Had a play with mine today so thought I'd add what little I discoveed to this great thread.
Mine hasn't run at all during the really cold snap, so today I checked the ambient sensor. With the engine running I had 12v on one terminal - that's good so I linked the 2 terminals as shown earlier in the thread.
The fbh still didn't fire so I pulled F5 in the engine fuse box for about 30sec. There's been a little confusion whether to do this with the engine running or not as F5 is also marked engine management - well I did mine with the engine running and it stayed running fine. On reinsertion the fbh whirred into life!
Then after a few seconds it stopped. Another reset and it stayed running for the 5 minutes I gave it, then after the engine was switched off it ran on for up to a minute before stopping.
All seemed good so I wanted to check it without the engine running, feeding 12v direct to the fbh sensor wire. It span up again and all seemed good.
A few things confuse me though. I haven't driven it yet and it's quite mild today, so these may clear themselves up.
A few people have said their fbh smokes and makes a loud roar when running, mine didn't smoke at all, the exhaust gasses didn't feel particularly hot either. It was actually fairly quiet.
The body of the fbh did get noticeably warm, and this started creeping down the exhaust (and the water pipes). This coupled with the fact it didn't stop on lock out makes me think it's probably working.
I tested the ambient sensor in the freezer, and it switched nicely, but could still be out of spec. Mines got a black plastic body - I think I read somewhere about a wide range sensor having a green body, I'll check but if so I'll get one of those to replace mine.
Right now I've got a wire link between the sensor terminals, hopefully the next journey will be warmer.
Bit of a ramble but it might help someone.
Edit:
I meant narrow range sensor, as in the excerpt from another forum (MG/ZT)
PROBLEM:
POOR HEATER PERFORMANCE FROM COLD ENGINE START UP
The customer complains of excessive delay in obtaining satisfactory heater output
following a cold start.
CAUSE:
Due to an excessively wide sensing tolerance, the outside air temperature sensor
may not activate the FBH until the ambient temperature is as low as 0 o C.
This renders the FBH inactive at temperatures where its additional heat output
would be beneficial in assisting with engine and heater warm up.
ACTION:
On customer complaint, replace the outside air temperature sensor with the new
narrow range sensor
PARTS INFORMATION:
JWL000020 Sensor - outside air temperature
NOTE: The body of the new part is now moulded in green to distinguish it from its
predecessor.