You can read the code anytime if it is stored (CEL on). CEL stays on if you have failures in back to back drive cycles. You can also read the code (pending code) if you got a warning on the last drive cycle. But the pending code will erase if the current drive cycle doesn't experience failure.

The easist way is to carry a OBDII reader with you to check the code when the failure occurs. Alternatively, you can drive to a gas station or auto parts store and they can probably read it for you.

When I cross referenced my failure code against this table, it clearly showed ETM internal failure.

http://www.volvoxc.com/resources/how-to/pdf/p_codes_conversion_table_standardized_dtcs.pdf
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2001 Volvo V70 T5 ASR Wife says we are Volvo for Life, 1992 BMW 535iA becoming The Ultimate Driving Machine