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BMW P0128 and 1D2008: thermostat stuck open
The engine takes too long to warm up, usually because the thermostat is stuck open. BMW has bulletins for both real failures and false codes.
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Codes on this page
| Code | Definition | Source |
|---|---|---|
| P0128 | Coolant thermostat (coolant temperature below thermostat regulating temperature) | Generic OBD-II |
| 1D2008 | Map thermostat (mechanical): jammed open | SI B12 33 14 (PDF) |
| 1D2009 | Characteristic map thermostat: clamped closed | SI B12 13 16 (PDF) |
P-codes are the generic OBD-II codes any reader shows. Six-character and four-character hex codes are BMW's own, read with a BMW-capable scanner; the wording is BMW's, from the source shown.
What it means
BMWs use an electrically heated "map" thermostat that the DME can open early. If coolant temperature stays below what the model expects, it stores 1D2008 (P0128 on generic tools). 1D2009 is the opposite: stuck closed, which can lead to overheating.
Can I keep driving?
Yes for stuck open; stuck closed means watch the temperature. Stuck open means slow warm-up, weak heat and higher fuel use, but no immediate harm. If the temperature warning comes on (stuck closed), follow the overheating advice.
Usual causes on BMWs
| Cause | Cars most affected | What you notice | Evidence | Repair prices (national average) |
|---|---|---|---|---|
| Thermostat jammed open | F22, F25, F26, F30–F36 with automatic transmission (bulletin); N20, N26, N52, N55, B46/B48, B58 | Check engine light with 1D2008. BMW extended thermostat coverage on many N20 and N26 cars. | SI B12 33 14 (PDF) SI B01 01 16 (PDF) SI B01 17 17 (PDF) | N52 / N54 / N55 electric water pump + thermostat Indie $758–$1,259 Dealer $1,841–$2,250 B58 heat management module (thermostat) replacement Indie $978–$1,506 Dealer $2,234–$3,037 |
| False code in cold, low-load driving (N63) | N63 in F01, F02, F07, F10, F12, F13 | BMW: the thermostat works and heat output is fine; don't replace it, reprogram the DME. | SI B12 18 13 (PDF) | — |
| N63R map thermostat faults | G12 750i, G30 (N63R) | 1D2008, 1D2009 and stroke sensor codes; BMW also extended N63R thermostat coverage to 15 years or 150,000 miles. | SI B12 13 16 (PDF) SI B01 01 21 (PDF) | — |
A bulletin isn't a recall and doesn't mean your car has the problem; open it for the exact models and build dates. Estimates are national averages built from BMW warranty labor time, regional shop rates and parts prices; open a repair for your ZIP. "Owners paid" is the middle half of sourced prices, shown where we have at least 3.
What to do
- Note whether the temperature gauge or heater is slow to warm up.
- Ask whether a software update or a warranty extension applies before replacing the thermostat.
Warnings it can bring up
Engine overheating or coolant warning Check engine light, no message
Related codes
Other BMW bulletins that mention it
- SI B17 01 15 Reduced Heat After Cooling System Repairs E90, E92, E93, F30, F31 and others · Feb 2015
- SI B18 02 17 Diagnosing selective catalyst reduction (scr) efficiency faults E70, E90 · Dec 2017
5 of the BMW bulletins we hold mention it in their text; these are the closest matches.
Sources
Bulletins are cited in the tables above. Explanations are ours; where a source is an owner report rather than BMW, it is labelled as one.