Ongoing Coolant Temp Sensor Problem & CEL - Stumped!

Hey everyone,

Hoping to get some collective wisdom on a really frustrating issue I've been having with my 2019 Tacoma TRD Sport (3.5L V6, 88,000 km). I've been battling a persistent Check Engine Light (CEL) and some very strange temperature gauge behavior that's now leading to rough starts and stops. I've already thrown a couple of parts at it, and I'm honestly at a loss for what to try next.

Here's a chronological breakdown of what's been happening, I'll try to keep it organized.

The problem first cropped up about 3 weeks ago. I noticed the temperature gauge on my dash was acting peculiar. Instead of smoothly rising to operating temp, it would slowly climb up to around the normal half way mark (80C), and then, without warning, it would suddenly plummet all the way down to the cold peg. At the same time, my OBD2 scanner would read a drastically low temperature, around 28C, right after the drop.
Along with this, I got my first CEL, and scanning it revealed two codes:

* P0128: Coolant Thermostat (Coolant Temperature Below Regulating Temperature)

* P0116: Engine Coolant Temperature Sensor Circuit Range/Performance

Given these codes and the temperature gauge behavior, it pointed pretty strongly to thermostat or ECT sensor issues. Based on the initial diagnosis, I decided to tackle the most likely culprits.

My first step was to replace the entire thermostat assembly. I then replaced the Engine Coolant Temperature (ECT) Sensor: This was a bit of a pain to access, but I managed to get it swapped out. I was really hopeful this would resolve the P0116 and the gauge issues.

After replacing both of these, I cleared the codes and took it for a drive. Unfortunately, the temperature gauge continued its erratic behavior – slowly rising, then dropping to cold. After driving for a bit post-repairs, the CEL came back on. This time, however, I'm getting a new code:

* P0125: Insufficient Coolant Temperature for Closed Loop Fuel Control

Crucially, the original temperature gauge issue persists exactly as before. It still slowly climbs to around the half way mark and then instantly drops to the cold mark, reading 28C on my scanner.

When I start it up, it often stumbles and sputters for a few seconds before settling into an idle. Similarly, when coming to a stop, it feels like it's struggling and almost wants to stall before the RPMs stabilize. This new symptom definitely feels related to the temperature issue, as the ECU isn't getting accurate temperature readings or something.

I'm truly at a loss here. Any thoughts, experiences, or diagnostic steps you can suggest would be immensely appreciated. I'm trying to avoid just throwing more parts at it without a clearer direction.

Thanks in advance for your help!

TLDR: A 2019 Tacoma (88,000 km) has a persistent Check Engine Light and an erratic temperature gauge that drops to 28C after reaching operating temp. Initially, codes P0128 and P0116 appeared, leading to thermostat and ECT sensor replacement. However, the issue persists, now with code P0125, and new symptoms of rough starts and near-stalls. Seeking advice for further diagnosis.

 

Sounds like you have a wiring or pin fitment issue, an intermittent open circuit in the temp sensor wiring would cause what you describe.

 

Hey, thanks for the reply, Dm93.

When I replaced the ECT I did observe the surrounding harness and the connector, it all seemed intact. I am curious if I should see/check the connection on the PCM side?

 

I'd plug a scanner in and monitor the coolant temp PID while wiggling on things, if it starts fluctuating your in the area of the problem.