Help with P0128 Error
 

Hey all,
Long time Z owner (used to have a boosted 350z) - newer to this forum but I've been lurking for a bit. That said, hoping for some communal expertise to help me with this one. I threw a code a couple of months back, P0128. My initial thought was that the thermostat was stuck open, no big deal - the engine took a tiny bit too long to get to temperature every once in a while, with no real negative impacts. I bought a new one but haven't replaced it yet. That said, really funky issue this morning, which is where I'm a bit lost.

In a nutshell, as my engine was getting to temperature coming into work, within just a couple of minutes the engine temp gauge started bouncing around. I mean, all over the place. It was like watching a light on a carnival game or something - it was changing constantly. Would bounce from completely cold up a few ticks, down a tick or two, up a few, down to cold again, all the way up to normal temperature, and then around some more. This was very rapid as well - there's NO way the actual engine temperature was bouncing around that much.

So, now I'm wondering if my thermostat is just fine and possibly the temperature sensor is broken or something - any thoughts/ideas/suggestions?

If I'm remembering correctly, the sensor is on the back of the motor between the intake manifold and firewall. It's less of a pain in the *** to change than the thermostat. Might as well give it a shot.

Trying to find it online, no luck so far - any chance anyone has pics/manual for the sensor location for the water temp sensor?

It could be:
* Coolant level. Check and adjust as needed.
* Thermostat. Since you already have a replacement, I'd go ahead and change it. See the FSM for R&R procedures.
* Cooling fan(s) stuck on or running too often. Verify operation.
* Bad coolant temperature (ECT) sensor or connection. Check connections and verify operation of sensor. See FSM.
* Bad Intake Air Temperature (IAT) sensor. Check connections and verify operation of sensor. See FSM. (If it's that far off, would probably trip another error.)

Edit: Since the ECT is jumping around, I suspect a bad connection (FSM should help you figure out if you're looking for a short or an open) or sensor.

Download the FSM. It has all the factory specs and procedures and lots of pretty drawings. Link in my sig.

Thanks guys - I looked the code up and saw all the possibilities, but this newest funky issue is leading me to lean towards the water temp sensor being the issue. No way engine temps can go from dead cold to at temp to halfway between to cold again in two seconds, and same thing with the fan - fan turns on, temp doesn't plummet in a second or two.

I can't access the DL for the FSM here at work, but thanks much for the link - hopefully I can get it when I get home.

I agree - sounds like coolant temp is the problem.

In my experience with industrial temperature measurement, RTDs and other resistive sensors _usually_ fail catastrophically and don't cause weird readings - they either work or they don't. I'd look at the wiring closely before deciding to change the sensor. A cheap VOM and the FSM should be enough to troubleshoot the problem; at this point, you are looking for erratic readings from the sensor or wiring, not necessarily accuracy.

Finally getting the chance to work on this over the weekend - was out of town - and have been continuing to read up on possibilities. It seems on a number of cars the coolant temperature send unit can go bad, with the symptoms being typical of the exact problem I'm having - coolant values jumping all over the place. However, I'm looking through Nissan parts/online retailers and can't find one for the 370z. Does anyone know if the send unit is built into the sensor unit (as is the case on some cars)? That would explain why I can't find the replacement part, if it happens to be the case on our cars.