Re:Fuel tank pressure sensor weirdness!

'16 Silverado 3500 comes in, hard P0452 Fuel Tank Pressure Sensor Low Voltage. All three wires to the FTP are rat chewed off at the FTP connector. Easy peasy, suggest a FTP pigtail and some labor. Slam dunk. I do a smoke test anyway because I'm paranoid, which passed.

After the repair, I go to run a Purge/Seal to check functionality (since the PCM won't allow it with a P0452 set). Vent is venting, purge is closed, I find that the FTP voltage is at 1.55V, which corresponds with atmospheric. But the FTP inH2O PID says -2.32 inH2O. :blink: Pull the gas cap, no change. Pull the vent line at the canister, no change. Purge and vent function correctly. Purge is not stuck open.

Started out with the Snap-On, so I switch to the Launch. Same thing. :silly: Two scan tools not reading that PID correctly? Not likely. So I install a manometer on the fuel filler neck. It reads atmospheric, while the scanner says -2.32 inH2O. What in the... The purge adds vacuum to the sealed tank, and the vent bleeds it to atmosphere when commanded. The only difference is the 2.32 inH2O reading.

Then I pinpoint check the FTP, plugged in, engine running. 5V, ground, and signal that matches the scanner PID. Shorting the signal to the 5V shows 5V on the scanner.

About this time, the customer is blowing up our phone, wanting their truck back. :lol: Without a fault or any other measurable problem present, I have to let it go. No codes set on the test drive. The question is, was I chasing a ghost? Or is there another problem waiting to eat my lunch?

Hello Tyler

Interesting... Maybe this will help.

The pressure sensor is a differential type sensor, not absolute. The 1.3-1.7v spec is what you should see with the fuel cap off. IOW, the voltage has nothing to do with BARO pressure, It is a comparison of two inputs.

I'm not clear as to how you are testing this. Did you compare the sensor voltage pid with the sensor pressure pid at the same time? If so, do they agree?

I primarily use a Snap On scan tool even though AM scan tools occasionally get pid data wrong. If the above test fails, then I would blame the tool. Both of them. In the last few weeks I've run into 2 GM vehicles where my SO tool got pressures wrong. I didn't want to fire up GDS2, so I worked around it.

You could also check to see if the evap test codes have run. For those who don't know, each DTC represents a test. One that failed. A capable scan tool will allow you to see if a particular test (code) has run. A monitor is a test (DTC) or group of tests for a particular system. In this case, I would not mess with the monitor since evap monitors are difficult to run. But you know that already.

As a side note and I hope you don't mind the comment...

I'm not sure why you performed a "smoke test" unless you mean you used a "smoke machine" to perform a flow test. The latter determines if there is a leak. Smoke only helps you find the leak you think you have and I find it is usually close to worthless with anything other than a huge leak

Flatrater wrote: I'm not clear as to how you are testing this. Did you compare the sensor voltage pid with the sensor pressure pid at the same time? If so, do they agree?

Sorry, I didn't make that clear. I observed the FTP voltage AND pressure PID at the same time. I have screenshots, just not with me. :blush: Voltage was 1.55V, pressure was -2.32 in H2O. Unless there's something different about this sensor compared to other GM FTP sensors I've seen, I feel that they don't agree.

I primarily use a Snap On scan tool even though AM scan tools occasionally get pid data wrong. If the above test fails, then I would blame the tool. Both of them. In the last few weeks I've run into 2 GM vehicles where my SO tool got pressures wrong. I didn't want to fire up GDS2, so I worked around it.

Maybe it's all BS, then. :silly: We have GDS2 available at the shop, but, like you, I just couldn't be bothered to get it out. Maybe I will if I happen to see this truck (or a similar truck) again.

You could also check to see if the evap test codes have run. For those who don't know, each DTC represents a test. One that failed. A capable scan tool will allow you to see if a particular test (code) has run. A monitor is a test (DTC) or group of tests for a particular system. In this case, I would not mess with the monitor since evap monitors are difficult to run. But you know that already.

I didn't think to check that! Unfortunately, the truck was full to the brim when it was brought to us. Thus, the EVAP Service Bay Test wasn't allowed. But that doesn't mean the cap off test didn't run... I'll check that next time.

As a side note and I hope you don't mind the comment...

I'm not sure why you performed a "smoke test" unless you mean you used a "smoke machine" to perform a flow test. The latter determines if there is a leak. Smoke only helps you find the leak you think you have and I find it is usually close to worthless with anything other than a huge leak

Nope, fair comment. I actually didn't include all the codes that were present when I first scanned the truck, mostly because I didn't feel they were relevant to my question about the FTP PID. This is the results of my initial code scan:



The P0455 was the main reason for the smoke test. Is it likely that we have a large leak AND an FTP sensor problem? Of course not. But I've been burned by multiple failures too many times to assume. I could have checked the Freeze Frame for the P0446 and P0455 to see when they set compared to the P0452? But the truck is already on the rack, and the smoke machine is handy. Like I said, paranoid.

Is there a pid in global obd2 for that sensor? Could it be odd processed data in enhanced mode?

Nissan Technician

Tutti57 wrote: Is there a pid in global obd2 for that sensor?

YES THERE IS. Why didn't I think to check that? :lol: Seeing the Global PID reading normally probably would have made me feel a lot better about the whole thing. Great idea, sir.

Could it be odd processed data in enhanced mode?

Pretty sure it was? I was just surprised to see two scanners with the same oddity.