P0125 and gas mileage decrease after tune-up

Hello!

I just had a friend (who works at Toyota) do a major tune-up on my vehicle from parts I bought from eBay. Water pump, oils, plugs/wires, filters, o2 sensors, etc. The water pump claimed to be OEM and came in Toyota packaging but he was doubtful that it was genuine.

Anyways, now I have have code P0125 - Insufficient Coolant Temperature For Closed Loop Fuel Control. My coolant level is fine and temps are looking normal. I happened to have to change the battery after this and the light still came back on after the "reset". How critical is it that I get a new thermostat?

Also, my gas mileage has decreased after this tune-up. I would think that after an oil change, fuel (and air) filter, spark plugs and wires, PCV valve, and o2 sensors, it should improve? However, he did say that the he had to reuse the gaskets for the o2 sensors. Any ideas? Could it be the CEL? Nothing else has changed.

Thanks!

Edit: It's a '97 automatic 2wd with the 5VZ and a clean title and about 224k miles. I have owned it for a little over a year. Previous owner was a reseller who got it from the auction.

 

Welcome to the Forum!

The more information we have about your vehicle the better!

Have you checked your coolant temp sensor ? Many times the cause the PO125 code.

My mileage was never better then 15mpg when I checked while loaded.

 

eBay parts are notoriously knock offs unless you find well vetted sellers. Well vetted means something along the lines of an actual Toyota dealership, or trustworthy vendor.

A non-genuine water pump sounds like you are just asking for a failure to occur. What kind of janky sensors didn't come with new gaskets?

Would have been far better to do that work in stages, but what's done is done.

Anyway, the decrease in milage could be attributed to any of the things you've done, and you shouldn't even consider comparison until the check engine light is off. When the light is on, the ECU is running a pre-mapped/non-optimized mode (open loop) where it ignores data from the sensors. During normal operation, once the systems all meet certain requirements (engine and oxygen sensors are up to temp, etc..), the ECU will switch to closed loop and optimize the AFR based on the MAF and exhaust sensors (in very over simplified terms).

 

If you plug in an obd scanner, what does it report for the coolant temp?

 

Yes, absolutely a cold cooling system will cause poor mpg. The pcm want to see the cooling system get up to operating temp quickly and stay there. The heat helps the fuel atomize (vaporize) as it goes into the cylinder. Resulting in a cleaner more efficient burn

Anything less than proper operating temp is a emission concern for the pcm, hence the check engine light

 

Thanks!

I've just updated the post to include vehicle details.

Yeah, it's just that since I bought it a little over a year ago (OP has been updated to include info) I hadn't done any maintenance on it so just wanted to knock it all out at once.

So what would you suggest? Just going ahead with the thermostat replacement? At this point I will definitely get OEM from the dealership.

I don't have one. But if I get one, do I just plug it in after a drive when the engine has reached its operating temperature? What should the temps read?

The rate at which the engine reaches operating temp has not changed at all. It always warmed up fairly quickly and it still does. Would you suggest to just go ahead with a new (OEM) thermostat?

 

What other members are saying, there are ways to pinpoint the issue thru real time engine data from ECM. They are saying don’t load the parts cannon and this saves you time/money.

lucky for you both thermostat/coolant temp sensor pretty cheap parts

check out some threads about ECT replacement on your engine
Lots of videos out there as well
Let everyone know what you find!