Question about 2002 Chevrolet Monte Carlo

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Low effeciency on catalys bank 1

Engine light comes on , check with scanner and comes comes up on scanner po 420 bank 1

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  • Chevrolet Master
  • 2,101 Answers

That is the upstream o2 sensor

Posted on Jan 09, 2013


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Posted on Jan 02, 2017

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it means you have a bod O2 senser the front one as your car has two or three for a v6 but it will be the first one one in the pipe on the front side of a gm product or if your exhaust pipe comes off the back i.e. 2.3 it will be the one in the manifold itself ohh do not forget to check the plug first sometimes an over enthusiastic mechanic unplugs or tampers with these to get you to do more work check the plugs well and wires they are easily fixed

Posted on Jun 14, 2008

  • 8 Answers

SOURCE: 2006 trailblazer po 411 code check engine light comes on and off over time is it a sensor or the air pump?

Check all your vacume lines one may have a crack in it

Posted on Apr 21, 2009

  • 3854 Answers

SOURCE: 2004 chevy aveo that the

P0128 - the thermostat may be stuck open or has anyone put a colder thermostat in the car,because once it starts to get cold it will throw a SES with a colder then stock thermostat. Replace the Thermostat should fix this problem.

PO700 is a code telling you there are codes in the TCM of some late model cars where the Transmission control module has been removed from the PCM the vehicles main computer , and basically it happens when the TCM has a issue within itself or the transmission that sets a code and it sends a request for the PCM to illuminate the SES or CHECK engine light. To get at the TCM codes requires in most cases the most up to date scanner software purchased after the cars model year and as of yet is still a little iffy. Standard OBD11 scanners will generally not be able to see these codes.

P1781 is set during engine start up when the ECM & TCM do not communicate properly during initial power up. It will only illuminate the service light if the condition occurs in 2 consecutive start cycles, but the DTC will be stored in the failure records after the first cycle.

Replace the Thermostat and go to a transmission shop that can read the DTC codes with a scanner and reset the ECM for clear the codes. Most Transmission shop will do that free of charge, if not turn the Ignition key's to the on position and and pull the ECM fuse out for 30 seconds and then place it back in. Turn the ignition keys to the off position for 10 seconds and then start the car, the engine may stall but let it. Just start the engine again, the ECM is just relearning the engine and go for a test drive. This will clear all the DTC history in the ECM like the day it cam off the assembly line. Good luck and keep me posted, be glad to answer ay more question.

Posted on Oct 30, 2009

  • 3854 Answers


The PCM copares switch rates between the front O2 sensors and the downstream catalyst moniters. The catylist moniters are often mistaken as O2 sensors, and they are, but that is not their job, or what they are called.
By comparing the switch rates between the front (before cat) and rear (downstream of cat) the PCM knows if the cat is affecting the exaust, and how much it is affecting the exaust.
If it sees little or no effect o the exaust gases that have passed through the cat, then it decides that the cat is not working anymore, and sets a P0420, or a P0430, depending on which bank it is.
To check this yourself, you will need to bring up data stream on your scanner, watch the upstream and downstream O2 parameters and see if they are following each other closely.
If the cat is working right, you should see a fairly rapid switch rate, probably 5 or 6 times a minute on the front O2, but almost a flat line just above .5 volts on the rear. If the rear is switching often you proabably need a catalytic converter.
You can try resetting the ECM by turning your ignition to the on position (engine not running) and pulling the fuse to your ECM for 30 seconds and then placing it back in. Once the fuse has been reinstalled, start the car and let it idle for 1 min and then go for a test drive.
The engine may stall when you restart it, let is stumble and stall. Just restart it, the ECM is relearning the sensors like it was the first start up the engine ever had then go for a test drive.
This should reset everything and turn your check engine light off, if the check engine light came back on, have it scanned again at Auto Zone, Parts source where they scan free of charge and address the issues the DTC codes points out.
The easiest thing to do first is a visual check of things. Visually inspect the exhaust system for leaks, check the catalytic converter for dents, holes, severe discoloration, and check for a rattle inside. If any of those syptoms are there, the converter likely needs replacement.
Then, visually inspect the downstream O2 sensor (behind the converter). Check for broken wires, obvious faults, etc. If all that checks out, you'll want to check the operation of the O2 sensor. To do that, you'll need access to a scan tool or oscilliscope. Check that the waveform is pretty steady. If the reading fluctuates then the sensor is likely bad and will need to be replaced as mentioned earlier.
Thank you for using Fixya and be safe.

Posted on Apr 18, 2010

  • 5332 Answers

SOURCE: check engine light code is

There is not a problem with a sensor there is a problem with the catalytic converter. P0420 sets when the downstream o2 sensor is either reading higher than expected or reading simular to the upstream O2 sensor. Either way it is usually the catalytic converter that is the source of the problem and rarely the downstream O2 sensor.

The Catalytic converter is located in the exhaust system under the car. Catalytic converters from either the dealer or a quality direct fit part from a reputable manufacturer are recommended. Cheap cats lead to lots of problems!

Posted on Feb 13, 2011

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1 Answer

Po 420 ?

code p0420 refers to cat converter system, bank 1 efficiency below threshold===causes --cat converter--wiring HO2S sensor 2

check that the cat converter is ok and replace the sensor. ( it is a HO2S sensor which will be different to a O2 sensor so get the right one )

Sep 12, 2014 | 1998 Chevrolet Astro

1 Answer

Bank 1 p0420

Sometimes replacing the sensor fixes the problem, but more often the catalytic converter is at fault.
Code 420 means the converter is not performing up to standard. The engine computer is getting too much activity from the sensor after the cat than it should be. The number 2 sensor should be less active than sensor number one before the cat.
You would need to use a scanner to watch the data flow from the sensors and measure the temp of the cat to verify the problem.

Feb 04, 2014 | Pontiac Cars & Trucks

1 Answer

What could PO 302, PO 300, PO 105, PO106, PO 110,PO 115& PO 120 CODES Mean on a Toyota Camry 2000 model sedan car?

what engine, what size. that is first question.
misfire, map ,maf, iat, ect,tps and cat fail and low coolant temps.
very odd mix there. did you pull connections, key on???????

it means they all fail a test.
DIY? or just curious?

login to
the question is , which is the most important.
all are cept 420, (last that, and bet goes way. later.)

reset the ECU, and see which come back.?
check all fuses.
then post codes that repeat, after fuse checks.

does the engine get fully hot running, 180F minimum?

Nov 27, 2013 | 2000 Toyota Camry

2 Answers

I just got the code po 420 how do i fix it

Most likely a worn out exhaust converter

PO 420 -- Catalyst System Efficiency Below Threshold (Bank 1)
that is the right or pass side

Rear or Monitor Sensors could be bad ,if converter is less
than ten years old

Aug 06, 2011 | 1996 Ford Explorer

2 Answers

Error codes po 134 po 153 po 154 po 155 po 171 po 174


Feb 23, 2011 | 2004 Suzuki Grand Vitara

1 Answer

PO 420 This is the code that showed up. How do I check it and tell what is wrong?

P0402: Catalyst System Efficiency Below Threshold Bank 1

The ECM or PCM monitors the switching frequency ratio of heated oxygen sensors 1 and 2. A three way catalyst converter(Manifold) with high oxygen storage capacity will indicate a low switching frequency of heated oxygen sensor 2. As oxygen storage capacity decreases, the heated oxygen sensor 2 switching frequency will increase. When the frequency ratio of heated oxygen sensors 1 and 2 approaches a specified limit value, the three way catalyst (Manifold) malfunction is diagnosed.


Possible Causes:
- Three way catalyst converter Bank 2
- Exhaust tube
- Intake air leaks
- Fuel injectors may be faulty
- Fuel injector leaks
- Spark plugs may be faulty
- Improper ignition timing
- Engine Control Module

Possible Solutions:
The most common cause is the three way catalyst converter Bank 1. Changing just the O2 sensor usually doesn't repair the problem.

Jan 15, 2011 | 2005 Honda Accord

2 Answers

How to fix the problem related to engine light on code PO 420 of Toyota corolla 2000, Engine 1.8L - golden03

Code PO 420 is telling you that the Catalyst System Efficiency Below Threshold (Bank 1), so the Cat converter inst working as it should according the the vehicles computer. This can be caused from the converter going bad from age and use. Or it can also mean you have a failing rear oxygen sensor, because this sensor is instrumental in keeping track of what the converter is doing. If the car is driven on several short trips, and hardly ever driven on the highway, you may be able to burn the deposits out of the converter and restore its function. To do this you should drive it on the highway for at least two hours. After this have the light reset. If the code comes back then have the converter and oxygen sensor tested. Most shops will simply want to replace them and see if it fixes the problem. Don't fall for this, because it is expensive, and you don't need them changed unless they are bad. Get the converter tested and the oxygen sensor tested prior to replacement. The problem could be as simple as a bad electrical connection on the oxygen sensor, that is why I say have them tested.

Dec 20, 2010 | 2000 Toyota Corolla

1 Answer

1997 mercury sable po 420 bank 1 which sensor do i change.

That could be the cataylic convetor you could try the oxygen sensor behind the cat the last one to the rear of the vehicle bank 1 sensor # 3 but P0420 is TWC system witch means low efficiency in the cat but it could be a sympthom of an other problem but if the code is 0420 thats where i would start

Nov 17, 2010 | Mercury Sable Cars & Trucks

4 Answers

Why does reduce engine power light comes on chevy

That is a hard problem to pin point without knowing what codes the computer has stored. It can be any number or sensors or engine problems.

You can go to most auto parts stores and they will pull the codes for you for free.

Jul 27, 2009 | 2005 Chevrolet Cobalt

2 Answers

Po420 twc system low bank 1 code

Vehicle needs a Primary or first position Oxygen sensor, bank one just means it's the O2 sensor before catalytic converter

Jul 02, 2009 | 1999 GMC Yukon

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