Codes P0128 and P420

JayWildChevy

Original poster
Member
Dec 12, 2014
35
Ahhh, more CEL! Fun fun...

My truck is throwing codes P0128 and P0420.

I noticed, yesterday that the thermometer reads that my truck stays at 150, even after 45 minutes of driving, 60-80mph. Which is where the P0128 is coming from.

What could be the cause of this? Is it just a bad ECT sensor? Code this be leading to causes for the P0420 or not related at all? I haven't had my sensors or converter tested yet.

IMG_27021_zpss9zscckf-1.jpg
 

freddyboy61

Member
Dec 4, 2011
276
If you have been running with a low temperature (or at least the PCM thinks it is that low), the P0420 can certainly be related because with low operating temps, the PCM will richen up the fuel/air mixture and the catalytic converter will start to clog.

Change both the thermostat and temp sensor, then see what happens to the temp reading.
 

IllogicTC

Member
Dec 30, 2013
3,452
I see you have an '06... the ECT sensor is going to be fun for you. It used to be right next to the thermostat, perfect 2-in-1 job opportunity. Now it's located at the rear passenger side, there are threads around here cursing about it and explaining what people ended up doing to get to it.
 

seanpooh

Member
Jan 24, 2012
461
Thermostat is the first fix. Since it's winter, it leads to adds to horrible MPGs do to the engine trying to warm up thus bring more gas.

It's easier to replace when you remove the alternator (more room, also make sure to have a 15mm ratcheting wrench).The ECT is optional but you might at well.

As for advice of changing the ECT, I have a 2002 so no help there.
 

djthumper

Administrator
Nov 20, 2011
14,950
North Las Vegas
on the 06 I would change the thermostat first and that should resolve the P0128 and low temp reading. Hopefully that will clear up the P0420, if it isn't too far gone.
 

JayWildChevy

Original poster
Member
Dec 12, 2014
35
Hey y'all!

Thanks for the advice!

I replaced the Thermostat, ECT sensor, and O2 sensor to solve the p0128 and p0420 codes! I'll see if my heat starts working correctly over the next 24 hours before I realize it was a separate issue or not! But, I am almost positive it will solve everything.

My truck is, now, running optimally!

My friend, that's a mechanic is impressed how great the engine is.

Thanks y'all!
 
Feb 16, 2012
202
Reviving this thread because, well I can.

Had a lot of work done on my TB in the last few months for a variety of issues. Included in them were, of course 0128 and 0420.

Replaced the thermostat and alternator (since I had to remove it anyway, and it is probably the original).

The 0128 has gone away, Check Engine light gone, took it to shop for inspection and it failed because 2 monitors were not "ready" yet.

So no codes for the moment until I get the monitors ready, but experiencing great deal of it "bogging down" as I accelerate.

I have read a variety of threads and some say to try to replace the O2 sensor(s), others say just do the cat.

'05 LS 4.2 ~130k
 

Mooseman

Moderator
Dec 4, 2011
25,257
Ottawa, ON
Try removing the O2 sensor and see if the bog improves. That would mean a clogged cat.
 

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