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EXHIBIT 31 CERTIFICATION OF CHIEF EXECUTIVE OFFICER I, John W. Hall, certify that:  1.
    
    I have reviewed this quarterly report on Form 10-Q of Onyx Acceptance
Corporation;2.
    
    Based on my knowledge, this report does not contain any untrue statement
of a material fact or omit to state a material fact necessary to make the
statements made, in light of the circumstances under which such statements
were made, not misleading with respect to the period covered by this
report;3.
    
    Based on my knowledge, the financial statements, and other financial
information included in this report, fairly present in all material
respects the financial condition, results of operations and cash flows of
the registrant as of, and for, the periods presented in this report;4.
    
    The registrants other certifying officer and I are responsible for
establishing and maintaining disclosure controls and procedures (as
defined in Exchange Act Rules 13a-15(e) and 15d-15(e)) for the registrant
and have: a)
    designed such disclosure controls and procedures, or
caused such disclosure controls and procedures to be designed
under our supervision, to ensure that material information
relating to the registrant, including its consolidated
subsidiaries, is made known to us by others within those entities,
particularly during the period in which this report is being
prepared;b)
    evaluated the effectiveness of the registrants
disclosure controls and procedures and presented in this report
our conclusions about the effectiveness of the disclosure controls
and procedures, as of the end of the period covered by this report
based on such evaluation; andc)
    disclosed in this report any change in the registrants
internal control over financial reporting that occurred during the
registrants most recent fiscal quarter that has materially
affected, or is reasonably likely to materially affect, the
registrants internal control over financial reporting; and 5.
    
    The registrants other certifying officer and I have disclosed, based on
our most recent evaluation of internal control over financial reporting,
to the registrants auditors and the audit committee of the registrants
board of directors (or persons performing the equivalent functions): a)
    all significant deficiencies and material weaknesses in
the design or operation of internal control over financial
reporting which are reasonably likely to adversely affect the
registrants ability to record, process, summarize and report
financial information; andb)
    any fraud, whether or not material, that involves
management or other employees who have a significant role in the
registrants internal control over financial reporting. Date: May 17, 2004 /s/ JOHN W. HALL  
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EXHIBIT 31 CERTIFICATION OF CHIEF FINANCIAL OFFICER I, Don P. Duffy, certify that:  1.
    
    I have reviewed this quarterly report on Form 10-Q of Onyx Acceptance
Corporation;2.
    
    Based on my knowledge, this report does not contain any untrue statement
of a material fact or omit to state a material fact necessary to make the
statements made, in light of the circumstances under which such statements
were made, not misleading with respect to the period covered by this
report;3.
    
    Based on my knowledge, the financial statements, and other financial
information included in this report, fairly present in all material
respects the financial condition, results of operations and cash flows of
the registrant as of, and for, the periods presented in this report;4.
    
    The registrants other certifying officer and I are responsible for
establishing and maintaining disclosure controls and procedures (as
defined in Exchange Act Rules 13a-15(e) and 15d-15(e)) for the registrant
and have: a)
    designed such disclosure controls and procedures, or
caused such disclosure controls and procedures to be designed
under our supervision, to ensure that material information
relating to the registrant, including its consolidated
subsidiaries, is made known to us by others within those entities,
particularly during the period in which this report is being
prepared;b)
    evaluated the effectiveness of the registrants
disclosure controls and procedures and presented in this report
our conclusions about the effectiveness of the disclosure controls
and procedures, as of the end of the period covered by this report
based on such evaluation; andc)
    disclosed in this report any change in the registrants
internal control over financial reporting that occurred during the
registrants most recent fiscal quarter that has materially
affected, or is reasonably likely to materially affect, the
registrants internal control over financial reporting; and 5.
    
    The registrants other certifying officer and I have disclosed, based on
our most recent evaluation of internal control over financial reporting,
to the registrants auditors and the audit committee of the registrants
board of directors (or persons performing the equivalent functions): a)
    all significant deficiencies and material weaknesses in
the design or operation of internal control over financial
reporting which are reasonably likely to adversely affect the
registrants ability to record, process, summarize and report
financial information; andb)
    any fraud, whether or not material, that involves
management or other employees who have a significant role in the
registrants internal control over financial reporting. Date: May 17, 2004 /s/ DON P. DUFFY  
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