Question regarding a honda odyssey 1999 check engine light codes.

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DTC P0420: CATALYTIC SYSTEM EFFICIENCY BELOW THRESHOLD
Perform PCM reset procedure. See SELF-DIAGNOSTICS - INTRODUCTION article. Start engine and run
at 3000 RPM with no loads and transmission in Park or Neutral until radiator fan comes on. Test drive vehicle
at 40-55 MPH for about 2 minutes. Decelerate for at least 3 seconds with throttle fully closed. Reduce vehicle
speed to 35 MPH, and hold it there until scan tool indicates FAIL or PASS for TWC test result. If test result is
not FAIL, problem is intermittent. System is okay at this time. If test result is FAIL, inspect Three-Way
Catalytic Converter (TWC) for plugging, melting, cracking, or exhaust system leaks. Repair or replace as
necessary.
DTC P1456: EVAP SYSTEM LEAK DETECTED
This code is set after symptom is detected during 2 consecutive driving cycles. Certain specific driving and
ambient conditions must occur before PCM will complete system checks. Additional test drives may still not
meet specific conditions needed to reproduce code. Test system using less than a full tank of fresh gas. If
possible, add one gallon of fresh fuel to tank just before starting test. Ensure extra gallon will not fill tank. DO
NOT deviate from vacuum and pressure in procedures. Excessive pressure or vacuum may cause fuel tank
failure.
Fuel Tank Filler Cap Check
Ensure fuel tank filler cap is a Gray OEM cap and is tightened at least three clicks. Check filler cap seal. If seal
is missing or damaged, replace filler cap. Check fuel tank filler pipe. If fuel tank filler pipe is damaged, replace
filler pipe. If filler cap and pipe are okay, go to EVAP PURGE CONTROL SOLENOID VALVE TEST .
EVAP Purge Control Solenoid Valve Test
1. Disconnect hose from EVAP purge control solenoid valve as illustrated. See Fig. 12 . Turn ignition on.
Apply vacuum to hose. If vacuum does not hold, go to next step. If vacuum holds, EVAP purge control
solenoid valve is okay at this time. Perform EVAP by-pass solenoid valve test. See EVAP BY-PASS
SOLENOID VALVE TEST .
2. Turn ignition off. Disconnect EVAP purge control solenoid valve 2-pin connector. Check for continuity
between ground and EVAP purge control solenoid valve harness connector terminal No. 2 (Red/Yellow
wire). If continuity exists, go to next step. If continuity does not exist, replace solenoid valve.
3. Disconnect PCM 32-pin connector "A". Check for continuity between ground and EVAP purge control
solenoid valve harness connector terminal No. 2 (Red/Yellow wire). If continuity exists, repair short in
Red/Yellow wire between EVAP purge control solenoid valve and PCM 32-pin connector "A" terminal
No. 6. If continuity does not exist, substitute a known-good PCM and recheck. See SUBSTITUTING
PCM in SELF-DIAGNOSTICS - INTRODUCTION article. If symptom or problem goes away, replace
original PCM.
EVAP By-Pass Solenoid Valve Test
1. Disconnect vacuum hose from EVAP 2-way valve as illustrated. See Fig. 13 . Using backprobe set,

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NOTE: If DTCs P0137, P0138 and/or P0141 are set, trouble shoot DTC P0420 last.

connect a fused jumper wire between ground and PCM 32-pin connector "A" terminal No. 3 (Blue wire).
Turn ignition on. Using vacuum pump, slowly apply vacuum to EVAP 2-way valve as illustrated. See
Fig. 13 . If vacuum holds, go to next step. If vacuum does not hold, go to step 4 .
2. Turn ignition off. Disconnect EVAP by-pass solenoid valve 2-pin connector. Check for continuity
between ground and EVAP by-pass solenoid valve harness connector terminal No. 2 (Blue wire). If
continuity exists, go to next step. If continuity does not exist, repair open in Blue wire between EVAP bypass
solenoid valve and PCM 32-pin connector "A" terminal No. 3.
3. Turn ignition on. Measure voltage between ground and EVAP by-pass solenoid valve harness connector
terminal No. 1 (Black/Yellow wire). If battery voltage exists, replace EVAP by-pass solenoid valve and
"O" rings. If battery voltage does not exist, repair open in Black/Yellow wire between EVAP by-pass
solenoid valve and No. 6 PCM CRUISE CONTROL (15-amp) fuse, located in left side under dash
fuse/relay box.
4. Plug upper port of EVAP 2-way valve. See Fig. 14 . Using scan tool, check Fuel Tank Pressure (FTP)
sensor voltage, or using backprobe set, measure voltage between PCM 32-pin connector "A" terminal No.
29 (Light Green wire) and 31-pin connector "C" terminal No. 18 (Green/Black wire). Slowly apply
vacuum, one stroke at a time. Go to next step.
5. Voltage should drop smoothly to about 1.5 volts and hold for 20 seconds. If voltage does not drop to
about 1.5 volts and hold, repair leakage from EVAP by-pass solenoid valve, EVAP 2-way valve, FTP
sensor and "O" rings. If voltage drops to about 1.5 and holds, EVAP by-pass solenoid valve and EVAP 2-
way valve are okay at this time. Perform EVAP control canister vent shut valve test. See EVAP
CONTROL CANISTER VENT SHUT VALVE TEST .
Fig. 12: Disconnecting Hose From EVAP Purge Control Solenoid Valve
Courtesy of AMERICAN HONDA MOTOR CO., INC.
Fig. 13: Locating EVAP 2-Way Valve Vacuum Hoses
Courtesy of AMERICAN HONDA MOTOR CO., INC.
Fig. 14: Locating Upper Port Of EVAP 2-Way Valve
Courtesy of AMERICAN HONDA MOTOR CO., INC.
EVAP Control Canister Vent Shut Valve Test
1. Disconnect vacuum hose from EVAP control canister filter as illustrated. See Fig. 15 . Turn ignition on.
Apply vacuum to hose with 5 strokes of pump. If EVAP control canister vent shut valve holds a vacuum,
go to next step. If EVAP control canister vent shut valve does not hold a vacuum, EVAP control canister
vent shut valve is okay at this time. Perform vacuum hoses and connections test. See VACUUM HOSES
& CONNECTIONS TEST .
2. Turn ignition off. Disconnect EVAP control canister vent shut valve connector. Check for continuity
between ground and EVAP control canister vent shut valve harness connector terminal No. 2 (Light
Green/White wire). If continuity exists, go to next step. If continuity does not exist, replace EVAP control
canister vent shut valve and "O" rings.
3. Disconnect PCM 32-pin connector "A". Check for continuity between ground and EVAP control canister
vent shut valve harness connector terminal No. 2 (Light Green/White wire). If continuity exists, repair
short in Light Green/White wire between EVAP control canister vent shut valve and PCM 32-pin
connector "A" terminal No. 4. If continuity does not exist, substitute a known-good PCM and recheck.

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See SUBSTITUTING PCM in SELF-DIAGNOSTICS - INTRODUCTION article. If symptom or
problem goes away, replace original PCM.
Fig. 15: Locating EVAP Control Canister Filter Vacuum Hose
Courtesy of AMERICAN HONDA MOTOR CO., INC.
Vacuum Hoses & Connections Test
Ensure fuel tank filler cap is tightened at least three clicks. Using scan tool, monitor Fuel Tank Pressure (FTP)
sensor readings. Start engine and allow it to idle for 5 minutes. If readings are more .16 in. Hg. pressure or less
than .16 in. Hg. vacuum, substitute a known-good PCM and recheck. See SUBSTITUTING PCM in SELFDIAGNOSTICS
- INTRODUCTION article. If symptom or problem goes away, replace original PCM. If
readings are not more .16 in. Hg. pressure or less than .16 in. Hg. vacuum, check the following components for
leaks to atmosphere and repair or replace as necessary:
􀁺 Fuel tank leak.
􀁺 A faulty fuel filler cap.
􀁺 Fuel fill pipe.
􀁺 EVAP 2-way valve malfunction.
􀁺 Fuel tank pressure sensor malfunction.

Ramon
P0420 relates to the catalytic converter. The computer sees little or no improvement in the exhaust before and after going through the converter. Either your oxygen sensors need replacement (3 total) or you need a new catalytic converter.
What is the mileage on the vehicle? Does it run well? Oxygen sensors generally start to fail after 70,000 miles. Catalytic converters usually fail because the engine is runnig rich and the owner ignores that fault for months. This causes the catalytic converter to overheat and fail.

P1456 relates to the evaporative emission system. This system holds the gas fumes in the system so they are not vented to the atmosphere. The most common cause of this code being set is failing to properly tighten the gas cap after refueling. Take the cap off and clean the sealing O-ring. Put it back on and tighten till it clicks 3 times. Make sure you have the correct gray colored gas cap. Clear the code and see if it resets within a few days. If it does, try replacing the gas cap.
Charlie