DTC P0451: Fuel Tank Pressure (Ftp) Circuit Range/Performance Problem

WARNING: This page is about a different car, the 2000 Honda Prelude. However, it is still accessible from the selected car via links, so may be relevant.

Perform PCM reset procedure. See PCM RESET PROCEDURE/CLEARING DTCS in SELF-DIAGNOSTICS - INTRODUCTION article. Remove fuel filler cap. Turn ignition on. Using scan tool, check FTP sensor voltage. Or, using backprobe kit, measure voltage between PCM 16-pin connector "D" terminals No. 11 (Green/Black wire) and No. 15 (White/Blue wire). See Figure . If about 2.5 volts does not exist, go to next step. If about 2.5 volts exists, replace fuel tank pressure sensor.
Disconnect vacuum hose located between EVAP 2-way valve and fuel tank pressure sensor at EVAP 2-way valve side. Connect vacuum pump to hose. Turn ignition on. Slowly apply vacuum to hose one stroke at a time. Using scan tool, check FTP sensor voltage, or using backprobe kit, measure voltage between PCM 16-pin connector "D" terminals No. 11 (Green/Black wire) and No. 15 (White/Blue wire). Voltage should drop smoothly from about 2.5 volts to about 1.5 volts. DO NOT apply vacuum after voltage drops to 1.5 volts or damage to fuel tank pressure sensor may occur. Go to next step.
If voltage does not drop to about 1.5 volts and hold, replace fuel tank pressure sensor. If voltage drops to about 1.5 and holds, substitute a known-good PCM and recheck. See SUBSTITUTING PCM under SELF-DIAGNOSTIC SYSTEM in SELF-DIAGNOSTICS - INTRODUCTION article. If symptom or indication goes away, replace original PCM.