As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
(LOG 6597901) AT GATE WITH APU ONLINE SMELLED AN ODD ODOR THEN THE LEFT PACK LIGHT ILLUMINATED. REPLACED RIGHT PACK ACM IAW B737 AMM 21-51. REPLACED PRIMARY AND SECONDARY HEAT EXCHANGER IAW B737 AMM 21-51. R/R BOTH LEFT AND RIGHT RECIRCULATION AIR FILTER IAW B737 AMM 21-25.