Emissions issues P0171 and P0174
Thread Starter
CF Junior Member
Joined: Dec 2009
Posts: 59
Likes: 4
Emissions issues P0171 and P0174
GMC Sierra 1500, 2004, 4.3L, 260,000 miles.
What would be the most likely cause of failure? Is there a common part that would cause both a bank1 and bank2 failures at the same time? Unfortunately I live in a county that requires emissions testing, and Im hoping its not a Catalytic converter issue.
Whats most likely issue for a C0291 Park brake unlatch relay circuit code?
Many thanks. I dont want to just start changing parts, till it goes away.
What would be the most likely cause of failure? Is there a common part that would cause both a bank1 and bank2 failures at the same time? Unfortunately I live in a county that requires emissions testing, and Im hoping its not a Catalytic converter issue.
Whats most likely issue for a C0291 Park brake unlatch relay circuit code?
Many thanks. I dont want to just start changing parts, till it goes away.
Last edited by Syncman; Feb 26, 2026 at 8:08 AM.
CF Senior Member
Joined: May 2010
Posts: 587
Likes: 124
From: Maryland
The first thing I would do is to check your short and long term fuel trims to verify that the engine is indeed running lean.
If your engine is running lean, then start by looking for a vacuum leak such as around the throttle body, the intake manifold, etc. If you can get hold of a smoke generator and perform an EVAP system smoke test. It can reveal "hidden" leaks in the EVAP system and pinpoint vacuum leaks as well. (FYI, I recently purchased one for about $65 on amazon and it is fantastic - wish I had purchased one years ago!)
Another thing to check is your fuel injection system pressure and MAF sensor operation.
As for the C0291 goes that is a DTC for the ABS motor relay circuit or low voltage problem and not a DTC for the parking brake unlatch relay circuit. I would check the electrical connections and grounds for the ABS pump.
Good Luck!
Good Luck!
If your engine is running lean, then start by looking for a vacuum leak such as around the throttle body, the intake manifold, etc. If you can get hold of a smoke generator and perform an EVAP system smoke test. It can reveal "hidden" leaks in the EVAP system and pinpoint vacuum leaks as well. (FYI, I recently purchased one for about $65 on amazon and it is fantastic - wish I had purchased one years ago!)
Another thing to check is your fuel injection system pressure and MAF sensor operation.
As for the C0291 goes that is a DTC for the ABS motor relay circuit or low voltage problem and not a DTC for the parking brake unlatch relay circuit. I would check the electrical connections and grounds for the ABS pump.
Good Luck!
Good Luck!
Thread
Thread Starter
Forum
Replies
Last Post
alno1995
Silverado, Sierra & Fullsize Pick-ups
1
Mar 1, 2010 6:52 AM