MB Medic Mercedes-Benz Repair Guides, Service, How To DIY
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In this guide (6 sections)
How to Troubleshoot Mercedes-Benz Air Conditioning, Read Sensor Data and Reset the Control Unit
If your Mercedes has this climate control panel, follow these steps to troubleshoot your Mercedes-Benz Air Conditioner System.
Does your car have this menu?This procedure belongs to the push button climate panels Mercedes fitted through the late 1990s and 2000s. If your panel has REST and AUTO buttons alongside up and down arrows for temperature, it is worth trying. Later models dropped the hidden menu, and on those the climate faults are read with a scanner instead. Trying the key sequence does no harm either way. If nothing happens, your panel simply does not support it.
Reading and Erasing the Fault Codes
Entering Diagnostic Trouble Code (DTC) Display Mode
- Turn the ignition on. Press the down arrow push button until “LO” is shown in the A/C-heater control panel temperature display window.
- Within 20 seconds, simultaneously press the REST and defrost push buttons for over 2 seconds. The Light Emitting Diode (LED) in the recirculated air push button will blink, and the display window will show “di R”.
- Press the AUTO push button repeatedly, recording each DTC, until all DTCs are displayed. Permanent malfunctions are displayed first. See DIAGNOSTIC TROUBLE CODE (DTC) IDENTIFICATION tables.
- If no permanent malfunctions are stored, the display window shows “End.” Press the AUTO push button again. Intermittent DTCs will be displayed. Intermittent malfunctions are recognizable by the degree (°) symbol displayed along with the DTC (example: “04 °”).
- If no intermittent malfunctions are stored, the display window shows “En° d.” Proceed to erase DIAGNOSTIC TROUBLE CODES.
Erasing Diagnostic Trouble Codes
- After the display window shows “En° d,” press the AUTO push button again. The display window should show “dE L” (delete).
- To erase DTCs, simultaneously press the down and up arrow push buttons for over 5 seconds. The display window should show “- –.” If not erasing DTCs, press the AUTO push button. The permanent DTCs will again be displayed. Proceed to exit DTC DISPLAY MODE.
Exiting DTC Display Mode
Simultaneously press the down and up arrow push buttons until the pre-selected “72 °F” temperature setting is displayed. Turn the ignition off.
Climate Control Fault Code List
These are the B codes stored by the climate control module. A code names a circuit, not a failed part, so treat each one as a place to start looking rather than something to order.
| Code | Component | Where to look |
|---|---|---|
| B1226 | In car temperature sensor (B10/4) | The sensor, or the aspirator motor that draws cabin air across it. Check channel 01. |
| B1227 | Outside temperature sensor (B14) | Sensor behind the bumper, its wiring, or the data line if the cluster disagrees. Check channel 02. |
| B1228 | Heater core temperature, left side (B10/1) | Sensor or wiring. Compare channels 03 and 04 against each other. |
| B1229 | Heater core temperature, right side | The partner to B1228. The two sides should read close together. |
| B1230 | Evaporator temperature sensor (B10/6) | This is the freeze protection sensor, and a fault here commonly shows up as warm air. Check channel 05. |
| B1231 | Engine coolant temperature sensor (B11/4) | Often a data line problem rather than the sensor, since this value arrives from elsewhere. |
| B1232 | Refrigerant pressure sensor (B12) | Sensor on the receiver drier, its 5 V supply, or its wiring. Check channel 07 first. |
| B1233 | Refrigerant temperature sensor (B12/1) | Sensor or wiring. Check channel 08. |
| B1234 | Sun sensor (B32) | The small dome on top of the dashboard. Check channel 12 by covering it with your hand. |
| B1235 | Emissions sensor (B31) | The air quality sensor that closes the recirculation flap in traffic. Check channel 11. |
| B1241 | Refrigerant fill level | The system believes the charge is wrong. Confirm on channel 07 before adding anything. |
| B1416 | Coolant circulation pump (M13) | The auxiliary pump that keeps hot coolant moving through the heater core. |
| B1417 | Duovalve, left (Y21y1) | Meters coolant to the left heater core. A classic cause of heat on one side only. |
| B1418 | Duovalve, right (Y21y2) | Same part, other side. Both halves sit in one unit on most models. |
| B1419 | Electromagnetic compressor clutch (A9k1) | Clutch coil, its power feed, or the command from the module. Applies to clutch type compressors. |
| B1420 | Idle speed increase | The request to raise idle when the compressor loads the engine. Usually a communication fault. |
| B1421 | Pulse module (N65) | Blower control. Pair this with channel 10 when the blower will not vary its speed. |
| B1422 | Serial interface (K1) to instrument cluster (A1) | A communication fault rather than a climate part. Explains missing coolant or outside temperature values. |
| B1423 | Switchover valve block (Y11) | Vacuum control for the air distribution flaps. Suspect a vacuum leak as readily as the block itself. |
| B1424 | Charcoal filter actuator, open (A32m2) | Recirculation flap actuator stuck or disconnected. |
| B1425 | Charcoal filter actuator, closed (A32m2) | The same actuator in the opposite direction. |
| B1432 | Not used on USA market cars | Ignore this one on a North American vehicle. |
| B1459 | Serial interface (K2) to instrument cluster (A1) | The second communication line. Same reasoning as B1422. |
| B1462 | Wide open throttle signal, diesel engines | Diesel models only. The signal that drops the compressor under full load. |
Read the intermittent list as wellPermanent faults come up first. Keep pressing AUTO past the End display and the module shows intermittent faults, marked with a degree symbol next to the number. Those are often the more useful ones, because a fault that comes and goes is exactly the kind that has you chasing a problem that is not there when you go looking for it.
Climate Control as a Diagnostic Tool
Reading Sensor Values
- Turn the ignition on. Simultaneously press the down and up arrow push buttons until the pre-selected “72 °F” temperature setting is displayed. Press the AUTO push button.
- Press the REST push button for more than 6 seconds. The display window will alternately show “01” and an in-car temperature of “72 °F”, or “LO” if there is an open circuit or “HI” if there is a short circuit.
- The next ascending test step is displayed by pressing the windshield air distribution push button. The next descending test step is displayed by pressing the footwell air distribution push button. See the SENSOR VALUE DIAGNOSTIC TROUBLE CODE (DTC) IDENTIFICATION table. Press the REST push button to exit reading sensor values mode.
This is where the menu earns its keep. The codes tell you something is wrong. The live values tell you what the car is actually seeing, which is usually what you needed to know. Here is each channel with the reading you are hoping for.
| Channel | What it reads | What a good reading looks like |
|---|---|---|
| 01 | In car temperature sensor (B10/4) | Should track the actual cabin temperature. LO means an open circuit, HI means a short. The sensor runs roughly 11.9 to 13.0 kilohms at 20 C, and the aspirator motor that draws air across it needs 12 V to work. |
| 02 | Outside temperature sensor (B14) | Should agree with the figure on your instrument cluster. When the two disagree, suspect the data line between the modules rather than the sensor. |
| 03 | Left heat exchanger temperature | About 11.9 to 13.2 kilohms at 20 C and 4.2 to 4.6 kilohms at 45 C. Reads cooler than engine coolant unless you are asking for full heat. |
| 04 | Right heat exchanger temperature | Same expectations as channel 03. The two sides should track each other closely, and a wide split between them points at one duovalve. |
| 05 | Evaporator temperature sensor (B10/6) | With the A/C running this should sit just above 0 C. Around 3.2 kilohms at 20 C. This is the freeze protection input, so a sensor reading falsely cold makes the system shut the compressor down and leaves you with warm air from a healthy system. |
| 06 | Engine coolant temperature | Arrives from the instrument cluster rather than from a sensor of its own here. |
| 07 | Refrigerant pressure in bar (B12) | Roughly 5 bar static with the system off at ambient temperature. With the engine running and the A/C on, expect about 9 to 15 bar, higher when the system is working hard. Around 8 bar while running points at a low charge. The sensor needs a clean 5 V supply to read at all. |
| 08 | Refrigerant temperature sensor (B12/1) | Displayed oddly: 23*9 means 23.9 C. Under about 13 kilohms at 20 C, about 5.5 kilohms at 40 C, about 1.8 kilohms at 70 C. |
| 09 | Not used | Nothing to see here. |
| 10 | Blower control voltage | Roughly 0.8 to 6.0 V, shown as 08 through 60. The channel to watch when the blower will not vary its speed. |
| 11 | Emissions sensor (B31) | Normally 4 to 6 V. A display of 31 means 3.1 V. |
| 12 | Sun sensor (B32) | About 0 V in darkness, rising to roughly 4.5 V in full sun. Cup your hand over it and watch the number fall. That is a complete test in five seconds. |
| 20 | Auxiliary fan control | Shown in milliamps, around 10 at maximum. |
| 21 | Engine speed | Should match the tachometer. |
| 22 | Vehicle speed | In km/h. Worth knowing that the system deliberately winds the blower back as road speed rises, because ram air does some of the work. That is why the fan seems to fade on the highway, and it is normal. |
| 23 | Circuit 58d | Percentage of battery voltage. This is the instrument lighting dimmer feed. |
| 24 | Battery voltage | 12.5 means 12.5 V. A low or wandering figure here sends you to voltage drop testing on the power and ground side, not to the climate system. |
The two channels to check firstIf you look at only two numbers, make them channel 07 and channel 05. Channel 07 tells you whether there is refrigerant in the system, which decides whether anything else is worth investigating. Channel 05 tells you whether the car thinks the evaporator is about to freeze, because if it does it will cut the compressor and hand you warm air from a system that is otherwise perfectly healthy. Between them those two settle most warm air complaints before you spend a cent.
Rear climate and module version channels
- 30 and 31 Left and right rear heat exchanger temperature, where a rear system is fitted
- 32 Rear evaporator temperature sensor
- 33 Rear blower control voltage, 0.8 minimum to 6.0 maximum
- 34 and 35 Left and right rear temperature selector
- 38 Software status, for example 37 for rear air conditioning
- 38 Hardware status. The factory sequence lists this number twice
- 40 Control module software version
- 41 Control module hardware version
- 42 Variant code, first digit
- 43 Variant code, second digit
Channel numbering and availability vary with the model and the panel generation, so your car may not offer every one of these.
Record all fault codes and sensor readings before clearing the climate control memory. Clear the codes only after completing the repair, then operate the system and scan it again.
Do You Still Need a Scanner If the Car Has This Menu?
Fair question, and the honest answer is that the hidden menu is excellent at what it does and blind to everything else. It is worth knowing about, and it will not cover every job.
| What you want | Hidden climate menu | Full system scanner |
|---|---|---|
| Climate control fault codes | Yes, on panels that support it | Yes, on every model |
| Live sensor values | Yes, the channels listed above | Yes, with plain language labels and graphing |
| Works on newer models | No. Later panels dropped the menu | Yes |
| Freeze frame, the conditions when the fault set | No | Yes |
| Command the compressor and flap motors to test them | No | Yes, through active tests |
| See related faults in the engine and body modules | No. Climate only | Yes |
That last row matters more than people expect. Plenty of air conditioning complaints are not air conditioning faults. A compressor that never engages because the engine module will not permit it, or a cooling fan sitting still because of a fault stored somewhere else entirely, looks identical from inside the climate menu. The menu shows you the climate module’s own view of the world and nothing beyond it.
When the hidden menu runs out of road
The UCAN-II Pro reads the climate module on models old and new, shows the same sensor data with proper labels, runs active tests so you can command the compressor and the flap motors instead of guessing, and reads the engine and body modules in the same session so you can see what else is going on. It covers what Xentry covers apart from online VIN coding. No subscription and no annual license.
Related Reading
Air Conditioner Troubleshooting Guide
How the system works and what the gauge pressures should be.
How the system works and what the gauge pressures should be.
W220 S-Class A/C Hidden Menu
The equivalent procedure on the W220 and W215.
The equivalent procedure on the W220 and W215.
A/C Compressor Clutch Problems
When the compressor will not engage at all.
When the compressor will not engage at all.
DIY A/C Recharge the Easy Way
Doing it properly once you have confirmed the charge is genuinely low.
Doing it properly once you have confirmed the charge is genuinely low.
Climate Control Buttons Not Working
When the panel itself is the fault rather than the system.
When the panel itself is the fault rather than the system.
Before you buy parts, scan every control module, not just the engine. It shows the root fault behind the warning lights and tells you whether the problem is still active. See how to do a full health check.
We sell the UCAN-II Pro. The function comparisons above come from each manufacturer’s own documentation so you can check them yourself.
Just got price for ac condenser $3200 for a 2003 SL500 (42,000mi) Mercedes 42,000 mi
Having 3 different MB models – all of them start displaying sensors after pressing REST, but here is where the different scrolling applies. (when you have climatronic with no REST button, you need to use advanced scanner, or replace climatronic with one having REST)
-on single zone R129 you scroll by pressing fan buttons
-on dual W210 you scroll by pressing left and right Auto
-on 4-zone W211 you scroll by pressing left temp adjustment.
4 zone displays more sensors
00 has to be inside cabin per my observation, so my guess 00 and 01 are front and rear of the cabin, but could never find official data.
Good info, but does not explain how to use the data.
Per my experience you need to read the sensors at least twice in different conditions.
1 on cold car before engine start and before sun can warm the cabin. All the temperature sensors should show the same, usually with +- 3F differences. Very common evaporator sensor failure will show temperature very high
2 a minute after restarting car that was sitting on hot parking lot. This will allow to see if compressor is building the pressure and how the cores work.