Since last fall, I have had the P0128 error code. It stays in the yellow & never turns on the check engine light. The code never goes away. I have tried clearing the code, but it keeps coming back. Since it never goes into red, does that mean there is no real issue, or is it something I should get fixed? I don’t want to mess with it if it’s a bug in the software, or will go away once the weather warms up here in Los Angeles.
That code indicates that the thermostat might be stuck open.
You want to fix this because it may be effecting your fuel mileage.
Tester
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What is on if the check engine light isn’t on? I’m guessing that yellow means ‘get it fixed’ and red means ‘get it towed’. It’s not a software glitch.
I’m guessing OP is checking for diagnostic codes pro-actively using their own scan tool. Yellow means code is pending. As mentioned above, coolant may not be reaching the proper operating temperature, which is an emissions issue. At this point it isn’t bad enough to turn on CEL.
Thermostats are located in a very corrosive environment and will often begin to warp over time, which allows coolant to flow into radiator before it should, lowering coolant temperature. Seems a good thing to have a slightly cooler engine, but EPA wouldn’t concur.
Problem is most likely warped thermostat, or less likely, faulty coolant temperature sensor. Make sure to change-out coolant with fresh stuff per manufacturer’s time schedule.
What do you mean by it stays in yellow?
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No, the engine runs more efficiently with the coolant hot and it does no harm as long as the cooling system is in good condition.
OBDII meter stays in the yellow. Never goes to either green or red.
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That’s the thing. I have been getting about 28 MPG on a car that’s 17½ years old. Best gas mileage ever. I really don’t check city driving, since I don’t do much of that. Would there be any other complications with letting it stay as is? It passes CA smog with just one code; it did last October.
Thank you. Changing the coolant is about due right now. Thanks for the reminder. If the rain has finally stopped, I’ll do that ASAP. That’s exactly the kind of advice I was looking for.
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What’s an OBD meter? Do you mean the warning light on the dash? It’s only yellow. It can’t be green or red. It has to be yellow per federal regulations. If it’s on, something is wrong.
I doubt CA passes a car for smog with a check engine light on, but I could be wrong. It’s been many years since I worked there.
I put the OBDII meter on the car every month or two just to see if there are any issues. Gas mileage is great & the cars runs with no issues, even though it will be 18 years old in October.
No light on at all on the dash or anywhere. It’s the meter you can buy for $20 & up that hooks up to a connection in your car that tells if there are any issues with the emissions, etc.
Tester
You are correct, automatic emissions test failure (at least in my area of Calif ) if check engine light is on. But it doesn’t appear OP’s check engine light is on. Just a pending code noted on a code reader. Calif emissions testing allows pending codes. (As long as the car’s own OBD II emissions tests show “complete” status. )
Note to OP: fyi, referring to an OBD code reader as an “OBD meter” may cause confusion when discussing this issue will mechanics. Suggest to use the term “OBD Code Reader” or just “code reader”.
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Thank you. The heater works perfectly, so the thermostat is not open, according to your article. I will flush the radiator, as once person suggested & see what happens after that. Wish me luck.
Check engine light is not on, so that’s why it passed the smog test in October. Like someone else said, it’s only a Pending Code (which on my code reader shows a yellow light).
That doesn’t mean a damn thing.
The temp out the vents can be 140 degrees and feel fine.
But if it requires that the coolant reach 150 degrees before the computer goes into the closed loop mode, the car is going to waste fuel.
.
Speaking of waste?
I’m not wasting my time anymore responding to this post.
Tester
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The thermostat is probably not stuck open. Often the seal on the poppet valve in the thermostat distorts and fails. This allows a small amount of coolant to pass through the thermostat during warm up. A thermostat that does not seal properly can delay full warm-up for several minutes. Best to replace the thermostat before the condition gets worse.
Thank you. That makes sense. I was wondering why it never turned on the check engine light. The leak would explain it, I guess. It also explains my gas mileage being as high as ever & the heater working normally.
I replaced the coolant & reset the codes. If the code returns, I’ll immediately get a new thermostat. I see there are several manufacturer options for it. Should I only go with ACDelco, or will any one work the same? Advance Auto: ACDelco $124, Motorad $33, Carquest $29. Auto Zone: Failsafe $39, Duralast $23, Mahle $49. Napa: Standard $21, Premium $29. If other than ACDelco is ok, do you agree Napa is the safest bet? I trust them more than any other auto parts store. Agree? Thanks for all your help. You also helped me on another issue from a year ago.