The other day, I loosened my radiator cap and forgot to re-lock it. About 10 mins into my drive home, my CEL came on (and about 2 mins later, I went "DOH!")

The code is the always super helpful and easy to chase P0420. I read that one of the causes can be "Faulty engine coolant temp sensor." Obviously, the cap is now back on, but I've only driven it on one 2-mile trip, so the light is still on. Is it worth it to start pulling O2 sensors/doing other diagnostics, or is this the likely cause?

Note: The engine did not overheat, or really even run that hot according to the temp gauge. (It was about 25 degrees outside).

1996 Chevy S-10 4.3L -- 185k

  • What vehicle/model/engine are we talking about here? This will make a difference in getting you an answer. Commented Dec 27, 2016 at 9:23

2 Answers 2


The cooling and emissions systems aren't directly related.

A loose radiator cap will cause the system to not pressurize, which will cause the coolant to boil at a lower temperature. You can't rely on your temp gauge for this. If the engine is not running at it's normal temperature, it is possible it will affect the emissions coming out of it.

Did you see steam coming out from under the hood?

Why did you loosen the rad cap to begin with?

  • No, there was no steam, no loss of power or anything like that. I haven't had the trick very long, but it seems to run cool (not cold) ~about 160 on the gauge. The lower coolant line wasn't fully seated and I had to depressurize the system in order to get in all the way on.
    – bbadgett
    Commented Dec 27, 2016 at 4:04
  • I know the systems aren't directly related, but it seems like opening the glove box wrong can cause a P0420... I just had it smogged last week so I'm hesitant to think it's truley something emissions related
    – bbadgett
    Commented Dec 27, 2016 at 4:08

In some cars the engine temp is used to trim fuel. In an older Toyota, I've seen a failing ECT sensor report implausibly. The ECM would adjust fuel trim for this incorrect ECT input and the engine would stall because it was starving for fuel. This was fixed by replacing the ECT sensor. Customer called me a few days later just to say they couldn't believe that $15 part really made his car stall everyday. It really comes down to how the programmer designed the ECM.

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