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Q: Service engine light keep popping up with error code
P0171 and P0174. on 2001 Cadillac Seville SLS

Repleaced MAF, PVC, air filter and checked for vacuum leak and found none. This weekend I am going to replace the Plenum boot between the intake manifild and the throttle control to make sure there is no leak.

When I raised the engine RPM pass 4000 I can feel one valve missing during the firing sequence.

Appreciate your help on this.
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this code can be very tricky to diag and repair at times. you most likely have a vac leak and on the 4.6l the boot does crack between thottle body and intake.when you remove it look on the bottom for small cracks or holes. if its ok i rec you get a smoke test which will show any vac leaks you have. ps repl the boot since it will be off and is suppect to leak at times
Thank yoo so much for your quick response. I did have the mechanic smoke test for vacuum leak and found one. I will check the Plenum boot. I have one more question, when I raise the engine speed rpm while the car is parked I can feel one cylinder speed hesitation. Do I need to chagne the park plugs or could it be a clogged injector. I did use the Chevron fuel injector cleaner and still the same proble.

Thanks in advance.
P0171 and P0174 are very similar codes. Basically this means that an oxygen sensor in bank 1 has detected a lean condition (too much oxygen in the exhaust). On V6/V8/V10 engines, Bank 1 is the side of the engine that has cylinder #1.

The P0171 is one of the more common trouble codes.

This code is triggered by the first downstream (front) O2 sensor. The sensor provides a reading of the air:fuel ratio leaving the engine's cylinders, and the vehicles powertrain/engine control module (PCM/ECM) uses that reading and adjusts to keep the engine running at that optimum ratio of 14.7:1.

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