What could cause a code P0420 and P0128 on a Chevy Trailblazer?

[Joe S]

I have a 2004 Chevy Trailblazer and I failed my smog test in Las Vegas due to a code P0420 and a code P0128. I know the P0128 has to do with the coolant thermostat and the P0420 I'm not sure. Any ideas of possible causes and checks I can do? I have an appointment at a repair shop on Monday and I want to be as informed as I can so I don't get ripped off.

Also, if the P0420 happens to be a bad catalytic converter, any idea on a cost to replace?

I'm a teacher not a mechanic, but I still don't want to get taken for a ride. It's almost impossible to find a good mechanic in Las Vegas.

I will tell you that I had a fender bender about a year ago. Had the bumper and passenger headlight replaced. My speedometer, temp, and fuel gauge stopped working and now the emission problems. I think the repair shop screwed me and it's past the warranty. Thanks insurance company!

[NOTOBAMA]

P0420 - Catalyst System Efficiency Below Threshold

The catalytic converter has an oxygen sensor in front and behind it. When the vehicle is warm and running in closed loop mode, the upstream oxygen sensor waveform reading should fluctuate. The downstream O2 sensor reading should be fairly steady. Typically the P0420 code triggers the Check Engine Light if the readings of the two sensors are similar. This is indicative of (among other things) a converter that is not working as efficiently as it should be (according to specs). It is part of the vehicle emissions system.

A code P0420 may mean that one or more of the following has happened:

Leaded fuel was used where unleaded was called for
An oxygen sensor is not reading (functioning) properly
The engine coolant temperature sensor is not working properly
Damaged or leaking exhaust manifold / catalytic converter / exhaust pipe
Retarded spark timing
The oxygen sensors in front and behind the converter are reporting too similar of readings.


P0128:
P0128 OBD-II Trouble Code
Technical DescriptionCoolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature)

What does that mean?This means that the engine's PCM detected that the engine has not reached the required temperature level within a specified amount of time after starting the engine. The intent of the P0128 code is to indicate a faulty thermostat. Similar codes: P0125

In determining the engine did not reach a normal temperature, it takes into account the length of time the vehicle has been running, the intake air temperature (IAT) sensor reading, the engine coolant temperature (ECT) sensor reading, and the speed of the vehicle.

CausesA code P0128 may mean that one or more of the following has happened:

Low engine coolant level
Leaking or stuck open thermostat
Faulty cooling fan (running too much)
Faulty coolant temperature (ECT) sensor
Faulty intake air temperature (IAT) sensor

[baldie]

po420 means catalyst system efficiency below threshold ( bank 1 ). you can buy a universal car a lot cheaper then the factory type.