EN

Severity: Low Code type: Generic (all makes)

P0597 Code: Thermostat Heater Control Circuit/Open

The electrically heated thermostat's heater circuit is open, so the ECU can't open it early under heavy load.

At a glance

Severity: 2/5 – Low2/5

Severity: Low

Meaning
Thermostat Heater Control Circuit/Open
System
Fuel & Air Metering · Powertrain
Safe to drive?
Yes, short term
Typical repair cost
$60–$900
DIY difficulty
Moderate

Many modern engines, including BMWs, several GM turbo four-cylinders and many VW and Audi TSI engines, use a map-controlled thermostat. Its wax element opens by itself at a fairly high coolant temperature, around 100 °C (212 °F) or a little more, to save fuel at light load; under heavy load or high outside temperatures the ECU switches on a small heater inside the element so it opens sooner and the engine runs cooler. P0597 means the ECU sees an open circuit where it expects current through that heater. The thermostat still opens mechanically, so the engine rarely overheats, but it can't be cooled down on demand, which can mean a hotter engine when towing or climbing and a little more knock retard.

Can I keep driving? Normal driving is fine. Avoid towing, long steep climbs and full-throttle runs in hot weather until it's fixed, and keep an eye on the temperature gauge or warning light.

How to read P0597

How to read P0597P10253974
  1. P Powertrain
  2. 0 Generic (SAE)
  3. 5 Speed, idle & inputs
  4. 97 Specific fault

Symptoms

  • Check engine light with no change in how the car drives
  • Coolant temperature staying at the high end of normal under load
  • Temperature rising more than usual when towing or in hot weather
  • Slightly reduced power on hot days as the ECU retards timing
  • Coolant seepage or white crust around the thermostat connector on some engines

Possible causes

Ranked by how often each cause is behind this code (editorial estimate).

  1. Heater element inside the thermostat burned open 36%
  2. Connector at the thermostat corroded, cracked or contaminated with coolant 26%
  3. Broken or chafed wire between thermostat and ECU 16%
  4. Missing supply voltage from a blown fuse or failed main relay feed 12%
  5. Thermostat housing leak letting coolant wick into the terminals 6%
  6. ECU output driver fault 4%

Diagnosis step by step

Diagnostic flow: P05971Let the engine cool and checkcoolant2Inspect the thermostat connector3Measure the heater resistance4Check supply and ECU control5Test the thermostat's response6Replace, bleed and verify

  1. Let the engine cool and check coolant

    Only open the cooling system on a cold engine. Check the coolant level and look for leaks around the thermostat housing, since a leak there often explains a wet or corroded connector.

    Tools: Flashlight, Gloves

  2. Inspect the thermostat connector

    Unplug the two-pin connector on the thermostat. Look for green corrosion, coolant inside the plug, burned terminals or a brittle, cracked housing, which is common on older engines where the plug sits next to hot coolant.

    Tools: Flashlight, Screwdriver

  3. Measure the heater resistance

    Measure across the two pins of the thermostat itself. Heater elements are typically somewhere around 10 to 20 ohms; an open reading (OL) confirms a burned-out heater, and the thermostat must be replaced as an assembly.

    Tools: Multimeter

  4. Check supply and ECU control

    On most systems one wire carries 12 volts with the key on and the ECU switches the other to ground. Check for battery voltage on the supply wire, then command the heater with a scan tool and confirm with a test lamp across the terminals that the ECU switches it on.

    Tools: Multimeter, OBD-II scanner

  5. Test the thermostat's response

    On a warm engine at idle, command the heater on and watch the coolant temperature in live data. Within a few minutes it should drop noticeably as the thermostat opens further; no change with a good circuit points to a mechanically sticking thermostat.

    Tools: OBD-II scanner

  6. Replace, bleed and verify

    After fitting a new thermostat, fill and bleed the cooling system exactly as specified; many of these engines have an electric water pump and a scan-tool or key-on bleed routine. Clear the code and drive until the engine has reached full temperature a few times.

    Tools: OBD-II scanner, Towels & shallow pan

Where the parts are

Ignition: coils and spark plugs per cylinder – P05971234567
Ignition: coils and spark plugs per cylinder
  1. 1Ignition coil
  2. 2Spark plug
  3. 3Fuel injector
  4. 4Camshaft sensor
  5. 5Crankshaft sensor
  6. 6Engine computer (ECM)
  7. 7Intake manifold

Repair options & costs

RepairDIY?Estimated cost
Repair or replace thermostat connector Yes $60–$200
Repair wiring to the thermostat No $120–$350
Replace map-controlled thermostat with coolant Yes $250–$700
Replace thermostat housing assembly (plastic housings) No $350–$900

Rough US estimates for parts and labor; shop rates vary by region. Not a quote.

Notes by brand

BMW

BMW six- and four-cylinder engines such as the N52, N54, N55 and N20 use a map-controlled thermostat, and failed heater elements and brittle connectors are common. Many shops replace the thermostat together with the electric water pump when the pump is also aging, since both are reached through the same work.

Chevrolet

The 1.4-liter turbo in the Cruze, Sonic and Trax uses an electrically heated thermostat in a plastic housing that is known to leak. Coolant reaching the connector can trigger P0597, so inspect the housing when you find this code.

P0597 on Chevrolet in detail

Field data

Chevrolet 7
BMW 2 528i Xdrive, X1 XDRIVE35I

Source: NHTSA Complaints API (US government open data), analyzed by DTC mention.

Emissions / smog test impact

Will fail

P0597 is an emissions-related code that lights the check engine lamp, so the car fails an OBD-II emissions inspection until it's repaired and the monitors have run again.

Frequently asked questions

Will my engine overheat with P0597?

Usually not. The thermostat still opens by itself at its mechanical temperature. What you lose is the ECU's ability to open it early, so the engine runs hotter under heavy load and in hot weather.

Why does my engine normally run so hot?

Map-controlled cooling systems deliberately run the engine near or above 212 °F at light load to reduce friction and fuel use. Temperatures in that range are normal for these engines and not a sign of overheating by themselves.

Can I fit a regular thermostat instead?

No. The ECU monitors the heater circuit, so a thermostat without a heater keeps setting P0597, and it also changes how the engine is cooled under load. Use the correct map-controlled part.

How is P0597 different from P0128?

P0128 means the engine warms up too slowly, usually a thermostat stuck open. P0597 is purely electrical: the heater circuit that lets the ECU open the thermostat early is open.

Sources