If you had to replace the cat because it was oil fouled (plugged) then the oxygen sensor should be considered a goner anyway. Always always always, plugged cat = failed sensor because the crap that fouled the cat reached the sensor first and a lot of it stuck there. Even if the plenum's never been blown and the cat's still good, if the sensor has more than 90,000 miles on it and you suspect it, it's time to swap it out. If the sensor has 100,000 miles on it, it's time to swap it out even if it appears to be working perfectly. (Some guys go over 150,000 miles on the original sensor before it fails, it's true, but it's easier to swap it at 100,000 at your convenience than to troubleshoot your way to it when it inconveniently fails.)

It might not solve your problem but it needs doing anyway if any of the above conditions are true.

FYI: The usual first sign of one going away is decreased fuel economy and performance that isn't cleared up by a tune up (and the plenum gasket is intact), then (usually several months) later the engine idle will fall to around 400RPM with the intake making a loud sucking noise right after the PCM goes closed loop a minute or two after cold start. A few minutes later, or after revving the engine for about 30 seconds, the problem clears up and doesn't come back until the sensor has an opportunity to cool. Leave it go long enough and it'll eventually start acting up when cruising lightly loaded on the highway. It's right around that time that the CEL finally comes on.

Testing the sensor requires either an oscilloscope or a scan tool that has the equivalent of one so you can watch the signal in real time, and a propane bottle (properly used!) with which to screw the mixture around so you can watch the sensor react. The signal is pulsating DC so a voltmeter won't tell you anything useful. If you happen to have the equipment to do it, a web search should turn up something like a reasonable procedure.

 

I didn't get around to working on the truck this weekend as I was sick but I was thinking about it... Wouldn't a bad o2 sensor throw a CEL?

 

Not necessarily. If the sensor has just gotten sloppy, but not completely failed, the PCM will not throw a CEL for it.

 

Here's an update a long time coming.
- Cleaned IAC - no change, wasn't super dirty to begin with
- Tested TPS - Looks good, the voltage smoothly followed the throttle
- Replaced front 02 sensor - no change
- I'm not burning oil and I have no residue when looking through the throttle body (plenum gasket)
- Went with the original plug wires that were on it when I bought the truck because two of the damn NAPA wires I bought broke - No change
- Spark plugs look fine, have replaced twice (NGK's) since I bought the truck in January.
- Compression test was 135 in one cylinder and 150 +- 10 in the rest
- Checked the crank position sensor and it is showing some resistance in the 1K-10K range (like the 1996 Service Manual I found online says to do), it looked new, I bought a new one and the resistance was there in the new one too. Is there another way to check it? It does have power to it from the pcm
- The engine wouldn't start last night after I turned her off after work so I checked the ignition coil (while mounted), thought I saw no resistance, thought I didn't see any spark when I took the coil to distributor cable off of the distributor and held it next to ground, took the ignition coil off and the resistance looked fine (.9-1.5 ohms, 12K secondary), put everything back on two hours later and she started right up. When coils go bad don't they usually just die and not work at all?
- Ran SeaFoam in the gas and through the brake booster vacuum - nice smoke, maybe a little improvement, threw a code when I WOT'd it afterwards
- Bought an ElmScan 5 compact to troubleshoot this thing
- The code is a P0300 (random misfire) - Not related to the SeaFoam, I've had this happen with WOT 2-3 times in the past (I don't drive it hard very often)
The CEL started flashing when I was really on it and accelerating but then when the CEL started flashing, the RPM's rose and I couldn't feel any more power so I let off on the gas a bit and everything seemed fine. I drove it a while further with no issues, turned around and drove it hard some more, the CEL blinked again while I was at WOT (not everytime though) and then I went home. At this point I was thinking HA! I'm going to finally be able to tell what the problem is but my computer battery had died when it was recording the data so I'm still kinda SOL. I was able to get the freeze frame data from when the CEL came but need some help in determining what it all means.
PID 0x02 - Freeze frame DTC - P0300
PID 0x03 - Fuel system status - Open-loop due to engine load
PID 0x04 - Calculated load value - 88.63%
PID 0x05 - Engine coolant temperature - 176 F
PID 0x06 - Short term fuel % trim - Bank 1 - 0%
PID 0x07 - Long term fuel %trim - Bank 1 - 0%
PID 0x0B - Intake manifold absolute pressure - 0 PSI
PID 0x0C - Engine RPM - 0 RPM
PID 0x0D - Vehicle speed - 57.79 MPH

From my previous drives with the ElmScan hooked up it looks like:
- Voltage read by Scan Tool - 12-15V
- Intake air temperature seems too high to me, sometimes it's getting to 175 F but it does change so I don't think the sensor is bad...
- Ignition timing advance for #1 cylinder, Absolute throttle position, Calculated load value, Intake manifold absolute pressure, Long term fuel % trim - Bank 1 all follow each other
-Ignition timing advance constantly maxes out at 40 when i'm on the highway
- Long term fuel % trim and Short term fuel %trim stay within +/- 8
- Oxygen sensor voltage (Bank 1 Sensor 1) and Oxygen sensor voltage (Bank 1 Sensor 2) varies from .1-.9V
- Intake manifold absolute pressure is about 8 psi on average

Under the On-Board Monitoring (Service $06) tab, it says 6 out of 9 failed this morning.
TID $11 - Manufacturer Defined - Value 7/Min 44
TID $13 - Manufacturer Defined - Value 0/Min 16
TID $15 - Manufacturer Defined - Value 0/Min 14
TID $51 - Manufacturer Defined - Value 0/Min 3
TID $52 - Manufacturer Defined - Value 0/Min 20
TID $53 - Manufacturer Defined - Value 0/Min 144

Then another check this morning when I was getting out of the truck at work turned up:
TID $11 - Manufacturer Defined - Value 7/Min 44
TID $13 - Manufacturer Defined - Value 48/Min 16 - PASS
TID $15 - Manufacturer Defined - Value 0/Min 14
TID $51 - Manufacturer Defined - Value 3/Min 3 - PASS
TID $52 - Manufacturer Defined - Value 0/Min 20
TID $53 - Manufacturer Defined - Value 0/Min 144

I'm getting noise from the exhaust too, sounds like my CAT is going out again. I need to find the root cause of this problem so I'm not throwing $350 away every 4 months for a CAT.

It's sad that I have this scan tool at my disposal but I haven't been able to figure out a way with all the data to pinpoint my problem and say "Hey! That thing is bad!" then replace it and be done with it.

I believe this is the Mode $06, mode 6, service mode $06 or whatever for the 1997 Dodge Ram but I'm not completely sure. My results are below the TID and description.

06 11:O2 Sensor 1/1 Half Cycle Counter -
48 6B D1 46 11 81 00 0A 00 2C 92

06 13:O2 Sensor 1/1 Big Slope Counter
48 6B D1 46 13 81 00 59 00 10 C7

06 31:O2 Sensor 2/1 Half Cycle Counter
48 6B D1 46 31 81 00 00 00 2C A8

06 33:O2 Sensor 2/1 Big Slope Counter
48 6B D1 46 33 81 00 00 00 10 8E

06 19:O2 Heater 1/1 Time to Reach Voltage - 11ms/bit
48 6B D1 46 19 01 09 51 14 5E B0

06 1a:O2 Heater 1/2 Time to Reach Voltage - 11ms/bit
48 6B D1 46 1A 01 0E 2E 3F 00 60

06 1b:O2 Heater 1/2 Time
48 6B D1 46 1B 01 00 00 FF FF E4

06 1c:O2 Heater 1/3 Time
48 6B D1 46 1C 01 0E 2E 3F 00 62

06 39:O2 Heater 2/1 Time
48 6B D1 46 39 01 00 00 14 5E 76

06 3a:O2 Heater 2/2 Time -
48 6B D1 46 3A 01 00 00 3F 00 44

06 3b:
NO DATA

06 3c:
NO DATA

06 15:Catalyst 1/1 Phase Response Counter - 1 count/bit
48 6B D1 46 15 81 00 00 00 0E 6E

06 21:Catalyst 1/1 Switch Frequency Ratio - 0.39%/bit
48 6B D1 46 21 01 00 00 00 C3 AF

06 22:Catalyst 2/1 Switch Frequency Ratio - 0.39%/bit
48 6B D1 46 22 01 00 00 00 C3 B0

06 35:Catalyst Slow O2 2/1 Response Counter -
48 6B D1 46 35 81 00 7E 0E 00 0C

06 41:EGR Monitor Fail Counter - 1 count/bit
48 6B D1 46 41 01 00 00 00 03 0F

06 51:Purge Monitor & IAC Shift - 1 step/bit
48 6B D1 46 51 81 00 00 00 03 9F

06 52:Purge Monitor & Adaptive Fuel Shift - 0.39%/bit (use LSB only)
48 6B D1 46 52 81 00 00 00 14 B1

06 53:Purge Monitor & RPM Shift - 1/8 RPM/bit
48 6B D1 46 53 81 00 00 00 90 2E

06 61:0.040" Leak Detection Pump Period - 11ms/bit
48 6B D1 46 61 81 00 00 00 00 AC


Thanks for your help

 

Check your fuel pressure.

 

inspect all vacuum lines....they get brittle with time and almost always crack when ur doin other under hood repairs.....just had this issue with my 99 5.9L....not a genius tho

 

I'll check the vacuum lines again, this time with a propane torch, and see if I can find any sort of leak. If I can't find a vacuum leak again, i'll have the wife take it to get the fuel pressure checked. When I started it up yesterday, the exhaust smelled heavily like gas so it's not burning properly and that's probably why my cat is going out again, yes? Any one know how to decipher these codes and readings and tell me what they actually mean? Why would the new and old crankshaft position sensor both show these readings that are out of spec? Do I need to get one from the dealer or is it ok? It would be very helpful to have a confirmed list of these TID's and CID's that I'm failing and passing. Thank you!

 

I checked all the vacuum lines and they are good. I haven't had a chance to take it in and get the fuel pressure checked yet but here's another symptom I noticed this morning. I was low on gas so I put it in neutral while coasting down a hill. when I did this, I noticed the rpms dropped real low, my gauges dimmed and so did my headlights.

 

Could be a failing IAC, a bad miss, low fuel pressure, or just a confused PCM. If the engine idles down really low, alternator output will drop, and with a bunch of stuff on, things will dim, and the volt gauge will reflect that drop. It shouldn't go below 12 volts though.... If it does, time to load test the battery.

 

Did you replace the crank sensor? What brand o2 did you install? Have you tried unscrewing it and going for a short drive to see if it makes a difference?