Here's a little problem for you.

Petrol (gas) station upright chilled display unit with twin evaporators, TXVs and solenoid valves.

Unit was running on R12, changed to R413a in Nov '00, compressor failed last week. Unit was not new when installed in early 2000, year date is 1990.

One of our A/C guys fitted a replacement L'Unite R404a condensing unit, two new TXVs, filter driers etc, last Friday. System charged up and left running with cabinet down to temperature.

Unit failed to hold temperature and would not run from Saturday morning, customer switched off evaporator unit.

I went to site this a.m. having been told that the LP pressure switch may not have set properly. Standing pressures were - suction 2.0 barg, discharge (receiver outlet) 10.0 barg. I lowered the LP pressure setting as the pressure switch was open circuit, sorry - pressure switch is combined LP/HP auto reset. The compressor started and immediately pumped down again; I hadn't powered up the evaporator yet DOH!

Switched on the evap unit, heard the solenoid valves open, a short rush of gas and then nothing. Returned to the condensing unit and found that it had tripped on the HP switch which was set at 25.0 barg; standing pressure on the suction was up to over 5.0 barg which at least showed that something had happened.

Where would you look next ? sorry no prizes for this one.