Mitsubishi Electric AC / Heat Pump Error P8 – Causes & Fix
The indoor coil isn't getting colder (or warmer) than the room after the compressor starts, so the unit stops.
At a glance
Severity: Moderate
- Meaning
- Pipe Temperature Error – Coil Not Responding
- Typical repair cost
- $100–$1,000
- DIY difficulty
- Moderate
P8 is a plausibility check, not a sensor failure. About 3 minutes after the compressor starts in cooling, the indoor board compares the room thermistor in the air intake (TH1) with the pipe thermistors on the indoor coil (TH2 on the liquid line, TH5 on the coil itself). If the coil is not at least about 5.4 °F colder than the room air and stays that way for another 6 minutes, the system stops and logs P8 – so the code never appears in less than roughly 9–10 minutes of running. In heating a similar test runs once the warm-start phase is over, this time expecting the coil to heat up. The check is skipped in dry mode. A small temperature gap usually means the refrigerant isn't doing its job (low charge, closed stop valve, blockage), but a pipe sensor that has slipped out of its clip, or piping and wiring crossed between two heads on a multi-zone system, fools the check just as well.
How it looks on the display
Model families & display variants
| Model family | Shows | Note |
|---|---|---|
| M-Series wall units (MSZ, MXZ multi-zone) with handheld remote | blinking lamp | The indoor unit shows a lamp blink pattern listed in your model's manual; the remote's recessed CHECK function can help read the stored code. |
| P-Series and ducted units (PKA, PLA, PEAD, SEZ) with wired controller | P8 | Shown directly on the controller, usually with the unit address. The stored history helps tell a one-off from a repeat fault. |
| Hyper-heat (H2i) systems in heating | P8 | In heating the check expects the coil to warm up after start-up; a unit that stays lukewarm in a cold snap can trip P8 when charge or valve problems limit capacity. |
Safety first · Consult a professional
Opening service valves, adding refrigerant or finding a leak legally requires an EPA 608-certified technician – many newer units use R-32, which is mildly flammable, so keep open flames away from a suspected leak. The terminals and the inverter board in the outdoor unit carry 208–230 V and stay charged after switch-off; only use the breaker and the outdoor disconnect.
- Unplug the appliance or switch off the breaker before opening any panel. Mains voltage can kill.
- Sealed refrigerant system: repairs require an EPA 608 certified technician.
Symptoms
- P8 on the wired controller, or the indoor unit's lamp blinking a fault pattern
- System runs normally for about 10 minutes, then the compressor stops while the fan keeps going
- Air from the indoor unit is barely cooler than the room in cool mode (or barely warm in heat mode)
- On a multi-zone system, a different room cools or heats than the one you switched on
- Frost on the small copper line or the valves at the outdoor unit, or an oily, dusty spot at a flare nut
- Unit seems to cool fine in dry mode but trips in cool mode
Possible causes
Ranked by how often each cause is behind this code (editorial estimate).
- Refrigerant shortage from a leak, most often at a flare connection or a service valve
- Pipe thermistor slipped out of its holder on the indoor coil, so it reads room air instead of pipe temperature
- Stop valve at the outdoor unit not fully opened after installation
- Refrigerant lines or S1-S2-S3 wiring crossed between two indoor units on a multi-zone or twin system
- Outdoor unit not really running – separate outdoor breaker tripped, compressor or fan not starting
- Room (intake) or pipe thermistor reading wrong because the sensor itself has drifted
- Cold supply air short-cycling straight back into the intake (shelf, curtain or ceiling too close)
Fix it step by step
Time the fault
Start the system in cool mode with a low setpoint and note when P8 appears. Around 10 minutes is typical for this code; if the unit stops within 1–2 minutes, you're more likely looking at a different fault, so check which code is actually stored.
Confirm the outdoor unit is working
While the indoor unit calls for cooling, go outside and listen for the compressor hum and the fan. If the outdoor unit is silent, check its own breaker and the disconnect – many P-Series and some M-Series installs power the outdoor unit separately. A silent outdoor unit means the indoor coil never gets cold, which trips P8 without any refrigerant fault.
Tools: Flashlight
Look at the lines without opening anything
At the outdoor unit, the insulated larger line should feel cool in cooling. Frost on the small line or on a valve body, or a dark oily film with dust stuck to it at a flare nut, points to low charge – note it and leave the valve caps alone.
Tools: Flashlight
Test one head at a time on multi-zone systems
Switch every indoor unit off, then run only one in cool mode for 15 minutes and check which room actually gets cold air. If the air comes out of another head, piping or signal wiring is swapped between the two branches – a classic after new installs or added zones.
Check how the indoor unit breathes
The room thermistor sits in the return air. A shelf just above or below the unit, a curtain or a tight ceiling can send cold discharge air straight back into the intake, shrinking the measured temperature gap. Give the unit free space and clean the filters while you're there.
Tools: Soft brush
How to reset P8
- Switch the system off at the remote or wired controller
- Turn off the breaker and outdoor disconnect for about 5 minutes
- Restore power and restart; watch whether P8 returns after roughly 10 minutes of running
Where the parts are
- 1Fan
- 2Temperature sensor
- 3CAN bus
- 4Outdoor unit
- 5Compressor
- 6Engine computer (ECM)
Repair options & costs
| Repair | DIY? | Estimated cost |
|---|---|---|
| Technician diagnostic visit | No | $100–$200 |
| Re-clip or replace pipe thermistor | No | $150–$350 |
| Open stop valves or correct crossed piping/wiring (installer) | No | $150–$500 |
| Leak search, flare repair and recharge | No | $400–$1,000 |
Rough US estimates; technician call-out fees vary by region. Not a quote.
When to call a technician
Book an EPA 608-certified HVAC contractor if P8 comes back after one power reset while the outdoor unit clearly runs. The tech will compare TH1, TH2 and TH5 readings with a thermometer on the pipe and check suction pressure before adding any refrigerant. If the system is new or zones were just added, call the installer first – open valves and crossed branches are installation work.
Frequently asked questions
Why does P8 only show up after about ten minutes?
The board waits 3 minutes after compressor start before it judges the coil, then gives the fault another 6 minutes to clear. A unit that trips right after starting has a different problem.
Does P8 always mean I'm low on refrigerant?
No. It's the most common reason, but the board only sees temperatures. A loose pipe sensor, a half-closed stop valve or swapped lines on a multi-zone system create the same small temperature gap with a perfectly good charge.
Can I add refrigerant myself?
No. Buying and handling refrigerant requires EPA 608 certification, and topping up without fixing the leak just brings P8 back. The charge on these systems is weighed in by the tech after the leak is repaired and the lines are evacuated.
Is P8 the same as P6?
No. P6 is freeze or overheat protection, often from a clogged filter or blocked coil. P8 means the coil isn't changing temperature enough in the first place.
Sources
- Official Mitsubishi Electric user documentation and support pages (facts only; wording is our own)
- Methodology: How We Build Code Pages · Data Sources We Use · Editorial Policy