OBD-II Code B3933: Air Conditioning Evaporator Temperature Sensor Circuit

What Does B3933 Mean?

The HVAC control module detects an electrical fault in the A/C evaporator temperature sensor circuit. The module disables the A/C compressor to prevent the evaporator core from freezing into a block of ice.

Technical definition: The SAE/ISO definition for B3933 is "Air Conditioning Evaporator Temperature Sensor Circuit." The HVAC module sets this code when the sensor's signal voltage drops below 0.443V (short to ground) or exceeds 4.55V (open circuit) for a specified duration.

Can I Drive With B3933?

Yes. Yes, you can drive the vehicle. This code disables the A/C compressor but does not affect engine safety. However, on affected 2007-2009 GM models 🎬 Watch: Why your Silverado A/C keeps freezing up., the evaporator core can freeze solid, blocking airflow and eliminating windshield defrosting capabilities.

Common Causes

• Unprogrammed HVAC Control Module (Very Common) — On 2007-2009 GM vehicles, replacing the HVAC module without reprogramming it triggers this code. The new module searches for an evaporator temperature sensor that does not exist on these specific models.
• Faulty A/C Evaporator Temperature Sensor (Common) — The internal thermistor fails, sending a defaulted -40°F signal to the control module. This is a standard wear-and-tear failure for vehicles equipped with this sensor.
• Damaged or Disconnected Wiring (Common) — Frayed wires, corroded pins, or a connector left unplugged after a dashboard repair breaks the circuit. Rodents chewing through the harness behind the glovebox is also a frequent cause.
• Clogged Evaporator Drain Tube 🎬 See how to quickly unclog your car's A/C drain line. (Less Common) — A blocked drain backs up condensation into the HVAC case. This standing water submerges the sensor and shorts its electrical connector.

Symptoms

• A/C blows warm air — The HVAC module disables the A/C compressor as a protective measure, resulting in zero cooling.
• A/C compressor does not engage — You will not hear the characteristic 'click' of the A/C compressor clutch engaging when turning on the air conditioning.
• Reduced airflow from vents — The evaporator core freezes into a solid block of ice due to a lack of temperature regulation, physically blocking air from passing through the vents.
• Musty smell or wet carpet — Standing water in the HVAC case from a clogged drain leads to mold growth and leaks into the passenger footwell.
• Live data shows -40°F/-40°C (scan-tool only — no driver-felt sign) — A scan tool reading of the evaporator temperature shows a defaulted value of -40 degrees, confirming an open or shorted circuit.

Diagnostic Flowchart

Tap your situation to follow the diagnostic path that matches what you're seeing on this code.

Common Fixes & Costs

• Reprogramming the HVAC Control Module — Parts: $0, Labor: $150-$300, ~1.5 hr book time (Professional)
• Replacing the A/C Evaporator Temperature Sensor — Parts: $25-$70, Labor: $200-$1000+, ~8 hr book time (Advanced)
• Repairing Damaged Wiring or Connectors — Parts: $5-$25, Labor: $100-$300, ~2 hr book time (Intermediate)
• Clearing Clogged Evaporator Drain Tube — Parts: $0, Labor: $0-$100, ~0.5 hr book time (Beginner)
• Replacing the HVAC Control Module — Parts: $200-$450, Labor: $150-$300, ~1.5 hr book time (Professional)

DIY vs Professional

• Reprogramming the HVAC Control Module — Beginner:
  Tools: J2534 Passthru device, Windows laptop, battery maintainer, GM TIS2WEB software subscription.
• Replacing the A/C Evaporator Temperature Sensor — Beginner:
  Tools: Trim removal tools, full metric socket set.
• Repairing Damaged Wiring or Connectors — Beginner:
  Tools: Multimeter, wire strippers, heat gun, weatherproof butt connectors.
• Clearing Clogged Evaporator Drain Tube — Beginner:
  Tools: Compressed air or flexible cleaning brush.

Used vs. New Parts: Buying Guide

When a used part is worth it: Never. The labor cost to access the sensor is too high to risk installing a used part. Used HVAC modules require expensive professional reprogramming.

Donor quality checklist:

• Avoid buying used electronic modules as they require costly VIN programming.

Decision logic:

• If The part is an electronic sensor with high labor cost for access → Always buy new. The small savings on a used part are not worth paying for 8 hours of labor twice.
• If The part is a control module that requires VIN programming → Buy a new OEM part from a specialist that pre-programs it to your VIN before shipping.

Warranty tradeoff: Used parts have a 30-day warranty. New OEM parts carry a 1-2 year warranty.

Worst-case if a used part fails: $1000 if a used sensor fails after a dash-out installation, requiring a repeat of the labor.

What Happens If You Wait — Timeline

1. 0-1 week: Code B3933 is set. The A/C compressor is disabled by the control module, resulting in no cold air. (MPG impact: 0%% · Added cost: $0)
2. 1 week - 2 months: If the compressor runs without regulation, the evaporator core freezes, causing reduced airflow and water dripping into the passenger footwell. (MPG impact: 0%% · Added cost: $50-$250)
3. 2-6 months: Sustained water leakage saturates carpet padding, leading to mold growth and a persistent musty odor. (MPG impact: 0%% · Added cost: $200-$1000)
4. 6+ months: Running the A/C with a freezing evaporator sends liquid refrigerant back to the compressor, causing catastrophic mechanical failure. (MPG impact: 0%% · Added cost: $800-$2500)

Cost of Not Fixing It

• Immediate: No air conditioning. The HVAC module disables the compressor. (Added cost: $0)
• 1-3 months: If the compressor runs without regulation, the evaporator core freezes solid, causing water to leak into the passenger footwell and breed mold. (Added cost: $200-$1000)
• 6+ months: Liquid refrigerant returns to the compressor ('slugging'), causing catastrophic internal mechanical failure. (Added cost: $800-$2500)

Diagnosis Steps

1. Check for Technical Service Bulletins (TSBs)
   Check for TSBs related to your vehicle. For 2007-2009 GM trucks and SUVs, TSB PIT5297 identifies that this code is caused by an unprogrammed HVAC module, not a bad sensor. This step saves hours of unnecessary dashboard disassembly.
   Tools: Alldata, Mitchell1, or Google Search (Beginner)
2. Scan Live Data for -40°F Default Value
   Use an advanced OBD-II scanner to view the live data stream for the 'A/C Evaporator Temperature Sensor'. A reading stuck at -40°F/-40°C confirms a hard circuit fault. Normal operation shows temperatures cycling between 34°F and 38°F.
   Tools: Advanced OBD-II Scanner (Beginner)
3. Verify HVAC Module Programming (GM Specific)
   If diagnosing a 2007-2009 GM truck, confirm the HVAC module is programmed to the vehicle's VIN. Unprogrammed modules default to searching for a non-existent sensor. Reprogramming the module clears the code permanently.
   Tools: GM Tech2 or J2534 Passthru device (Professional)
4. Visually Inspect and Wiggle Test Wiring
   Locate the sensor wiring behind the glovebox. Watch the live data scanner while gently wiggling the harness and connector. If the temperature jumps from -40°F to a normal reading, you have located a broken wire or loose connection.
   Tools: Advanced OBD-II Scanner, Flashlight (Intermediate)
5. Test Circuit Voltage (5V Reference)
   Back-probe the sensor connector with the ignition on. You must see a 5V reference voltage on the signal wire. Readings below 0.4V indicate a short to ground, while exactly 5V indicates an open circuit.
   Tools: Multimeter, Back-probe pins (Intermediate)
6. Test Sensor Resistance (Ohms)
   Disconnect the sensor and measure its internal resistance. A working NTC sensor reads roughly 9,800 Ohms at 32°F and 3,000 Ohms at 77°F. A reading of infinite resistance (OL) confirms the sensor is dead and must be replaced.
   Tools: Multimeter, Thermometer (Advanced)
7. Check for Shorts with a Fused Jumper
   Apply a known voltage to the signal circuit using a fused jumper wire. If the scanner reading changes, the wiring is intact and the sensor is bad. If the fuse blows immediately, the harness has a dead short to ground.
   Tools: Fused jumper wire, Multimeter (Advanced)

When This Code Triggers (Freeze-Frame Conditions)

• Ignition Status: On (The fault is detected immediately when the ignition is turned on as the HVAC module performs a self-test.)
• A/C Request Signal: On (The code sets when the A/C is requested by the driver, and the module cannot get a valid reading to permit compressor operation.)

Related Codes

• B3933 02 — Indicates a short to ground in the evaporator temperature sensor circuit. The signal wire voltage will read near 0V.
• B3933 05 — Points to a short to battery or an open in the sensor circuit. The signal wire will show battery voltage or a flat 5V.
• B0164 — Code for the passenger compartment temperature sensor. B3933's sensor is deep in the HVAC case, while B0164's is behind a small grille on the dash face.

Climate & Environmental Factors

• High Humidity: Increases condensation on the evaporator core. If the drain clogs, moisture backs up and submerges the sensor, causing electrical shorts.
• Airborne Contaminants: Volatile Organic Compounds (VOCs) from air fresheners mix with condensation to form corrosive acids. This eats away at the sensor wiring over time.

How to Talk to a Mechanic About This Code

Say this: "I have a B3933 code and no A/C. If it's a 2007-2009 GM truck, please check the HVAC module programming first. Otherwise, please check the sensor's live data and wiring before quoting a full dashboard removal."

This directs the mechanic to the most common and cheapest fixes first (programming, wiring) before they assume the most expensive one (dash-out sensor replacement).

Avoid saying:

• 'My A/C is broken, please fix it.'
• 'My check engine light is on, can you look at it?'
• 'Just replace the evaporator sensor.'

Questions to ask before authorizing the repair:

• What was the live data reading from the old sensor?
• Did you perform a 'wiggle test' on the wiring harness to rule out a loose connection?
• For a GM truck, do you have the J2534 pass-thru device to reprogram the HVAC module?
• What is the warranty on the labor if the dashboard has to be removed?

Where to Take It: Dealer vs Independent vs Chain

• Dealer:
  Best for: 2007-2009 GM trucks/SUVs where HVAC module programming is the likely cause., Vehicles under warranty.
  Downsides: Highest labor rates., May be less willing to perform wiring repairs and more likely to replace entire harnesses. (Typical cost: +50% vs. baseline)
• Independent Shop: Best fit for most scenarios. For 2007-2009 GM trucks, you MUST confirm the shop has a J2534 pass-thru device to perform GM-specific module programming.
  Best for: Out-of-warranty vehicles where the cause is likely a failed sensor, wiring, or clogged drain., Getting a second opinion on a high dealership quote.
  Downsides: Must verify they have advanced scanners to read B-codes and live data., May not have the specific equipment to program GM modules. (Typical cost: +0% vs. baseline)
• Chain Shop: AVOID for B3933 diagnosis. Their business model is not suited for the diagnostic nuance this code requires.
  Best for: Simple A/C refrigerant recharge services, not diagnostics.
  Downsides: Technicians lack the training for complex electrical diagnostics., Highly likely to misdiagnose a programming/wiring issue as a 'bad sensor,' leading to unnecessary repairs. (Typical cost: -10% vs. baseline)

When to Walk Away From the Repair

If the estimated repair cost exceeds 50% of the car's private-party value, consider selling the vehicle.

• Car worth $8000, fix is $1000: Fix it. The repair cost is less than 20% of the car's value and restores a major comfort feature.
• Car worth $4000, fix is $1500: Borderline. The repair is nearly 40% of the vehicle's value. Get a second opinion before proceeding.
• Car worth $2500, fix is $1500: Walk away. The repair cost is 60% of the car's value. It is not economically sensible to proceed.

What Scan Tool You Need for This Code

Minimum: A scanner that reads manufacturer-specific Body ('B') codes and views live sensor data. Basic readers that only show 'P' codes are useless for this.

A cheap $20 reader will not see B3933. You must see the live data from the 'A/C Evaporator Temperature Sensor' to confirm if it is stuck at -40 degrees.

Budget: BlueDriver Pro (~$100) — Confirms the B3933 code is present and displays the live data stream for the evaporator temperature sensor to perform a 'wiggle test'.

Mid-range: Foxwell NT510 Elite (~$150) — Provides full OE-level diagnostics and bi-directional control, allowing you to command the A/C compressor clutch to turn on.

Professional: Autel MaxiSys MS906 Pro (~$1200) — Offers full bidirectional control and advanced coding functions. Combined with a J2534 pass-thru device, it performs the necessary GM HVAC module programming.

Rent vs buy: You can borrow a scanner from AutoZone for free, but you must verify their specific loaner tool reads Body (B) codes. Buying a budget scanner is a worthwhile investment.

How to Clear the Code After You Fix It

1. Use an OBD-II scan tool to clear the 'B' (Body) trouble codes.
2. On GM vehicles, perform an HVAC actuator calibration by removing the HVAC fuse for 60 seconds, reinstalling it, and starting the vehicle without touching climate controls for 40 seconds.
3. Operate the A/C system through all modes and temperature ranges to confirm proper operation.

Drive cycle (~10 minutes): Start the engine and let it idle. Turn on the A/C and allow it to run for 10 minutes. If the repair was successful, the code will not return.

Readiness monitors affected: None. B-codes do not affect emissions readiness monitors.

Watch out for:

• Disconnecting the battery does not clear the code from the HVAC module's memory.
• Failing to let the HVAC module self-calibrate after a power reset causes incorrect blend door operation.

Will This Fail Emissions / State Inspection?

No — by itself this code doesn't fail OBD inspection (but it can keep readiness monitors from setting, which causes a separate fail).

• California: This code will NOT cause a smog check failure. California's test looks for an illuminated Check Engine Light and emissions-related readiness monitors.
• New York: An OBD-II scan is part of NY's inspection, but it targets emissions codes ('P' codes). A B-code does not cause a failure.
• Texas: Texas inspections check for emissions-related faults. A body control module code like B3933 will not trigger a failure.

Most Commonly Affected Vehicles

• Chevrolet Silverado, Suburban, Tahoe, Avalanche (2007-2009) — Subject to TSB PIT5297. These models do not have an evaporator temp sensor. The code appears if the HVAC module is replaced and not programmed.
• GMC Sierra, Yukon (2007-2009) — Affected by the same HVAC module programming issue outlined in TSB PIT5297. Models from 2010 onwards do use the physical sensor.
• Cadillac Escalade (2007-2009) — Included in GM TSB PIT5297 regarding unprogrammed HVAC control modules causing a false B3933 code.
• Hummer H2 (2008-2009) — Included in GM TSB PIT5297 for the B3933 code being caused by an unprogrammed HVAC control module.
• Chevrolet Camaro (2010-2015) — Commonly experiences actual sensor failure. The sensor is easily accessible on the passenger side next to the blower motor.
• Chevrolet Sonic (2013-2014) — TSB PI1044 addresses poor A/C performance linked to this sensor. The recommended fix is replacing the sensor (Part No. 95018085).
• Chevrolet Cruze (2008-2016) — The sensor is located behind the passenger glove box, pushed into the side of the HVAC case.
• Cadillac SRX (2008-2009) — Prone to actual sensor circuit failures requiring dashboard access to repair.

Manufacturer-Specific Notes

• General Motors (Chevrolet, GMC, Cadillac, Hummer): Per TSB PIT5297, this code on 2007-2009 trucks is caused by replacing the HVAC control module and failing to program it. These specific models do not use an evaporator temperature sensor.
• Ford: Ford vehicles use an identical 'Evaporator Air Temperature Sensor' but set different diagnostic trouble codes (e.g., B1057, P0536) for the exact same failure.

Real Owner Stories

2008 Chevy Silverado - The Phantom Sensor

Owner replaced a faulty HVAC control head with a used one. A month later, the A/C stopped working and code B3933 appeared.

Outcome: After finding TSB PIT5297, the owner realized the replacement HVAC module needed programming. He used a Tech2 scan tool to perform the 'Replace and Program' procedure. The code cleared and the A/C worked correctly.

Lesson: On 2007-2009 GM trucks, B3933 is almost always a programming issue after an HVAC module replacement. Always check TSBs before disassembling the dash.

2010 Chevy Camaro SS - An Easy Fix

A/C suddenly started blowing warm air. A scan revealed code B3933.

Outcome: After removing the panel under the glovebox, the sensor was accessible. The owner twisted and pulled it out, installed the new one, cleared the code, and the A/C immediately blew cold.

Lesson: On the 5th Gen Camaro, this sensor is very easy to access and replace. Don't assume all B3933 repairs require dashboard removal.

2013 Chevy Cruze - Misdiagnosis Avoided

A/C worked intermittently and eventually failed, setting code B3933.

Outcome: Instead of replacing the sensor, the owner cleaned the connector pins and ensured it was securely seated. The code did not return.

Lesson: Always perform a 'wiggle test' on the sensor's wiring while watching live data before committing to a sensor replacement.

2011 GMC Yukon - Clogged Drain Causes Electrical Gremlins

A/C stopped blowing cold air, accompanied by a musty smell and dampness on the passenger floor mat. Code B3933-02 was present.

Outcome: The drain tube was clogged with debris. After clearing it with compressed air, trapped water drained out. Once the area dried, the code cleared and did not return.

Lesson: If B3933 is accompanied by a musty smell or wet carpet, clear the evaporator drain tube first. A clog causes water to back up and short the sensor.

How to Prevent This Code From Triggering

• Regularly clean the A/C evaporator drain tube. (Once every 1-2 years.) — Prevents water from backing up into the HVAC case and shorting out the sensor connector.
• Dry the evaporator core before shutting off the vehicle. (Daily habit in humid climates.) — Turn the A/C compressor off but leave the fan blowing a few minutes before arriving. This dries the core, preventing corrosive mold growth.
• Replace cabin air filters regularly. (Every 12,000-15,000 miles.) — Maintains proper airflow across the evaporator, preventing it from freezing and trapping debris that clogs the drain tube.

Frequently Asked Questions

My mechanic says my 2008 GM truck doesn't have this sensor, but I have the code. Why?

Your mechanic is right. 2007-2009 GM trucks do not have this sensor. An unprogrammed replacement HVAC module triggers this false code. Programming the module to your VIN fixes it.

Why is the labor cost so high to replace a cheap sensor?

The sensor is buried deep inside the dashboard's HVAC housing. Mechanics must remove the entire dashboard assembly, which takes 6 to 10 hours. The labor dominates the repair bill.

My mechanic replaced the sensor, but the code came back. What's next?

You likely have a wiring issue or an unprogrammed control module. Perform a 'wiggle test' on the wiring harness while watching live scanner data. If the wiring is intact, the HVAC module requires professional reprogramming.

Can I just reset the code to get my A/C working again?

No, resetting the code only provides a temporary fix. The HVAC module runs a self-test on every ignition cycle. The code will return immediately and disable the A/C again.

Can I replace the evaporator temperature sensor myself?

It depends on the vehicle model. On a 2010-2015 Camaro, it is an easy DIY job accessible behind the glovebox. On most trucks, it requires advanced dashboard removal.

Will my A/C work at all with this code?

No, the vehicle's computer disables the A/C compressor to protect the system. This prevents the evaporator core from freezing into a solid block of ice.

How do you test an evaporator temperature sensor?

Watch the live data on a scan tool; a reading stuck at -40°F indicates a fault. Alternatively, measure the sensor's resistance with a multimeter. A reading of infinite Ohms (OL) confirms the sensor is dead.

What is the difference between an evaporator temperature sensor and an ambient temperature sensor?

The evaporator sensor lives inside the dashboard and measures the A/C core temperature to prevent freezing. The ambient sensor sits behind the front grille and measures outside air temperature. They control entirely different aspects of the climate system.

Key Takeaways

• On 2007-2009 GM trucks and SUVs, code B3933 is almost always caused by an unprogrammed HVAC control module, not a faulty sensor.
• A live data scan tool reading of -40°F (-40°C) for the evaporator temperature confirms a hard electrical fault in the sensor circuit.
• Do not replace the sensor without first performing a wiring 'wiggle test' and checking for a clogged evaporator drain tube.
• Replacing the sensor on most vehicles requires 6 to 10 hours of labor for full dashboard removal, making accurate diagnosis critical before authorizing repairs.