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johnt, ASE Certified Technician
Category: Chevy
Satisfied Customers: 85
Experience:  27 years experience foreign and domestic
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I have a 2005 Chevy Silverado that shows a PO455 code. I removed

Customer Question

I have a 2005 Chevy Silverado that shows a PO455 code. I removed and re-tightened the gas cap but the code didn't go away. Does the need to be cleared? If so, how do I clear OBD code?
Submitted: 7 years ago.
Category: Chevy
Expert:  johnt replied 7 years ago.

The gas cap is not the only possible cause , this code shows a large evap system leak.

Other potential causes are as follows.

  • Fuel filler cap is very loose, missing or the wrong part
  • Fuel tank, fuel filler neck or fuel sending unit 'O' ring is leaking
  • Fuel tank pressure sensor is damaged, disconnected or it failed
  • Fuel tank vapor line(s) is clogged, damaged or disconnected
  • Purge valve vapor line is clogged, damaged, or disconnected
  • Purge solenoid is not opening (it may be damaged or sticking)
  • Vent solenoid is not closing (it may be damaged or sticking)
  • PCM has failed

If in fact the gas cap was the problem the code will reset itself after the next test cycle

runs . Here are the parameters for the test to run.


  • At least 17 hours have elapsed since the last drive cycle met the test conditions.
  • MIL is off.
  • No Emission related DTC's are present.
  • Accessories turned off (A/C, fan motor, headlights and rear window defroster).
  • BARO signal over 74 kPa.
  • Battery voltage between 10-18v.
  • ECT and IAT signals between 39-86ºF.
  • Difference between the ECT and IAT signals at startup less than 14ºF/
  • Estimated ambient temperature at the end of this drive cycle between 39-86ºF.
  • Fuel level between 15-85%.
  • Vehicle speed less than 75 mph.

You can reset with a scan tool , the large parts chains will usually scan and clear

the codes for free. Also check the o'ring on the gas cap for cracks.