November 27, 2021
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Chevrolet Tahoe P0134 P0154 Fresh Rebuild, will Foxwell N510 Elite help?


Coolant seemed to be slowly losing (internally), suspected head gasket, ending up adding Bars HG1 along with other tune-up items: cap, plugs, wires, rotor, fuel filter. Did compression (found low cyl 5), But otherwise working, and yes, the Bars HG1 did seem to help significantly….fast forward to Jan 2020.

What I did do:

Jan 2020: CEL and Flashing CEL, limped it back home about 5 miles….P0300.. no overheating, no external leaks..another round of Bars HG1, .replaced plugs and O2 Sensors front and back…no help…..time for teardown. turns out bad head gasket as well as both heads have crack…time for a rebuild.. replaced heads, full machining, valve job, rebuild kit install.

Jun 2020:

Finally get the rebuild done..[resolve a P0300 a few days ago (see my other post- had two plug wires swapped) and now that that is resolved] engine seems to be running smoothly with probably 10 miles on the fresh rebuild now but P0134 and P0154 pops up (takes a while for this code to show) Ok…

GM scanner to use:

Foxwell N510 Elite for GM


The testing result with Foxwell N510 Elite:

I seem to be in open loop all the time according to the scan tool, ( but I do get up to 93C, typically fluctuating from about 90-95C, via scan tool.). For both cold and hot engine..I get the following readings
b) STFT B1/B2, LTFT B1/B2, STFTB1S1, STFTB2S1 all at 0
c) STFT B1S1 and STFT B2S2 both at 99.2%
d) Oxy Sens Output Voltage B1S1, B1S2, B2S1, B2S2 all at 0.445 V

Possible reason:

I think you have a bad O2 sensor or sensors, either before or after the catalytic converter, so your vehicle won’t go into closed loop mode. That is what OBD II code P0154 would indicate. The powertrain control module (PCM) provides a baseline voltage of about 450 mV on the oxygen sensor signal circuit. When cold, the PCM detects the internal resistance of the sensor is high. As the sensor warms up the resistance is lowered and it starts producing voltage based on the oxygen content in the exhaust.

When the PCM determines that the time it took for the sensor to warm up is greater than one minute, or that the voltage is inactive, (not reading outside 391-491 mV) it may view the sensors (O2) as inactive or open and sets the P0134 code, thus you can’t get a closed-loop mode. You may have an issue with motor oil burning, or your system is not calibrated properly and is running too rich.


I am starting to think it’s possible that the ECM is malfunctioning.

I broke down and ordered one yesterday, went out today, will be here by friday.. I will probably just wait until then and swap it out (its coming preprogrammed) before going back and revisiting all the measurements. From all the forum discussions and my own research..I only now feel that I sort of understand the O2 sensor it will be alot easier for me to recheck things. I also ordered an O2 sensor extension cable which i will butcher to allow me easy access to line data 9so I dont have to back probe) so I can check directly the sensor output really easily.

I also went through wiring diagrams in detail so feel much better about that as well. I found where the relevant grounds are:
G101 and G119 for Battery, right front wheel house and right front of engine, respectively
G105 and G117 for Block, braided lines, right rear of engine compartment and right rear of engine, respectively
G119 – O2 sensor heater circuits right front of engine, Fig 52 Ground Distribution, Pg104,
G120 – PCM right top of cylinder head, Fig 36 Engine Performance,Pg 72, or G131 – PCM near thermostat housing, Fig 52 Ground Distribution, Pg 104 (there seems to be a discrepancy in the diagrams for the PCM C3 pins 17/18 ground pins from the document I consulted, unfortunately, its a pdf and the name/source of the document is not shown)

The only ones that were touched were those that went to the block, so I will likely check first if the new PCM doesnt solve things. And if not, I will confirm O2 sensor activity with the jumper myself. And then possibly figure out how to remove the cover from C1 so I can probe right at the connector while its still connected to see if any signal is making it back.


Well….PCM arrived early, put it in, but now I cant get the engine to idle. It cranks, turns over but then immediately dies… (no issues with the old PCM starting, so I put that back in for now)..

followup: apparently I need to do a passlock procedure on it….in progress

So I got the engine working with the new PCM…and I saw activity on B1S1 and B2S1 for a little while in the scan tool. Generally, they were higher voltages… but then it stopped and read a solid 0.450 V


Did another round of scan tool monitoring today. Watched sensor voltage graph as engine warmed up. I think same response as yesterday so maybe PCM is not getting killed. I get a very slow responding B1S1 followed by slow responding B2S1. Not really oscillatint..starting high (0.8v +) and then slowly meanders down. I think if I hit the gas quickly it will come down to below 0.45v but generally stays high and then after 5 just sits at 0.45v steady. Didnt see activity out of B1S2 or B2S2.


my 3rd set of sensors arrived today. I was sort of waiting for them before getting back underneath as its just too frickin hot here. I waited until 11 PM and went out, swapped all 4 sensors (and added some 2 ft extenders to make my life easier in the future) and amazingly enough…precats were bouncing around full range… and system went into closed loop. It was the frickin sensors…had 2 sets of **** sensors..3rd ones had excellent reviews and are working fine. Tomorrow I will drive around a bit and see if I can get all my I/Ms to become ready so I can smog it…but major hurdle passed.


looks like a spoke to quickly.. upstreams working…. downstreams now getting P0140 and P0160.. i suspect more bad sensors…

Finally! Solved!

new Bosch 13474 post cat arrived today..installed…seems to be working fine…after multiple sets of bad sensors

Credits to @ oldandintheway