terminals in it. There is no relay. My general rule is to keep to what is in the vehicle and not the manuals which may not have been updated. Also, since they worked previously without the relay, I am sure the TIPM cover is correct and it is for (8.0l only).

So, I go check AllData and Mitchell's to see what they say about it. Both say (California) and nothing about (8.0l only). WTF? This truck is a California emission vehicle. Does it need the relay or not? Right now I say it does not.

My current plan is to test resistance across the heater control wire from the PCM and ensure it is under .5ᾨ. If they are good, then It is a PCM issue and I will be swapping out the PCM. Here is why I am thinking the PCM is bad:

When in idle the B2/S2 O2 sensor long term fuel trim is pegged at 12,000%! Yes that is an actual number on the Launch. The Autel is 99%. When I step it up just off idle it drops to the correct ranges.

BTW, the new sensors are running very close to zero when you add long term and short term fuel trim together. The truck runs perfect as well. I think the PCM has a fault, but am going to finish the diagnostics first on the wires and connectors.

Since I am getting RIDICULOUS LTFT on B2/S2 at idle on both scan tools, I think that the PCM has a fault. Otherwise, I have never heard of numbers like 12,000% or even 99%. I believe the max is 25%.

Well, throw some darts at me or let me know what you think about this. It is one of those things where you wake up in the middle of the night and wonder if you diagnosed everything correctly and WTH is that ridiculous number coming from! Grrrr!

Thanks in advance for your input.
Dave

 

I think you should be getting 12 volts on the heater circuit...... High resistance somewhere (dirty contacts in a possible relay, dirty connectors.... etc.) will cause that. Find out what that's all about first, clear codes again, see if they come back.

 

Yes, that is exactly what I thought. Tomorrow I will have time to check the Heater Control wire for resistance. I will also test voltage at the PCM output to both those heater control circuits.
The PCM PWM heater control circuit should be 12vdc or battery voltage looking at both the square wave amplitude on a scope or just with a Multimeter.I will let you know what I find tomorrow.

Thanks for the reply.

 

What did you find causing all 4 oxygen sensor heater errors? PCM failed to measure heater circuit current? I have a 2005 Dakota failing intermittently with same codes. No issue with PCM supplied heater voltage or ground circuit. No PIDs to see heater circuit current measurements.

 

I saw your request to Action70 and your post here. Having the same problem with a 2005.
Did you figure it out?
Hoping you might be able to help me diagnose as well.

 

I saw your request to Action70 and your post here. Having the same problem with a 2005.
Did you figure it out?
Hoping you might be able to help me diagnose as well.

 

I think he gave up, junked it and just bought a new car.