Trouble Code P0128: Faulty Thermostat

look up:   OBDII Diagnostic Trouble Codes

Although I haven’t experienced this issue thus far, apparently a few of the electrically assisted (also called “map-controlled”) thermostats that is unique to the 124 Spider (this is not the thermostat used on the Fiat 500, although some of those have issues as well) have been troublesome (sticking in the open position or opening too soon), which results in the engine warming up to normal operating temperature too slowly.   There are a few clues, that you may observe eventually, that you may have one of these faulty thermostats:

  • the blue engine coolant light (which triggers OFF at 130° F) remains ON for longer than about 5 – 7 minutes after starting your engine that has been off for more than an hour.
  • the coolant temperature gauge takes more than 5 – 7 minutes to get to normal operating temperature.
  • the CEL (Check Engine Lamp) is ON, and the error code P0128* will come up if an OBD-II diagnostic tool is hooked up to your vehicle.

Consider this as an inconvenience issue rather than a critical issue, but don’t ignore it either. This is obviously a warranty item that your dealer will fix for you, but the thermostat is very difficult to get to (at the rear of the engine adjacent to the firewall), and they seem to be on back-order, so you are advised to ask the dealer to pre-order the following parts and verifying their arrival (on hold in your name) prior to your service appointment for the repair, so that your vehicle isn’t kept for a couple of weeks waiting for said parts:

  • replacement thermostat (Part # 68320858AA) >> different part # than the original thermostat. Item #6 in parts diagram:
  • vacuum pump gasket (Mopar # 68140726AC) >> in case the vacuum pump needs to be removed in order to access the thermostat.
  • Mopar # 68334695AA Antifreeze / Coolant

It is OK to drive the vehicle in this condition while you are waiting for your service appointment, but as the manual advises you to do when the blue coolant lamp is ON, go easy on “engaging the turbo”  😉  for the longer duration that that lamp is illuminated.  Some with marginal thermostats have had good luck preventing the error code P0128 trigger, by engaging the AC after starting the engine, the extra load aiding in warming up the engine quicker.

Here are the specs on our Mazda-sourced thermostat:

thermostat specs

♦♦♦♦♦  UPDATE  ♦♦♦♦♦

Apparently, in most cases, there wasn’t anything actually wrong with the electrically assisted (also called “map-controlled”) thermostats, but rather the ECU mapping, and Fiat is currently advising dealers to flash the 124 Spider’s ECU (also known as the Power Control Module (PCM) Part # 68231492__) to firmware level AI, to alter the engine operating temperature vs. time algorithm somewhat, so that the CEL (and the P0128 trouble code*) isn’t illuminated for slow thermostat behavior, especially for the North American mapping.  Electric coils heating the wax inside the thermostat are controlled by the ECU to schedule opening prior to the coolant reaching melting temperature.

*     P0128 Trouble Code:  Indicates that the coolant temperature has not achieved the required thermostat regulating temperature (engine operating temperature) within a specified amount of time after starting the engine.

3 thoughts on “Trouble Code P0128: Faulty Thermostat

  1. If I comprehend the 124 Spider forum comment authors correctly, it seems to be a batch of questionable thermostats introduced more recently at Mazda, but the MX-5s on the same assembly line using the same thermostats (as far as i know) haven’t experienced the same issues, unless their PCMs are programmed with a much looser algorithm and therefore just don’t trigger the error code unless they never close. Early build vehicles also have firmware AD, which is considerably older than level AI, yet I’m not aware that those vehicles experiencing many of the thermostat errors, which seems to signify that the problem does indeed lie primarily with the thermostats.

    What is really disappointing though is that the replacement parts, with a different part number signifying a change in design, are also resulting in the error code(s).


  2. My CEL came on within the first 1500 miles on the 124 spider. My dealer is happy to make the repair but need the car for 7-9 days, WTF!! The only loner vehicle they had to loan me was the 4 door 500 which is not comparable in any sense of the word, its embarrassing to drive that piece of automotive garbage. I cant believe they are designed by the same company.


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