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Dave Nova
Dave Nova, GMC Technician
Category: GM
Satisfied Customers: 19450
Experience:  ASE Master Certification. GM World Class Certification
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GMC GMC Envoy 2004 GMC Envoy Service engine soon light on Light

Customer Question

2004 GMC Envoy: Service engine soon light on

Light came on solid a few days ago. Owners manual said to check the gas cap and make sure it was tight. Did that, and the light remains on.

Today, I went out, and the light was off. Went driving for about a half hour, and it came back on, with reduced engine power.

What could be causing this?
Submitted: 5 years ago.
Category: GM
Expert:  Dave Nova replied 5 years ago.
Hello and thank you for using JustAnswer. My name is David and I will do my best to help you with your GMC problem.

Usually the check engine light coming on in conjunction with the reduced engine power message is due to a fault in the electronic throttle controls. This could be a problem in the accelerator pedal position sensor, or in the throttle position sensors within the throttle body. Having the codes scanned will give you a better idea of the cause of the failure.
Customer: replied 5 years ago.
Sorry....I guess I should have stated the reduced engine power light didn't come on, just that it was driving that way. I could only get up to about 40 for a while. I pulled over for a few minutes, started driving again, and then got up to 55, but after driving for about 10 minutes, it just started slowing down.

If the problem is what you stated in your answer, any idea how much it would cost to fix? Do dealerships charge to have a code scanned? How about auto part stores?


Expert:  Dave Nova replied 5 years ago.
The auto parts stores will scan your codes for free, dealerships will charge you a diagnostic fee (usually $50-100). If you want to have the codes scanned at the auto parts store and let me know the code numbers, I will be glad to help you figure out more what the problem is. Low engine power can be the result of low fuel pressure, electrical problems with a sensor or wiring, or could be a restricted catalytic converter. Having the codes will help us to narrow it down.
Customer: replied 5 years ago.
Hi. I just went to Advance Auto Parts, and they did a scan. Here's what came up:

P0171: System too lean...Bank 1

P0172: System too rich...Bank 1

The guy was confused on this one, as he said it should be one or the other.

P0016: Cranshaft position

Camshaft Position

Correlation Bank 1

Sensor A
Expert:  Dave Nova replied 5 years ago.
Normally you would only have either P0171 or P0172, not both. Does your Envoy have the 6 cylinder or V8?
Customer: replied 5 years ago.
6 cylinder
Expert:  Dave Nova replied 5 years ago.
It would be best to clear the codes, (the auto parts store can do this with the scan tool) then drive the vehicle until the light comes back on and have the codes checked again. You will hopefully only have one code reset and this will isolate the problem further.

If the P0171 lean code comes back, the problem will likely be a failing fuel pump. Low fuel pressure will cause this code and will reduce the engine power.

If the P0016 code comes back then the problem will be in the camshaft actuator controls. Most likely a camshaft position actuator solenoid. This is located in the passenger side of the cylinder head.

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