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Ron Z.
Ron Z., - Chevy Tech -
Category: Chevy
Satisfied Customers: 18966
Experience:  18+yrs experience. State Inspector and Chevy Diagnostics
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2004 suburban: check engine light..tighten..fuel cap..diagnostic test

Resolved Question:

2004 suburban check engine light turned on, the following day message indicated to tighten fuel cap, after tightening cap the check engine light turned off. 3 days later light on again. On star ran diagnostic test that indicated problem with emission system. Could this be related to fuel cap?
Submitted: 6 years ago.
Category: Chevy
Expert:  Ron Z. replied 6 years ago.

Do you happen to have a "p-code"? (I think OnStar can provide this)


Customer: replied 6 years ago.
Do I contact on star to get this?
Expert:  Ron Z. replied 6 years ago.
I believe so. The problem I have is- there are literally dozens of parts/sensors in the emissions system that are monitored by the PCM. Without a p-code, I have no "starting point" for my diagnosis, and to take a "guess" isn't very efficient. With the p-code I can usually pinpoint the problem. The "p-code" we are looking for is the letter "P" followed by 4 numbers. ie. P0455 (this would be a faulty fuel cap code)
Customer: replied 6 years ago.
On star gave p0442 emissions control system problem
Expert:  Ron Z. replied 6 years ago.


Ok- P0442 is "EVAP (emissions) System Small Leak Detected"


This is actually a very tough one to diagnose. Seeing as we are looking for a "small leak", it's almost impossible to see it "by eye" or to say "This is what has the leak". The best, XXXXX XXXXX way to diagnose this problem is to have the EVAP system "smoked". This is where a machine forces smoke through the EVAP system, and the leak will reveal itself with a stream of smoke. Most shops charge around $40-$65 for this service, but it's money well spent, as you'll drive yourself nuts trying to find a small leak in the entire EVAP system.

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