Danfoss Compressor Won't Start, Trips on Overload, or Won't Hold Pressure? Diagnosing Scroll, Reciprocating, and Control Failures Before You Replace the Wrong Part

Danfoss compressor and control failures usually announce themselves through one of four symptoms: a light-commercial AC unit that hums but won't start, a commercial refrigeration rack losing capacity, a pressure switch nuisance-tripping the compressor offline, or a case that won't hold superheat no matter how many times it's charged. Each points to a different failure category — and confirming which one you're dealing with before you order a part saves a return shipment and a second truck roll. Below, we break down the most common Danfoss compressor, pressure switch, and expansion valve failures by category, with the OEM part numbers technicians actually order to resolve them.

This guide covers troubleshooting and part identification only — always follow manufacturer service procedures and have refrigerant work performed by a licensed technician.

Scroll Compressor Won't Start or Trips on Thermal Overload (Light-Commercial AC, R407C Systems)

On 5-ton residential and light-commercial split systems running R407C, a scroll compressor that hums without starting, trips repeatedly on internal overload, or draws high locked-rotor amps almost always traces back to one of three things: a failed run capacitor, a compressor that's lost internal winding insulation from age or a prior electrical event, or a start-assist component that's no longer providing enough torque under load.

Diagnostic steps:

• Confirm line voltage is within the 208–230V single-phase range before condemning the compressor.
• Check the run capacitor first — a compressor that hums and trips on overload but shows normal winding resistance is frequently a bad 55 µF run capacitor, not a failed compressor.
• If the compressor has high torque-starting demands (long line sets, low ambient starts), verify a start capacitor (88–108 µF) and potential relay are present and functioning — their absence is a common cause of chronic hard-starting that eventually takes out the compressor itself.
• Megohm-test the windings to ground. A compressor that trips on overload with good capacitors and correct voltage, but shows low winding-to-ground resistance, has failed internally and needs replacement.

Resolution: The Danfoss 121L3070 is the current-generation 5-ton hermetic scroll compressor for this application — a direct replacement for the earlier 120U-series compressors (120U3070, 120U0281, 120U1521, 120U1546) and cross-references to York OEM part 015-03406-004. It's engineered specifically for R407C, runs on 208-230V single-phase 60Hz power, and uses brazed connections (7/8" suction, 1/2" ODF discharge) with PVE lubricant already charged from the factory. The hybrid scroll wrap design and internal motor protection are built to reduce the same overload-trip pattern that often kills older reciprocating and early scroll designs — and Danfoss rates it at roughly 3 dB quieter than older reciprocating equivalents, worth noting on light-commercial jobs where noise complaints are common.

Reciprocating Compressor Losing Capacity or Short-Cycling (Medium-Temp Commercial Refrigeration)

Commercial refrigeration racks and walk-in coolers running 4-cylinder reciprocating compressors present differently than residential AC failures. The complaint is usually gradual capacity loss, inconsistent case temperatures, or a compressor that short-cycles under load rather than an outright no-start.

Diagnostic steps:

• Check discharge and suction pressures against the rating conditions for the refrigerant in use — capacity loss on a multi-refrigerant-rated compressor is frequently a valve-plate or ring-wear issue that shows up as reduced volumetric efficiency before the compressor fails outright.
• Verify oil level and condition. Reciprocating compressors in parallel rack configurations depend on correct oil equalization between units — a compressor that's short on oil in a rack setup usually means the oil equalization line or port has a restriction, not that the compressor itself failed.
• Confirm the contactor and overload protection are sized correctly for the compressor's actual Locked Rotor Amp (LRA) and Maximum Continuous Current (MCC) ratings — undersized protection causes nuisance trips that get misdiagnosed as compressor failure.
• If the unit is IP54-rated for a demanding mechanical room environment, inspect the terminal box for moisture or contamination before assuming an electrical fault is internal to the compressor.

Resolution: The Danfoss MTZ160-4VI (also identified as MTZ160HW4AVE) is a 4-cylinder hermetic reciprocating compressor built for medium-temperature commercial refrigeration and rated across a wide refrigerant range — R404A, R134a, R407C, R407F, R448A, R449A, R452A, and R513A — which matters given how many of these systems are mid-transition between legacy and next-generation refrigerants. It delivers 107,631 BTU/h on R134a and 87,421 BTU/h on R404A/R507A under standard rating conditions, running on 380-400V 3-phase 50Hz or 460V 3-phase 60Hz power. It carries an LRA of 130A and MCC of 36A — confirm contactor and breaker sizing against those numbers before installing. It uses 175PZ polyester oil, connects via 1-3/4" Rotolock suction and 1-1/4" Rotolock discharge with ODF sweat sleeves, and includes a 3/8" NPT oil equalization port for parallel rack setups. It's a direct replacement for the earlier MTZ160HW-4AVE and ships with a complete mounting kit (grommets, bolts, nuts, sleeves) for vibration-dampened installation.

Pressure Switch Nuisance-Tripping the Compressor Offline

A low-pressure switch that trips the compressor or fan intermittently — especially under normal operating conditions — is one of the most common "phantom failure" complaints on commercial refrigeration and HVAC systems. Before replacing the compressor or condemning the refrigerant charge, confirm the switch itself isn't the actual point of failure.

Diagnostic steps:

• Check the switch's regulation range and differential setting against the system's actual operating pressures. A switch set too close to normal operating pressure will nuisance-trip even on a correctly charged, correctly functioning system.
• Inspect the bellows and mechanical connection for physical damage or contamination — vibration fatigue is a common failure mode on switches mounted directly to compressor manifolds.
• If the switch is an older 060-200166, note that Danfoss has moved to a current-generation successor part — mismatched replacement parts or switches with drifted calibration are a frequent source of intermittent trips that get misdiagnosed as refrigerant or compressor problems.

Resolution: The Danfoss 060-200191 KP1 pressure switch is the current-generation replacement for the 060-200166, built for low-pressure monitoring on commercial refrigeration and HVAC systems. It uses a SPDT contact function for compressor protection or fan cycling, with a regulation range of -0.2 to 7.5 bar (6 inHg to 108 psig) and an adjustable differential of 0.7 to 4.0 bar (10 to 58 psi) for precise setpoint calibration. It connects via 1/4" flare fitting, carries an IP30 enclosure rating for compact cabinet installations, and is rated for a maximum working pressure of 245 psig (290 psig test pressure). It's compatible with R134a, R404A, R448A, R449A, and R513A, and — per Danfoss's own spec — remains usable on legacy R22 systems within its -0.2 to 7.5 bar range, provided the pressure settings fall within spec. It's an auto-reset design, meaning no manual reset is required once pressure returns to the differential setpoint. Rated for ambient temperatures from -13°F to 149°F, it holds up in outdoor condenser installations as well as indoor mechanical rooms.

TXV Hunting, Flooding, or Failing to Hold Superheat

A thermostatic expansion valve that can't maintain stable superheat shows up as evaporator flooding (liquid slugging back to the compressor), starving (reduced capacity, high superheat, frosted or iced coils), or hunting between the two. On commercial refrigeration systems running R404A or R507A, this is frequently a valve-specific issue rather than a charge or airflow problem — especially on older installations where the original valve has exceeded its service life.

Diagnostic steps:

• Confirm the refrigerant in the system matches the valve's rated refrigerant — a TXV charged and calibrated for one refrigerant blend will not hold stable superheat on a different one, even if physically compatible with the port sizes.
• Check the orifice sizing against the system's actual tonnage. An undersized or oversized orifice on an otherwise healthy valve produces the same hunting/flooding symptoms as a failed power element.
• Inspect the capillary tube and remote bulb placement and insulation — a poorly insulated or misplaced bulb reads ambient temperature instead of suction line temperature and causes chronic superheat instability that looks like a valve failure but isn't.
• If the valve has been in service for years on a system that's had refrigerant changes over its life, a stress-fatigued diaphragm or degraded power element charge is a common root cause of intermittent hunting that gets worse over time.

Resolution: The Danfoss 068U2287 (also listed as 068U228700) is a stainless steel TUAE thermostatic expansion valve designed for precise liquid injection on R404A and R507A systems. It uses an external equalization connection (1/4" ODF) with 3/8" inlet by 1/2" outlet solder ODF connections, rated to a maximum working pressure of 500 psig (34 bar) and effective across a -40°F to 50°F operating range. The bimetal solder connections allow installation without wet-wrapping, and the 59-inch capillary tube gives flexibility for remote bulb placement on tight equipment layouts. It's bi-flow capable and uses an exchangeable orifice assembly (sizes 1-8, sold separately or as part of a kit) so the valve body can be field-matched to different tonnage requirements without replacing the whole assembly. The laser-welded power element is built to extend service life by reducing diaphragm stress over time.

Danfoss Component Failures at a Glance

Why Legacy Refrigerant Compatibility Matters Right Now

All four of these components serve systems running refrigerants directly affected by the AIM Act's ongoing HFC phase down — R407C, R404A, and R507A are all HFCs subject to the EPA's production and import caps, and reclaimed-refrigerant costs for these blends have continued climbing as the cap schedule tightens toward its next major step in 2029. A rule finalized by the EPA and effective July 27, 2026 clarified continued allowances for existing HVAC and refrigeration equipment to stay in service on these refrigerants — which means more of these systems, not fewer, will need correctly sourced compressor, switch, and valve replacements over the next several years rather than wholesale system replacement. With refrigerant costs rising and legacy equipment staying in the field longer, getting the compressor or control part right the first time matters more than it used to — a mismatched or generic part on one of these systems risks another service call and another refrigerant recovery/recharge cycle, not just a wasted part.

A note on service: GSIstore supplies OEM and cross-reference parts for HVACR technicians — we don't perform installation, service, or repair. The diagnostic steps above are for licensed technicians working on their own jobs.

Confirm Fit Before You Order

Danfoss part numbers span multiple compressor families, refrigerant ratings, and voltage configurations, and the same base component often has several revisions over its production life. Before ordering any part listed above, confirm the part number against your unit's rating plate or original OEM part number — a part number match on refrigerant type and physical connections doesn't guarantee full compatibility with every application of that part family. When in doubt, cross-reference against your unit's data plate rather than the compressor's general application notes.

FAQ

Is the Danfoss 121L3070 compatible with R22 systems? While R407C is a common R22 retrofit refrigerant, this compressor is charge-optimized with PVE oil specifically for R407C. Always verify that system expansion valves and seals are compatible with the lubricant before installation.

What electrical components does the 121L3070 need to start reliably? This is a single-phase unit requiring a 55 µF run capacitor. For applications with high-torque starting requirements, a secondary start capacitor (88-108 µF) and potential relay may be needed.

Can the MTZ160-4VI be used in a parallel compressor rack? Yes — it's equipped with a 3/8" NPT oil equalization connection, making it suitable for manifolding in multiple compressor racks.

Does the 060-200191 pressure switch require a manual reset after tripping? No. It's an automatic-reset design — the circuit restores once pressure returns to the differential setpoint, with no field reset needed.

Does the 068U2287 TXV include the orifice, or is it sold separately? The 068U2287 is the valve body and power element only. The orifice is typically sold separately or as part of a kit, sized to match your system's tonnage.

Part numbers referenced above are for identification purposes. Confirm compatibility with your specific equipment model before ordering — a part number match on refrigerant type or connection size does not guarantee fit across every application within a part family.