DTC P2451: General Description

Courtesy of HONDA, U.S.A., INC.

Courtesy of HONDA, U.S.A., INC.

The powertrain control module (PCM) automatically starts the evaporative emission (EVAP) system diagnosis after the vehicle has been turned to the OFF (LOCK) mode, and has met all other enable conditions (initial engine coolant temperature [ECT Sensor 1] and off for at least 5 hours). The EVAP system diagnosis is performed by reducing pressure between the EVAP canister and the atmosphere. If pressure is reduced with the fuel vapor containment valve open, the entire EVAP system will be diagnosed, and if the fuel vapor containment valve is closed, only the EVAP canister side will be diagnosed.

Leak check pressure sensor operation flow

• Reference pressure measurement : Malfunction detection in the EVAP leak check module itself and determination of early completion of pressure reduction when normal.
• System pressure reduction : Pressure in the EVAP system is reduced to measure the ultimate pressure.
• Reference pressure re-measurement : Leak is detected by comparing with the ultimate pressure of the system pressure reduction
• Reference value is to be measured twice considering change in the characteristics of the vacuum pump during detection.

Reference pressure is the pressure reduction by the reference orifice (#40 equivalent) in the EVAP leak check module used to check for leak by comparison with the system pressure reduction.

Fuel tank pressure (FTP) sensor middle stuck and fuel vapor containment valve close stuck are detected by changes in the fuel tank internal pressure under system total pressure reduction. Since the same phenomenon is used to detect two malfunctions, normality judgment is to be performed to discriminate the two.

Here is an overview of the malfunction detection :

Judgment 1:  Normality judgment for the FTP sensor middle stuck by comparison of the fuel tank internal pressure before and after wake up. (P0451) (Under a sealed condition, the Fuel tank internal pressure changes according to the temperature of the fuel tank)

• If the temperature changes before and after wake up, and if the FTP sensor is normal, the FTP sensor output changes. (Judged as normal)
• While the FTP sensor output does not change if the temperature does not change before and after wake up or if the FTP sensor is stuck middle, malfunction judgment is not performed since the fuel tank internal pressure may accidentally become the same.

Judgment 2:  Detection of the fuel vapor containment valve close stuck by the judgment by the leak check pressure sensor before leak detection. (P2451) (When the fuel vapor containment valve is opened, the canister side pressure also changes due to the fuel tank internal pressure)

• If the fuel tank internal pressure and the barometric pressure are different and if the fuel vapor containment valve is normal, the leak check pressure sensor output changes. (Judged as normal)
• While the leak check pressure sensor output does not change if the fuel tank internal pressure and the barometric pressure are the same or if the fuel vapor containment valve is stuck close, malfunction judgment is not performed since the fuel tank internal pressure may accidentally become near the barometric pressure.

Judgment 3:  Malfunction judgment by pressure reduction of the total system. (P2451) (During the detection of leak in the total system, the fuel vapor containment valve is opened to reduce pressure)

• When the fuel tank internal pressure does not change, the fuel vapor containment valve close stuck is detected.