DTC P1456 - Evap System Diagnostic Aid

SOURCE:
Honda Service News August 2003

TITLE:
Find and Fix Those Pesky DTC P1456 Problems

APPLIES TO:
All Models

SERVICE TIP:
Has this happened to you? You're chasing down a DTC P1456 [EVAP control system leakage (fuel tank system)] problem. You've done all the published tests, and everything passes. So you return the vehicle to your customer, and you get a comeback for the same problem.

The reason could be thermal expansion. Since testing requires the engine to be at its normal operating temperature, thermal expansion of the fuel tank, couplers, hoses, and valves can seal up a leak and let the system pass the test. When your customer starts the vehicle, the ECM/PCM tests the integrity of the fuel-tank side of the EVAP system within 30 seconds of a cold start. So if there's a leak in the system, it's going to show up here, while the engine is still cold.

The best thing to do in this case is to keep your customer's vehicle overnight so the EVAP system can cool to the same temperature at which your customer starts the vehicle. The next morning, turn the ignition switch to ON (II), and check the fuel tank pressure sensor voltage. If the voltage is at or near 2.5 volts, there may be a leak in the system. To check for a leak, do the vacuum tests in S/B 99-075, "Troubleshooting DTC P1456 and P1457", and the DTC P1456 test in the April 2002 S/N article "DTC P1456 or P1457 Still Set? Test for Internal Leaks."