I have a 1998 V40-T4. A few weeks ago I renewed the thermostat as it appeared to run cool and the temp gauge now runs spot on 3 oclock. Since then the lambda light comes on when reducing speed in a high gear with no noticable difference in performance. The code reader says P0138 which is O2 sensor circuit high volts (bank 1 sensor 2). Can anyone tell me what this means and how to rectify it? Also, when warm, the engine takes about 4-5 seconds to fire but is instantaneous when cold. Could this be related? Thanks in advance Orpheus

Did a quick google on this and it has been recommended to disconnect the battery as a potential fix which should clear the problem. Does the light go off then come back on or is it always there now? Also see here....can you get the codes read at all or read them yourself?

Hi, DTC code P0138 shows a problem in the exhaust and emission conmtrol system and is for heated oxygen sensor prior- or after the catalysator guess sensor 02 is this one after the catalysator. This sensor cannot be repared must be exchanged. The only thing what u can do is to check of its power supply is disrupted. regards .

Sounds like the coolent temp sensers faulty aswell to me, it will explain why it won't start when its warm and putting lamba probe fault code in, When a cars cold it uses more fuel, its overfueling on start up which is why its not starting right and if the ecu doesn't know what temp engine is it will assume its cold and overfuel all time hence the lamba light for 02 sensor, Or thats what it sounds like to me

Thanks for the replies guys. I can erase the code and extinguish the light using the reader so does disconnecting the battery do the same or something additional? As suggested I will check out the coolant temperature sensor. I assume it is different from the one that feeds the temperature gauge because this now reads normal. If it is overfueling on warm starts would you expect the light to come back on straight away? It only comes on slowing down in a high gear.

I have renewed the coolant temp sensor, cleaned both O2 sensors, fitted new volvo spark plugs. So far the lambda light hasn't come back on but I still have to crank for about 5 seconds on warm starts. The O2 sensors were sooty black and the new spark plugs are also sooty black so something is causing it to run rich. It idles fine, has plenty of power and no hesitations when accelerating. I have looked for air leaks - found none. I have no codes coming up on the reader. Any advice on how to correct the rich mixture would be apreciated

Did you just do the plugs or the coil packs as well? I guess it 'may' be worth checking the exhaust out too

Just the plugs not the coil packs. There are no leaks on the exhaust and it passed the MOT emissions in December with the old thermostat running cool

sadly an MOT onyl really counts on the day mate. Just cause an exhaust system passess emmisions tests one day doesn't mean it couldn't fail an hour later