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Discussion Starter #1
I have a 2012 GMC Terrain SLT. 105,000 miles

I am getting code P0420 and P0420pd MiL On Monitors 1 Inc Evap 7 Ready Misfire Fuel Comp Catlyst 02 Sensor 02 Hrt EGR.

I have changed both 02 sensors. One of them defiantly needed to be changed. But the check engine is back with the same 2 error codes. This has been going on for months. Changed one 02 sensor then the next. The check engine light will come on every few days to every few weeks.

3 times in the last month the vehicle has now stalled when first starting up and not driving away immediately. Not sure if this is related but also a concern.

Any thoughts on what to try next. We like to fix things ourselves than pay too much at a garage.


30 Posts
There seems to be a lot of fantasy and misinformation on the internet about this code and other emissions codes, and wrongful attribution of problems and cures; so let's be clear about what this code is and why it appears.

P0420 Permanent Code is the ECU saying the "second" O2 sensor(s) after your first catalytic converter(s) are not sending the signals the computer expects in relation to the signals the first O2 sensor(s) located in the exhaust manifold, are sending.
Here are my responses to some myths I've seen and heard on websites and Youtube:
1. O2 sensors are not a kind of temperature sensor that generates voltage.
2. You cannot "fool" the computer by applying voltage to the O2 sensor lead.
3. Catalytic converters DO NOT store oxygen, and furthermore, do not release oxygen when it's needed!

What isn't a myth is this:
O2 sensors generate voltage based on the oxygen DIFFERENCE between the outside air, and the exhaust gas stream. (The greater the difference the higher the voltage)

First, P0420 can only be cleared by the ECU (engine computer) after several drive cycles where no issue is detected; you cannot use a scan tool to clear the error. Second, to understand this particular error code you need to know what the computer wants to see in various conditions. So the 1st O2 sensor generates aDC voltage between 0v and 1v, after it is sufficiently hot, based on the oxygen content of the exhaust, compared to the outside air. In a fuel lean condition, there will be more oxygen in the exhaust stream since not all of it was used in combustion (because you're running lean), and therefore you will have a low voltage value generated by the O2 sensor. When the ECU adds excessive fuel to the air/fuel mix, creating a rich condition, there will be less oxygen left after in the exhaust stream and therefore a higher voltage is generated by the O2 sensor. This lean/rich/lean/rich condition created by the computer, occurs a few times a second in a PROPERLY WORKING SYSTEM, and can be seen by monitoring the voltage of O2 S1; you will see the voltage swing from 0 to possibly as high as 1.2 at idle and while driving.

Now how does this relate to what O2 S2 (the down-stream/after the catalytic converter sensor) is generating?
First let's understand what the catalytic converter is doing:
In almost every driving condition, EXCEPT coasting with the throttle closed (not stepping on the gas pedal), the catalytic converter will use the free oxygen and that, that has bonded to nitrogen (making the dreaded smog-creating NOx molecules) during the high temperatures and pressures particularly during a lean combustion condition, and break off the oxygen from that NOx bond, and use that oxygen to further burn any hydrocarbons left over in the the exhaust stream, and add an oxygen to the poisonous carbon-monoxide molecule, to make CO2 (the gas consumed by plants).

So what does the computer want to see from O2 S2 ?
What the computer is looking for in most driving and idling circumstances is for a high voltage signal from O2 S2, no matter what the first O2 sensor signal is. When O2 S2 has a persistently high voltage, that means most of the free oxygen has been used, and that only happens if:
1. the catalytic converter isn't destroyed by excessive oil burning, coolant leaking into the exhaust stream, burned out by overheating (excessive lean condition for a long time) and is otherwise doing its job.
2. there wasn't excessive air (extra lean) in the original combustion mix.

And #2 is usually the culprit in most circumstances, because it is easy to create an "extra-lean" condition.
How? Well here's a list:

1. Dirty air filter (always try replacing this first)
2. Bad o-ring seal on the PCV inlet pipe that connects to the air box.
3. Lose clamps on air-box connections with the filter housing or throttle-body.
4. Cracked PCV pipes (yes, these are plastic pipes, not rubber hoses, and they do crack; especially when some technician unwittingly crushes them as he's leaning over the engine.)
5. Dirty MAF sensor. (Try cleaning this sensor that is found between the filter box and the air-box.)
6. Bad o-rings on the oil level dipstick. (these are usually cracked, compressed, or missing)
7. Bad seal on the oil filler cap.
8. Bad throttle-body o-ring.
9. Bad intake manifold seals.
10. Other bad gaskets/seals in on the engine.

Unlike old engine designs, new engines are supposed to be a sealed environment, with air entering only in a controlled and metered way. You can test for major leaks with a can of starter fluid, just don't spray it near the exhaust manifold. If you have a leak at any of the air handling, throttle-body, intake manifold, or vacuum pipes, the starter fluid will make the engine idle accelerate temporarily. Starter fluid has almost no ability to test the dipstick seals, the oil filler cap seal, bad valve cover gasket, or other general engine seals, because it would take to long for the starter fluid ether to get to the combustion chamber.

As far as a destroyed catalytic converter described earlier, this can be inexpensively checked by removing the first O2 sensor in the exhaust manifold, and putting a bore-scope into that hole and viewing the catalytic converter's "screen". First, it should not be black with carbon build-up; if working properly, it gets hot enough to burn off everything that touches it, and should look gray or dark-tan in color, not black. The screen should be unclogged, and not burned away. Either of those problems are an indication of excessive oil consumption.

P0420 code will clear itself if after several drive cycles the computer sees O2 S2 creating a voltage consistently above 0.5v and only going below that during a throttle closed "coasting". Under load (driving down the highway, climbing a hill, acceleration) the voltage of O2 S2 should be consistently high and not drop below 0.5. But a vacuum leak can cause this to happen because under load, when vacuum drops, vacuum leaks have less impact on the computer's lean/rich fuel calculation, than when not under load and the high vacuum is now pulling in un-metered air and leaning out the mixture . A sure sign of a vacuum leak is that the LTFT (long-term fuel trims) will be high, like over +10% during most driving situations.
Also, in particular to the evap code, check that you don't have a cracked evap tube; these are plastic in certain places just like the PCV tubes, and can crack with age or a misplaced hand.
And keep those defiant O2 sensors in line!
Good luck.
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