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P0420 and engine coolant temp sensor. Weird problems

I have a 2007 Chevy equinox AWD. The issue started as a P0420 check engine code. Car seems to run normally other than when the code is popped the car shifts somewhat sluggishly.

Things I have replaced so far:
Both 02 sensors(the downstream was pretty clogged with carbon so worth the new part)

Catalytic converter(since returned because I’m sure it wasn’t the problem)

Spark plugs and cables

2 separate Engine coolant temp sensors(this is my main issue)

From my understanding, if the computer is getting innacurate readings of the engine’s temperature it could cause the engine to run rich. That could cause the P0420 code, right?

The reason I thought this could be the problem is on a long drive I noticed that my temp gauge was pegged and pulled over immediately. I opened the hood and everything seemed to be the normal operating temperature. So I figured it was the engine coolant temp sensor. I replaced the part twice and after resetting the code the gauge pegs and I get a message that says engine hot ac off.

The weird part of this is that as soon as the check engine code pops the temperate gauge functions properly and the car no longer gives the message engine hot ac off. I have reset the code three times and the same issue occurs.

I’ve check the wiring harness to the sensor to the best of my ability. I’m not sure what else to do. Any advice from some more qualified folks would be greatly appreciated.

Have you verified the engine coolant temperature with a separate infrared or laser thermometer . . . ?!


No. I don’t have those kinds of tools at my disposal. The gauge reads pegged from cold start. It only reads this way immediately after resetting the check engine code. As soon as the code shows up the gauge starts reading normally again.

You can get a good enough laser thermometer for around $15. Good investment. Has multiple uses. Great for using light to let dog or cat chase it. If you confirm that your coolant temperature is always where it is supposed to be, it helps you chase down why your car does not know what temperature it really is. This can help prevent buying unnecessary parts

Why are you telling ME this . . . ?! :confused:

Considering I’m the one who mentioned the laser thermometer to op, one should assume I myself possess one

Sorry was meant for OP

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I use the laser pointers from Petco to play with the cats . . . $5 tops, I believe. The laser pointers at Petsmart are more expensive, for some reason

and later on, when the batteries are weak, I buy a 4-pack of 357 batteries at Dollartree . . . can’t really beat that

To keep it car-related . . .

dollartree is also a great place to buy the batteries for key fobs. They tend to have the commonly used sizes

2xCR2016 for $1

2xCR2025 for $1

2xCR2032 for $1

the CR2032 are often also used for those fake red plastic candles, the ones you use this holiday season

several years ago, I was performing a rather large engine repair job at home, and I was reassembling the oil pump. I went to dollar tree and got petroleum jelly for $1 and packed the pump with that, before buttoning it back up

From what I see the dash coolant temp gauge uses a separate coolant temp sensor, and there’s another sensor dedicated for the computer’s use. Since one of the problems is the gauge is showing max temp even when the coolant is known to be cold, that’s probably caused either by a bad connection between the sensor and the gauge, the gauge itself is failing, or there’s a electrical system problem, most likely in the dashboard console. GM vehicles of this era tended to have some console problems often related to actuators or solder connections. I expect the gauge problem is something like that.

The coolant gauge problem is probably unrelated to the p0420 or the sluggish shifting. If the computer’s temp sensor failed (the one not related to the gauge), that might cause drivability symptoms, but seems unlikely. First b/c those sensors are simple thermistors and rarely fail. And if the coolant temp sensor failed it would usually cause rich or lean codes, not p0420. In any event the computer’s temp sensor is easy to test; a shop would just measure its resistance and compare to the spec. The scan tool temp display probably uses that sensor too, so that’s another way a shop would test that sensor.

Sometimes those coolant temp sensors have a switch built into them too. That switch might be connected to the computer to use to issue warning messages. The switch component of the sensor is a much more common failure item b/c it has a moving part. From what I see the coolant temp sensor used by the computer on the nox may have a switch included in that ass’y also. How many connector pins does that part have?

George, I thought you’d have some comment about dollar tree, given that you like to be economical, in regards to auto maintenance

I bought a laser pointer one time from a Japanese version of Dollar Tree. Or maybe it was at a flea market, can’t recall. It comes in quite handy sometimes, for example if I want to point something out to someone (like on a circuit board) and my finger would get in the way otherwise. I buy those coin-shaped batteries at the flea market too. They sell a 6 x 6 array on a sheet of cardboard, assorted battery numbers, for $4 I think. I use them all the time to replace batteries on calculators, watches, other electronic gadgets. Purchasing even one of those batteries at the drugstore would cost more than $4, so its indeed a pretty good deal for a cheapo like me … lol … as far as for these bargains being auto related, hmmm …, well I use those batteries for the multi-meter I use to test electrical system on my vehicles. And years I bought a small digital clock for $1 at a place like that, I used to glue it to the dash of my truck so I knew what time it is. One day it fell off and broke into like 10 pieces, and now I have to use the clock display on the radio.

It’s a good deal, no doubt about that

I’ve come across those assortment, as well

Unfortunately, most of them are sizes I’ll never use, and I don’t like letting stuff go to waste

Presumably, this isn’t the original radio . . . ?

Had a similar problem with a Chevy Trailblazer and it was the thermostat. The engine never came up to temp and finally the cat was destroyed. Just something to look at.

From memory and since I see it so many times the P0420 is “Catalyst efficiency below threshold” correct me if I am wrong.

Most common causes? A leak in the exhaust system and or bad O2 sensors. However coupled with the temp sensor issue you may or may not be having… it can relate to the 420 code. If the engine thinks its cold all the time it will enrich the fuel mixture…which will coat the O2 sensors as well as the catalyst matrix with excess soot…which will cause all sorts of problems that basically snowball. The Cat wouldn’t be able to properly scrub the exhaust of Nox emissions if it was heavily coated with soot. You mentioned how your old O2 sensors were heavily sooty with deposits. Well that same soot would also in the cat as well. Cats rarely go bad, but everything has its limit.

If the engine cannot measure its temp accurately you could be running very rich all the time. Is your fuel mileage very poor? Do you have any exhaust leaks?

First order of business is to suss out the temp issues you may or may not be having and also be sure you have no exhaust leaks. The engine computer cannot adjust the fuel mix correctly as long as temp or exhaust leaking is a possible problem and that is pretty important. After you sort out any temp/ exhaust issues then you can move toward the 420 and other sensors/Cat/exhaust issues.