Codes P0420, P0101 and P0300

DENALI TROUBLE

MEMBER
2002 GMC YUKON
1.6L
V8
AWD
AUTOMATIC
220,000 MILES

Truck started flashing a service engine soon light on the road this morning. About 3 miles later I started losing speed. From 70 slowly down to 40 with the RPM rising as the speed dropped. A loud whining sound came from under the hood as I pulled off the road and the truck shut off. Will not turn over now as the battery is dead. Used a scan tool from auto zone and it gave me three codes P0420, P0101, and P0300. Plugs and wires were changed four months ago, alternator was changed about two years ago. No other work has been done in the last four years. It sounded really rough as the speed was dropping and I am pretty sure it backfired a few times.

Friday, January 30th, 2015 AT 9:57 AM

1 Reply

KASEKENNY

MECHANIC
18,907 POSTS

The most likely cause of this is a restricted exhaust. You can run through the code diagram that I provided or have an exhaust shop test the back pressure to confirm this.

What is happening is the P0420 code is telling you that the converter is not reducing the harmful emissions enough so it sets the efficiency code. The P0300 code is a multiple cylinder misfire so when the exhaust is restricted it is causing misfires potentially on all cylinders at different times but not consistently on one cylinder. The P0101 is saying that the air coming into the engine is not sufficient. Normally that is because the sensor is faulty but in this case with the other codes and the lack of power it is because the air just can't pull in the air due to the restricted exhaust.

Think of this in pluming terms. If a sink drain trap is clogged then the water will drain slowly. Imagine the water is the air trying to go through the engine and the clogged drain represents the restricted exhaust.

Run through this testing or simply pull the o2 sensors and then drive and see if you get some of the power back. If so, the converters needs to be replaced.

Then you need to inspect the head gasket because oil and coolant are the most likely cause of clogged exhaust systems. I would hate to have this happen again due to a failing head gasket or a worse engine failure.

https://www.2carpros.com/articles/head-gasket-blown-test

Let me know if you have questions. Thanks

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