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OBD2a/b Code 91 Issue & Fix for Civic & Integra
Author_ Katman | Created_July 2004 | Last Update_January 2005
[w/ additional help from Honda-tech member EE_Chris and Trey Sullivan]

SYNOPSIS

This article covers the 'Code 91' (P-code P0453) issue associated with OBD2 civic and integra vehicles.

This problem stems from the type of ecu being used in certain OBD2 civic & integra's. When performing an engine swap on certain 96-00 civic's & 96-01 integra's, depending on what US OBD2 ecu is being used, you may get this pesky code 91 (fuel tank pressure (FTP) sensor) problem. Some 96-98 civics & 96-99 integra's do not have this FTP sensor in the fuel tank which poses as a problem.

This partial list below shows which civic & integra vehicles are FTP equipped and non-equipped:

Vehicles equipped with an FTP sensor:
 1996: Civic EX / D16Y8 (All coupes), Integra GSR & possibly LS
1997: Civic HX D16Y5 (All coupes; California sedan: LX)
1997: Civic LX D16Y7 (California coupe; California sedan: LX)
1997: Civic EX D16Y8 (All coupes; California sedan: LX)
1998: All Civic models
1999: All Civic models
 (the above list came from a Helms manual)

Vehicles un-equipped with an FTP sensor:
1996: Civic DX/CX hatchbacks, Integra RS
1997: ?
1997: ?
1997: ?
1998: ?
1999: CX/DX hatchbacks
Pretty screwy, eh? This is why there is plenty of confusion on Honda message forums regarding the code 91/FTP sensor. I've seen numerous times people say "The FTP only started in 99" or "None of the '96 models had them". It's all a bit confusing, but there are some solutions for this problem. Read on.

SUGGESTED FIXES

  1. Change the ecu
    The easiest fix, is to use an ECU (keeping within your particular OBD2 version) that doesn't look for FTP sensor. Mainly, a JDM ecu would be of choice, because they are programmed not to look for an FTP sensor.
    NOTE: using a JDM OBD2 ecu will not allow you to pass smog, in California at least.

  2. Change the gas tank
    This is the next best possible solution for alleviation of code 91. Swap out your gas tank for one respective your vehicle type, that has an FTP sensor. Such as running a 99 Si coupe fuel tank on a 97 civic CX or DX. You will more than likley have to wire in the FTP sensor along with adding the FTP equipped gas tank too.

  3. Fool the ecu
    We fool the ECU by providing a similar voltage that the FTP would normally send -- this happens to be around 2.5v. You need to acquire a 10k linear potentiometer: Radio Shack Part Number 271-1715.
    The outter 2 pins are going to be +5v reference and ground (does not matter which), while the center pin will be your output feeding back to the ECU pinout.

    (Before you hook up the pot's center pin to the ECU, use a multimeter to adjust the output to 2.5v.)

    Here are the 96-98 civic & integra ECU pin assignments:
    D10: +5v
    D15: FTP input
    D11: GroundHere are the 99-00 civic & 99-01 integra ECU pin assignments:
    C28: +5v
    A29: FTP input
    C18: Ground

  4. Convert to OBD1
    This will totally eliminate any code 91 issues altogether. This is the most expensive way out of all 4 of these solutions because you would obviously need to purchase an OBD1 ecu and OBD2a or OBD2b to OBD1 ecu jumper harness. OBD1 programming does not look for an FTP sensor at all.
  5. The Trey Way
    I stumbled upon the 'Trey way' (as I like to put it) after reading his OBD2a EK B-series swap article. He mentions a temporary trick in defeating the Code 91 issue. An excerpt from his swap article mentions this:
    " I found a way to get around the code but isn't a true fix. This only happens when the car has been sitting for a long time like over night. You have to put the key in the ignition and turn it to the accessory position (radio on, but the car is off), and count to 10 slowly then crank the car. That will pressurize the fuel lines and the code will not come on."

ADDITIONAL INFORMATION

There have been reports, that while this fix works on a relatively full tank, empty tanks seem to trigger a code. Even though you've now provided the input, sometimes the ECU looks for a change in that input and you're providing something constant now.

CONTRIBUTIONS / PROPS

Special thanks to Honda-tech members EE_Chris and Trey (Sullivan) for providing these handy fixes!

The information in this article applies to:

  • All US 96-00 civics & 96-01 integra's with B-series engine swaps utilizing an ecu that looks for a fuel tank pressure sensor (normally associated with US OBD2a/b ecu's).
  • Hope this info helps you poor chaps out!