Hi all, Long time lurker, first time poster.

I swapped a 2004 L59 5.3 flex fuel into my 73 c10 and the engine is bone stock: cam, fuse box, exhaust manifolds, even the air intake tube and filter box is oem. It's mated to a TH350.

I have chased down many issues, but I cannot figure out the P0135/P0155 to save my life.

My symptoms:
Both codes show as pending after a single ride after clearing codes, and are then confirmed after a second ride.
Cold startup behavior is not as smooth as I would expect - not really sure if O2's affect this or not since they're not warm yet... The engine fires quickly and revs to around 1600, pauses for a second, and then drops to 1100 and pauses again before settling down at 900 rpm or so where it slowly drops to 650-ish as the engine warms.
PCM has a 12587603 operating system I got from PCM Hacking (using PCM Hammer and Tunerpro). Nothing has been changed in it except for VATS removal and DTC codes turned off.

My diagnostics so far:
Grounds are fantastic - I've been into the harness several times and all grounds are less than 0.5 ohms back-probed from connectors that have grounds, and each ground wire at the PCM plug to batt (-) terminal, chassis, and each cylinder head.
I have FULL battery voltage to the fuse box (this was a previous problem I fixed)
I put each front o2 sensor 12v wire on a single dedicated 15 amp fused circuit through a relay activated with key on (confirmed they're active while cranking). Fuse is still good.
Both high and low signal wires from each sensor and heater low signal wires have great continuity (<0.5 ohm) from o2 plug to PCM plug and do not fluctuate when I jiggle the wiring.
Both front o2 sensors are new AC Delco units - Bank 1 has 5.7 ohms across heater and bank 2 has 5.8 ohms across heater.
My cheapo scan tool (harbor freight special) can see the o2 sensor voltages switching, so I'm pretty confident the sensors are responding to the PCM's switching commands...
Exhaust is sealed - I blew air from the shop vac through the tail pipe, sprayed soapy water and didn't see any obvious leaks near the o2's... seems like this is more of a heater issue... not sure
Intake manifold gaskets are the newer updated Felpro units; IM was torqued correctly and then retorqued after 50 miles.
I smoke checked the intake from the throttle body and found no leaks that I could discern - I also tried the carb cleaner trick with no luck.

Considerations:
My battery is 12 years old... it cranks and holds voltage still, but could a voltage drop during cranking be enough to irritate the PCM? I can pick up another battery this weekend if needed.
I thought the PCM may have a bad o2 heater driver, so I picked up another green/blue PCM at pull-a-part and loaded the exact same calibration in it. The only difference is 1 pcm keeps the P0135/P0155 codes as 'Pending' and it never goes to 'Confirmed'. The other one (newer to me) confirms the codes quickly.
Neither plug at either o2 sensor appears dirty - I suppose there could be some corrosion deeper that I can't see, but I've shined my light in there and found nothing.
All spark plugs look great and the truck seems to run and drive well

I'm open to checking or rechecking anything, I just need some diagnostic guidance. I would rather not fire the parts cannon either until I can isolate what isn't working - I really want to understand what in this system is causing my hiccups.

Thanks!

Edit: The wiring harness itself going to each O2 has been inspected no chaffing, rubbing, burning, etc. has been found - they all appear normal.

 

Those codes set because the PCM doesn't see O2 sensor current on the OI2 Heater Ground Wire at the PCM. That should be pins 72 and 74, but check the diagram you used. Turn the key on. You should see power on both of those pins with the PCM connector unplugged. If not, chase down your power or ground problem.
Joe

 

TWELVE YEar old battery ??
I see that you are indeed reluctant to throw parts cannon.
Impressive, really.
Curious: what's the date code on your tires?

 

JoeRJr - I did not know that. I will check first thing tomorrow - thanks

Full Power - I actually laughed out loud - you're very perceptive. Truth is - I don't know. I'll have a look tomorrow though.

 

A scanner should be able to see the amperage reading.

 

Ok I confirmed pins 72 and 74 on the C2 green connector are both the low heater lines for bank 1 and 2 front o2 heaters. With the connector unplugged from the PCM, and key on engine off, I get 11.83 v from each pin to batt (-) terminal. I also get 11.83 v across the battery terminals with key on engine off. So full battery voltage - albeit low.

Key off, if have 12.6 v across the battery terminals, so I’m assuming the battery voltage is not holding under load? How many volts or amps is the pcm looking for?

Joe, I don’t think I have a tool or software capable of measuring the amperage you showed; I have OBDXplorer on my laptop so I’ll look for that measurement.

Full Power - the DOT code is “DOT T7HL H2 A” and that’s it. A quick google search suggests January of 2010. The truck was a gift and these tires were already on it. They’re on my list of things to replace.

 

I wouldn't worry about a small difference in voltage like that. It proves you have complete circuits from the power wire, through the heaters and all the way back to the PCM. Thgat is NOT what I was expecting. For your next move it would be really convenient to have a scanner that can read those values and cycle the O2 heaters.
Those codes can also set if power is shorted to those wires. I looked up the spec. If there is less than a quarter amp, or over 3 amps, that code sets.

Maybe try something real quick. Unplug the O2 sensors and power should go away on those 2 pcm pins. If not, you definitely have power on the ground wire. If you still have power there, fix it.
If not, do you have a multimeter that can measure amps? It's not as easy as reading the scanner but it'll give you the reading we need to proceed.

Joe

 

Great idea Joe. I’ll check if my multimeter measures amps, and if not, I can borrow an ammeter from my father in law.

 

Update: I unplugged both o2 sensors and check voltage on green c2 connector (unplugged from pcm) with key on - 0.00 volts. I believe this eliminates a short.

plugged sensors back in and confirmed battery voltage at the pins again.

My multimeter DOES measure amps so I unpinned the low signal wire at pin 72 and set my multimeter to 10 amps measurement with leads on common and 10a. I Hooked red lead to the o2 heater low wire (depinned) and black lead to a spare pin I installed into the pin 72 spot so my multimeter is in series. The green c2 connector was reinstalled on the pcm. With key on, I got 0.00 amps. Same for pin 74.

I figure the signal is not being grounded somehow so it’s either a driver inside the pcm or the pcm ground pins not making a good connection. I checked ALL ground leads on both pcm connectors again to batt (-) post and chassis ground and got 1.0 ohm or less.

Is it safe to assume the os heater driver inside the PCM is bad? Maybe…

I swapped pcm’s to the spare that I had and got the same reading. Seems odd that BOTH PCM’s would have bad O2 heater drivers so either that’s an age problem, or I’m trying to measure amps incorrectly.

With 11.6 volts to the sensor, and 5.8 ohms across the O2 heater, I would expect around 2 amps of current.

I measured the current with key on and engine off. I should have gotten a reading, right? What do you guys make of this?

 

It is possible that the PCM drivers are out on a GM PCM. I've never seen it in all my years working on GM cars though. Because you tried a second PCM with the same results, I'd say it really can't be 2 bad PCMs. If that ends up being the case, you'd better buy a hard hat just to walk around outside, LOL.

That leads to 2 other possibilities. The first is most likely, and that is the fuse being blown in the multimeter. Try measuring the amperage by touching one meter lead directly to ground instead of the pcm pin. If it still doesn't read amperage the fuse is blown or you have a voltage drop problem on power side of the circuit, meaning you have battery voltage there until you load the circuit, then it goes bad.
Don't power up the O2 heaters with the ammeter for more than about a minute at a time. You can smoke them.

Before I believed you had 2 bad PCMs I'd suspect one of the PCM grounds is disconnected. Unfortunately GM doesn't list internal ground paths on their controllers, so you're going to have to check them all. Do that with an old fashioned test light. One with a bulb. disconnect the pcm connectors and test the grounds by seeing if they can light the test light with other end connected to power. It's not a perfect load test, but it'll do.

Joe

 

lol no hard hat for me hopefully.

Just so I’m clear, I connect the clamp end of my test light to batt (+), disconnect both pcm connectors, and touch the test light needle to each of the ground leads in the blue and green pcm connectors, correct?

And a wire with a dim light, or not illuminated at all, is what I’m looking for?

 

That's it exactly

 

Ok latest diagnostics are done. Test light showed full bulb brightness on all ground pins on both blue and green connectors. I confirmed with the negative batt post and brightness was the same.

Also, I think your suspicion was spot on Joe. I tested current from c2 pins 72 and 74 with lead to ground (briefly) and still showed 0.00 on both pins so I think my multimeter fuse is blown. I’ll grab another one.

Anything else I can try in the meantime?

Edit: I pulled out the 10a fuse from the multimeter and checked it for continuity and got nothing. Fuse is blown and more are on the way.

 

If you're feeling ambitious, we can skip one step while we wait for the fuse. Use your test light or multimeter to watch the B+ wire of the O2 sensor heaters. Test it right at the connectors.
With the key on it should be roughly battery voltage.
Use that jumper wire you made back at the pcm and temporarily jumper the ground side to a good ground. Play it safe and don't keep it grounded for more than about 15 seconds. Don't do it again until you give it a 15 second rest.
With the ground side grounded you should still have B+ on the power side of the O2 heater.
This will confirm or eliminate a voltage drop problem to the O2 heaters.
Joe

 

Oh that’s a great idea…. I’ll try it out tomorrow.

I really appreciate your help on this Joe.

 

I did 3 more tests today:

Current through O2 sensor on bench:
Pulled B1S1 sensor out and wired it to the battery with my momentary button on B+, button to ammeter, and ammeter on B-. When I hit the button, the current went to 1.75 and then started coming down to roughly 1.62 amps. I Only held for 8 to 10 s but confirmed I have amps through the O2 heater on a bench.


-Reinstalled sensor and plugged in to harness

Current test at PCM connector:
Ammeter fuse came in. I Pulled the B1S1 heater low wire (pin 72) out from pcm plug and hooked it to my momentary switch, switch to ammeter, ammeter to B-. I got the same behavior: current went to 1.75 amps and slowly dropped to around 1.62 amps. I only held the button for 4-5 seconds, but I confirmed I have current from the fuse, through the o2 sensor heater, back to the pcm plug, through my ammeter to ground.


Voltage drop test on supply side:

With the multimeter I back probed the pink supply wire down at the O2 sensor plug to ground. Key on I had 11.8v which slowly dropped to 11.67v over 8-10 seconds (o2 was not grounded yet). I Hooked up my momentary switch from pin 72 pin (still out of pcm connector) to B-, hit the switch and volts dropped from 11.67 immediately to 11.37, but then climbed to 11.42 over the next 5-8 seconds.

Does a 0.4v drop count as a voltage drop problem or is this expected? And since the volts climbed slightly after dropping, does that mean the circuit is solid? Volts climbing sounds like the resistance dropped which is what I saw with the ammeter.

I did notice the pin 72 was kind of ‘open’ a bit probably from me installing and removing it several times. I closed it back up gently - maybe a connection there wasn’t solid? Not sure. But if GM’s spec is 0.25 to 3 amps, and I have it confirmed to the pcm plug, it kind of sounds like either the pcm itself os bad or I have a grounding gremlin that didn’t show up with my test light.

I actually did a 4th test too. I swapped in my original (to me) pcm and tested current with key on between pin 72 and my jumper to the pcm pin - no dice.

Is the pcm supposed to ground tbe o2 sensor heater immediately with key on, or does it wait until it senses the engine running to ground the sensors?

I think tomorrow I’ll spend some time cleaning and confirming ground cable and strap connections but I’m not sure what else to measure.

 

Did you clear the codes before seeing if the PCM grounds the heater? If it has any O2 codes it'll disable the heaters.
The diagnostics for that code says to turn the key on and wait up to 15 seconds for the pcm to ground the connection. Then it will cycle it on and off at 1 second intervals.
The manual states to watch the amperage on a scan tool, but you're doing the same thing.

 

I did not… I wasn’t aware that was required but I’ll do that.

 

I left that part out.....
Sorry.

 

Oh no worries; your ideas have gotten me farther than I was before.

I went on a short ride, came back to both P0135 and P0155 pending. Cleared the codes, then connected my ammeter in series between O2 heater low and pcm pin 72. Waited 60 seconds with key on, engine off. Nothing - 0.00 amps.

Then I cranked the trunk up and got current! It fluctuated between 0.76 and 1.12 (best I could tell just watching the readout). Both of these are inside the range you mentioned, Joe. Without having a scanner to watch the amperage in real time on a drive, I can only assume the current is dipping below the 0.25 or peaking above the 3 amp range to trigger the code.

With current confirmed between o2 low and pcm pin 72, I’m led to believe the pcm is in fact good. Is there a time component to the o2 heating up and the pcm sensing it? Maybe it’s heating to slowly. I keep thinking I have a bad ground somewhere