You can generally keep driving. The thermostat still opens on its own, so overheating is unlikely, but the MIL stays on, you may fail emissions testing, and long-term you lose the fuel-economy and load-cooling benefit — fix it at your convenience. P0597 means the engine computer found an open (broken) circuit driving the small electric heater inside a map-controlled thermostat. The heater lets the ECM force the thermostat open earlier under load; with the circuit open, that command is lost and the thermostat simply acts like an ordinary wax thermostat.
What P0597 means
Some engines — mostly VW, Audi, BMW, Mercedes, Mini and a few GM/Opel and Chrysler applications — use a map-controlled (electrically heated) thermostat. Its wax pellet contains a small heating resistor that the ECM energizes through a low-side driver, often with a pulse-width-modulated (PWM) signal. When the engine is under high load or high speed and a cooler coolant target improves knock margin and power, the ECM heats the wax so the thermostat opens sooner and coolant temperature drops from a warm part-throttle target (around 105 °C) toward roughly 90–95 °C. P0597 sets when that driver detects no current flow at all — an electrically open circuit — meaning the heater winding, its wiring, connector, or supply/ground path is broken. Because the fault is a broken control circuit (not P0598 short-to-ground low, nor P0599 short-to-power high), the ECM can no longer command early opening, so the thermostat reverts to purely mechanical operation.
Symptoms
- Check Engine Light (MIL) on, frequently with no drivability complaint at all
- Coolant may run a little warmer than optimal under sustained high load because early opening can no longer be commanded
- Possible small drop in fuel economy or a slight loss of the emissions/warm-up benefit the mapped thermostat provides
- No overheating, no coolant loss, and normal temperature-gauge behavior in ordinary driving
- Companion coolant-temperature codes such as P0128 or P0125 may appear if warm-up timing is affected
- A scan-tool actuator (output) test for the thermostat heater fails to draw current or reports the circuit open
Common causes
- Open or burned-out heating element inside the thermostat/coolant-regulator housing (winding failed open)
- Broken or corroded wire in the heater control circuit between the ECM and the thermostat connector
- Loose, corroded, or backed-out terminal at the thermostat electrical connector
- Loss of the switched 12 V / ignition supply feeding the heater's high side (blown fuse or open feed wire)
- Damaged connector or chafed harness near the hot, vibration-prone thermostat/engine area
- Open low-side driver circuit or a failed driver inside the ECM/PCM (least common, verify wiring first)
Severity & driving advice
Severity: Low — Not an overheat risk — the thermostat still opens mechanically; main impacts are the MIL, an emissions-test failure, and a small efficiency loss.
Can I drive? You can generally keep driving. The thermostat still opens on its own, so overheating is unlikely, but the MIL stays on, you may fail emissions testing, and long-term you lose the fuel-economy and load-cooling benefit — fix it at your convenience.
Diagnostic approach
- Record freeze-frame and scan for companions — Read all stored codes and freeze-frame data. Note whether P0597 stands alone or is joined by P0598/P0599 (short faults) or coolant-temp codes P0128/P0125. An open code with no short codes points to a break in the circuit rather than a shorted wire.
- Confirm the vehicle actually uses a mapped thermostat — P0597 only sets on engines fitted with an electrically heated (map-controlled) thermostat — common on VW/Audi 2.0T and BMW. Verify the coolant-regulator housing has a two-pin electrical connector before condemning parts; a conventional wax thermostat cannot set this code.
- Inspect the connector and heater element resistance — Unplug the thermostat connector and check both terminals for corrosion, spread pins, or coolant intrusion. With the connector off, measure resistance across the heater element's two pins; a healthy element reads a low, finite value (typically only a few ohms). An OL/infinite reading confirms an open element.
- Verify supply voltage to the heater — With the key on, back-probe the supply side of the connector and confirm battery/ignition voltage is present. No voltage points upstream to a blown fuse or an open feed wire rather than to the thermostat itself.
- Check control-wire continuity to the ECM — With the key off and the ECM connector unplugged, measure continuity of the control (ground/PWM) wire from the thermostat connector to the correct ECM pin. Infinite resistance means the wire is open; wiggle the harness while measuring to expose an intermittent break.
- Command the heater and watch for current — Reconnect and use a bidirectional scan tool (or Guided Fault Finding on VW/Audi) to command the thermostat heater on. If the driver energizes but no current flows and P0597 immediately resets, the open is in the element or wiring; only after wiring and element test good should the ECM driver be suspected.
- Repair, clear, and confirm the fix — Repair the open (replace the thermostat/coolant-regulator assembly for an open element, or repair the harness/connector). Clear the code, then run the engine through a warm-up and a load cycle and confirm P0597 does not return and coolant temperature stabilizes normally.
FAQ
Will P0597 make my engine overheat?
No. P0597 is an open in the thermostat's electric heater circuit, not a failure of the thermostat's mechanical action. The wax pellet still expands and opens at its normal temperature, so cooling continues. You lose only the ECM's ability to force the thermostat open earlier under load.
What is a map-controlled or heated thermostat?
It is an electronically assisted thermostat whose wax element carries a small heating resistor. Under high load the ECM energizes the heater so the wax melts sooner and the thermostat opens early, lowering coolant temperature for better knock margin and power; at part throttle it stays closed longer for economy and emissions.
How is P0597 different from P0598 and P0599?
All three cover the same thermostat heater control circuit. P0597 is an open circuit (no current flow). P0598 is circuit low, usually a short to ground. P0599 is circuit high, a short to power or overvoltage. Which one sets tells you what kind of electrical fault to look for.
Can I just replace the thermostat to fix P0597?
Often yes, because the internal heating element frequently fails open and the element is not serviceable separately — you replace the whole map-controlled thermostat/coolant-regulator assembly. But first confirm the wiring, connector, and supply are good, so you do not replace a good thermostat when the real break is in the harness.
Why does this code show up mostly on European cars?
Map-controlled thermostats are most common on German makes — Audi, BMW, Mercedes, Mini, Opel/Vauxhall and Volkswagen. Vehicles with an ordinary wax thermostat have no heater circuit, so they cannot set P0597 at all.