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Every 2-3 tanks of gas, my "check emission system" light comes on and the code that comes up is P0420. I went to get the car inspected yesterday and the light was not on. It failed. The gas cap was good. No code came up. But the two faults were Heated 02: Not Ready and Evap Sys: Not Ready. I should also mention that when the check emission light comes on, I'll disconnect the battery and it resets. It had been about a week since I last did that and performed the inspection yesterday. I'm not a mechanic and am not familiar with car or technical terms, but I would like some advice from anyone who might have experience with this issue. I don't want to spend hundreds just trying to diagnose the issue then have to shell out whatever it costs to fix it on top of that. I understand it might not be a straight forward one-answer fix. But as much insight as I can get on this problem would be helpful. Thanks!
 

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P0420 relates to a bad oxygen sensor. These sensors monitor your air/fuel ratio before and after the cats. Have you noticed any issues with the car using more gasoline than normal?

When you clear the check engine light by removing the battery. Youre resetting most electronics which includes Evap Sys.
Youre seeing Not Ready because the system was still performing its checks against that system.
How far have you driven from when you last reset the CEL ( check engine light ) and had the car inspected?

It takes quite a bit of mileage for all systems to complete their checks.
 

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It had probably been 300-450 miles since I last disconnected the battery. How do I know when it's ready? What's a safe number of miles before I try getting it inspected again. If I can at least get the Evap Sys to read ready, It will pass.
 

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Most cars are back to "ready" status after a single 50 mile trip.

I use a small cheap bluetooth code reader connected to my phone, which will show status of these systems.
 

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On my 2009, probably same on all, if you turn ignition switch forward to on position, DO NOT START CAR, the check engine light will go off after about 10 seconds. If it flahes 5 times, car has not fully completed readiness cycle. My p0420, after replacing cat and o2 sensors took about 600 miles to disappear.
 

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On my 2009, probably same on all, if you turn ignition switch forward to on position, DO NOT START CAR, the check engine light will go off after about 10 seconds. If it flahes 5 times, car has not fully completed readiness cycle. My p0420, after replacing cat and o2 sensors took about 600 miles to disappear.
600 miles in the bronx ? were you driving in circles ? :D lol
 

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600 miles in the bronx ? were you driving in circles ? :D lol
lol, no. i reset light but code was still in cpu. mixed driving and alot of highway. even did the recomended drive cycle a few times. it just took that long for it to clear. idk why.


You will need to drive your Honda for at least 200-300 miles through various driving conditions in order to complete the required Emission Monitors. Sometimes simply driving an extend length of time does not necessary mean the emission monitors will run. Your Honda’s emission monitors are internal self-tests which the Engine Control Unit performs to ensure your is not polluting and is running efficiently. The entire process of self-testing various emission control systems is called a Drive Cycle. Follow the emission drive cycle trace below. Obey all traffic laws while performing drive cycles. Perform at your own risk.
HOW TO PERFORM A HONDA DRIVE CYCLE
The following is a Honda OBDII Drive Cycle Pattern.
COLD START: Begin from Cold Start, and ignition in OFF position for at least 1 hour. ensure your Honda’s fuel tank is between 1/4 and 3/4 full and engine coolant temperature must be below 122°F and within 11°F of the ambient air temperature. Do not leave the key on prior to the cold start or the heated oxygen sensor diagnostic may not run.
IDLE TIME: Run your Honda’s engine (do not drive) for 2 1/2 minutes with the air conditioner on and rear defroster on. During this period the ECU will be running the Oxygen Sensor Heater self-test, Air Injection System Passive Air test, EVAP Purge "No Flow" test, Misfire Detection.
ACCELERATE: Turn off the air conditioner and rear defroster and begin driving. As soon as possible apply half throttle until 55mph is reached. The ECU will now run self-tests on the Misfire Detection, Fuel Trim, and EVAP Purge Flow.
HOLD STEADY: Now hold your vehicle speed steady at 55mph for three minutes. During this time the Oxygen Sensor O2 response, Air Intrusive (AIS System), EGR Flow, Purge Valve, Misfire Detection, and Fuel Trim diagnostics will be performed.
DECELERATE: During this step you want to slowly let off the accelerator pedal. Do not brake or shift your Honda in order to slow down. It is important to let the vehicle coast along gradually slowing down to 20 mph. The ECU will once again self-test the EGR System, EVAP Purge System and Long Term Fuel Trims.
ACCELERATE: Go ahead and accelerate again at 3/4 throttle until 55-60mph is reached. This will perform the same self tests as during the first acceleration step.
HOLD STEADY: Hold steady speed once again. This time at 55mph for five minutes. During this time, in addition to running the self-tests as during the first HOLD STEADY step, the ECU will perform the catalyst monitor diagnostics. If the catalyst (Catalytic Converter) is marginal or the battery has been disconnected, it may take 5 complete driving cycles to determine the state of the catalyst,or in order to Complete the CAT Monitor.
DECELERATE: This will perform the same diagnostics as in the first Deceleration. Remember not to press your Honda’s brake or shift gears during deceleration.
This should complete the drive cycle on your Honda. To ensure all monitors are complete use a scan tool to view the monitors. You may want to complete the drive cycle 2-3 times.
 
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