5 unique Check Engine Codes (12 codes, total) --- Transmission not getting above 3rd / 4th gear


Asked by Oct 24, 2014 at 09:44 AM about the 2009 Chevrolet Avalanche LT2 4WD

Question type: Maintenance & Repair

I have a 2009 Chevy Avalanche LT-2 (4WD), purchased in 2012.  The check engine light
came on, had it scanned and they told me it was a loose / faulty gas cap.  So, I
changed the gas cap, light went out and then recently came back on.  When I had it
scanned today, the light was on, but the truck appeared to work fine (mechanic may
say otherwise, but from a layman's perspective, it operated as I would expect it to).

The scan came back with 12 total codes, 5 of them being unique: U0100, P0449,
P0700, U0073 and P0445.  What appeared strange, to me at least, when I got out onto
the highway, after the engine light scan, the truck could get up to 3rd or 4th gear and
then just stayed there, not shifting into 5th or 6th.  Flashers went on and couldn't get
much beyond 45 - 50...not a fun speed on an at-speed 495 (DC Metro Area's 8+ lane

According to the Zorac OBD Check Engine Code Android App (not trying to advertise
for them, just that it seemed to work well for me in trying to find code information):
1. Chevrolet P0445
Evap Emission Ctrl Sys Purge Ctrl Valve CircShorted

2. Chevrolet U0100
Lost Comm With ECM / PCM A

3. Chevrolet P0449
Evap Emission Ctrl Sys Vent Valve / Solenoid CircError

4. Chevrolet U0073
Control Module Communication Bus Off

5. Chevrolet P0700
Transmission Control Module (TCM) Requested MIL Illumination

Aside from, "Take it to your mechanic, yesterday!" any recommendations?  I know that
disconnecting the battery won't fix the issue, but would that clear the codes long
enough to safely get the truck to the shop?  There are enough back-roads that I could
avoid the highways on the way home, would have this morning had the transmission
acted up before I hit the highway (not trying to drive dangerously nor ignorantly).

Many thanks in advance!!

4 Answers


have you checked to see if adequate air is getting in and out of the engine it sounds like its a vacuum leak

1 of 1 people found this helpful.

Thank you, Crumbob. I'll take a look. I appreciate the answer.


There was a blown emissions sensor that needed to be replaced. The transmission fluid needed to be completely flushed & replaced. Although not required at the time, it looks like I'll need my coolant & power steering fluid fully drained & replaced, too. ~$750 - 800 for the sensor & transmission work. Quoted ~$325 - 350 for the coolant & power steering fluid replacement. In the ensuing weeks, there have been no subsequent issues...*knock on wood*.... My normal mechanic wasn't able to do the work, but our local Shell handled the task. According to my mechanic, Shell did good work and didn't overcharge on the work.


My check engine light came on this morning. I have a 2005 Avalanche. Any reasons ?

4 of 4 people found this helpful.

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