P0442 EVAP Leak Monitor .040 Leak Detected (Medium Leak)
P0455 EVAP Leak Monitor Large Leak Detected
P0456 EVAP Leak Monitor .020 Leak Detected (Small Leak)

What this code means is that the computer has tested the evap system and was not able to draw a vacuum on the fuel tank. it does this by turning on the evap vent valve which closes the fuel tank vent off, It then opens the vacuum solenoid and allows engine vacuum to be drawn on the fuel tank. A sensor in the fuel tank monitors the Pressure/Vacuum levels on the tank and reports this to the computer.If the computer does not see a change in pressure/vacuum on the tank then code P0455 will set.

Check under the hood to make certain no vacuum lines are disconnected, check your fuel cap if it isn't sealing correctly or is missing or the wrong one this will cause that code to set, You may have a bad vent valve in the gas cap causing the problem.

If you first get a P0442 or P0456 then a while later get a P0455 it is a sign a hose is going bad.
It is common for the hoses over the gas tank on Jeeps and some other chrysler cars to go bad, which requires removal of the gas tank to repair.

Could also mean a piece of charcoal from the vapor canister is stuck in a line. Blowing the lines out can fix it.

If you get a P0455 (large leak) at the outset it more likely a stuck valve or a hose which came loose.

A smoke machine is the fastest way to finds leaks in these systems but not a common tool for do it your selfers.

Technical Service bulletin (TSB) 2500401 points out that, while most leaks will be external such as a loose gas cap or damaged hose, internal leaks also are possible. The most likely source of an internal leak is the Duty Cycle Purge valve (DCP). This valve could be stuck open due to corrosion, dirt from outside or debris from inside the system, such as charcoal from the vapor canister. The DCP valve is tested by on-board diagnostics only after the Purge Flow Monitor is passed; if that monitor fails, the DCP valve will not be tested. The bulletin offers helpful tips for troubleshooting and understanding the logic of the EVAP diagnostic routine.

P0442 (Evaporative Emission Control System Leak Detected (medium leak))
A 0.040 leak has been detected in the evaporative system. (A faulty gas cap may cause this code).
Note: The respective Powertrain Diagnostic Procedures Manual will direct the technician to determine if the vehicle evaporative system has an external system leak. Examples of some external system leaks are: a loose gas cap, a disconnected evaporative system hose, or a loose hose connection. Remember that the evaporative system may also have an internal system leak. This internal leak may be caused by an intermittent or permanently stuck open Duty Cycle Purge (DCP) valve. This leak source is considered to be internal because any escaping emissions enter into the engine system and not directly into the atmosphere, unlike an external leak. An internal leak may cause one of the above DTC?s. Possible causes of a stuck open (intermittent or permanent) DCP valve are: 1. Corrosion due to system moisture or possible water intrusion into the system.

Also see TSB 25-001-02A for Jeep Cherokee

Check under the hood to make certain no vacuum lines are disconnected, check your fuel cap if it isn't sealing correctly or is missing or the wrong one this will cause that code to set, You may have a bad vent valve causing the problem. What this code means is that the computer has tested the evap system and was not able to draw a vacuum on the fuel tank. it does this by turning on the evap vent valve which closes the fuel tank vent off, It then opens the vacuum solenoid and allows engine vacuum to be drawn on the fuel tank. A sensor in the fuel tank monitors the Pressure/Vacuum levels on the tank and reports this to the computer.If the computer does not see a change in pressure/vacuum on the tank then code P0455 will set.

Does customer regularly commute on a steep grade (down hill grades in excess of 10%? Has the Leak Detection Pump Operation and Diagnosis booklet (Publication No. 81-170-000606) been used to assist in the diagnosis?

Review and if necessary, perform TSB 18-010-02. Be sure to completely read the TSB. The fix for the condition is a PCM replacement, not a reprogramming of the controller. Be sure that there is not any leaks in the system. This TSB applies in a very limited number of cases and usually in mountainous regions. TSB 18-010-02 Is a PCM replacement correct an erroneous MIL illumination for SAE P0456 that sets when driven on steep grades.

Also see See TSB's 25-001-02A, 18-002-03 and 25-004-01. for Jeep Cherokee

TSB 25-001-02A Says:
Raise the vehicle on an appropriate hoist. Locate the LDP vent valve. Remove the vacuum hose from the top of the vent valve (Fig. 1). Attach a hand vacuum pump to the vent valve and apply 20" Hg. vacuum. Observe the vacuum for 10 seconds. If the vacuum holds between 15.5" Hg. and 20" Hg., refer to the appropriate Powertrain Diagnostic Procedure Manual, MDS2, or the Leak Detection Pump Operation and Diagnosis booklet (Publication No. 81-170-000606) for further diagnosis. If the vacuum drops below 15.5" Hg., obtain the appropriate LDP vent valve using the Mopar parts catalog and perform the Repair Procedure.

May need a new Leak Detection Pump (LDP) vent valve (behind left rear wheel on Cherokee).

Links:
EVPAP Code Blues at Search-autoparts.com