<PAGE>

                                                                  EXHIBIT 4.3.31

THIS WARRANT AND ANY SECURITIES ACQUIRED UPON THE EXERCISE OF THIS WARRANT HAVE
NOT BEEN REGISTERED UNDER: (A) THE SECURITIES ACT OF 1933, AS AMENDED, IN
RELIANCE UPON THE EXEMPTIONS FROM REGISTRATION PROVIDED IN SECTIONS 3 AND 4 OF
SUCH ACT AND/OR REGULATIONS D PROMULGATED THEREUNDER; OR (B) ANY STATE
SECURITIES LAWS IN RELIANCE UPON APPLICABLE EXEMPTIONS THEREUNDER. THESE
SECURITIES MUST BE ACQUIRED FOR INVESTMENT ONLY FOR THE ACCOUNT OF THE INVESTOR
AND MAY NOT BE SOLD OR TRANSFERRED IN THE ABSENCE OF REGISTRATION UNDER THE
SECURITIES LAWS OR AN OPINION OF COUNSEL ACCEPTABLE TO THE CORPORATION OR ITS
REPRESENTATIVES THAT SUCH SALE OR TRANSFER WOULD NOT VIOLATE APPLICABLE
SECURITIES LAWS OR REGULATIONS.

WARRANT NO. _____                                           TO PURCHASE ________
                                                          SHARES OF COMMON STOCK
                                                              ($0.001 PAR VALUE)


                      CLASS "AM" WARRANT TO PURCHASE SHARES

                               OF COMMON STOCK OF

                                  PROBEX CORP.

                            (A DELAWARE CORPORATION)

                        PURCHASE PRICE PER SHARE: $1.243

EXPIRATION DATE: 5:00 P.M., DALLAS, TEXAS TIME, ON SEPTEMBER 13, 2006

         THIS CERTIFIES that, for value received,



is the registered owner and is entitled, subject to the terms and conditions of
this Warrant, until the Expiration Date, to purchase the number of shares of
common stock, $0.001 par value (the "Common Stock"), of Probex Corp. (the
"Corporation") set forth above from the Corporation at the purchase price set
forth above. The number of shares of Common Stock which may be received upon the
exercise of this Warrant and the price to be paid for each share of Common Stock
are subject to adjustment from time to time as hereinafter set forth.

1.   EXERCISE OF WARRANT. Subject to the provisions hereof, this Warrant may be
exercised in whole or in part until the Expiration Date, by delivery of this
Warrant to the Corporation with the exercise form duly executed and payment of
the purchase price (in cash or by certified or bank cashier's check made payable
to the order of the Corporation) for each share purchased.

2.   CORPORATION'S COVENANTS AS TO COMMON STOCK. Shares deliverable upon the
exercise of this Warrant shall, at delivery, be fully paid and non-assessable,
free from taxes, liens, and charges with respect to their purchase. The
Corporation shall at all times reserve and hold available sufficient shares of
Common Stock to satisfy all conversion and purchase rights of outstanding
convertible securities, options and warrants.
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Class "AM" Warrant No. ____
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3.   METHOD OF EXERCISE; FRACTIONAL SHARES. The purchase rights represented by
this Warrant are exercisable at the option of the registered owner in whole at
any time, or in part, from time to time, within the period specified above;
provided, however, that purchase rights are not exercisable with respect to a
fraction of a share of Common Stock. In lieu of issuing a fraction of a share
remaining after exercise of this Warrant as to all full shares covered hereby,
the Corporation shall either (1) pay therefor cash equal to the same fraction of
the then current Warrant purchase price per share or, at its option, (2) issue
scrip for the fraction, in registered or bearer form approved by the board of
directors of the Corporation, which shall entitle the holder to receive a
certificate for a full share of Common Stock on surrender of scrip aggregating a
full share. Scrip may become void after a reasonable period (but not less than
six months after the expiration date of this Warrant) determined by the board of
directors and specified in the scrip. In case of the exercise of this Warrant
for less than all the shares purchasable, the Corporation shall cancel this
Warrant and execute and deliver a new Warrant of like tenor and date for the
balance of the shares purchasable hereunder.

4.   ADJUSTMENT OF EXERCISE PRICE. If on or prior to September 13, 2003, the
Corporation shall (i) issue, (ii) enter into any binding agreement to issue, or
(iii) commence any public or private offering for the issuance of, shares of
Common Stock, warrants to purchase shares of Common Stock or debt or equity
securities which are convertible into shares of Common Stock, in an aggregate
amount of at least 100,000 shares of Common Stock (excluding any options to
purchase Common Stock issued pursuant to any employee stock option plan of the
Corporation), and the issuance, exercise or conversion price (the "Issuance
Price") of such Common Stock is less than $1.243 per share, then the exercise
price per share pursuant to this Warrant shall be adjusted (and the Corporation
shall, after occurrence of any event requiring such adjustment, notify the
holder of this Warrant of the adjustment) so that the exercise price shall be
equal to the Issuance Price. For purposes of this section, a public or private
offering shall be deemed to have commenced if the there is substantial agreement
as to the material terms of such offering.

5.   REDEMPTION. The Corporation has no redemption rights pursuant to this
Warrant.

6.   LIMITED RIGHTS OF OWNER. This Warrant does not entitle the owner to any
voting rights or other rights as a shareholder of the Corporation, or to any
other rights whatsoever except the rights expressed herein. No dividends are
payable or will accrue on this Warrant or the shares purchasable hereunder
until, and except to the extent that, this Warrant is exercised.

7. EXCHANGE FOR OTHER DENOMINATIONS. This Warrant is exchangeable, on its
surrender by the registered owner to the Corporation, for new Warrants of like
tenor and date representing in the aggregate the right to purchase the number of
shares purchasable hereunder in denominations designated by the registered owner
at the time of surrender.

8. TRANSFER. Except as otherwise provided, this Warrant is transferable only on
the books of the Corporation by the registered owner in person or by attorney,
on surrender of this Warrant, properly endorsed. However, because this Warrant
has not been registered under the Securities Act of 1933, as amended (the
"Securities Act"), and applicable state securities laws, this Warrant may not be
sold or transferred in the absence of an effective registration of it under the
Securities Act and all other applicable securities laws or an opinion of counsel
acceptable to the Corporation or its representatives that such sale or transfer
would not violate applicable securities laws or regulations. Any Common Stock
purchased upon exercise of this Warrant shall also be subject to the same
restrictions on transfer and will contain the same or similar transfer legend
found on the face of this Warrant.
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Class "AM" Warrant No. ____
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9.   REGISTRATION RIGHTS.

     (1) Each time that the Corporation proposes for any reason to register any
of its Common Stock under the Securities Act in connection with the proposed
offer and sale of its Common Stock for money either for its own account or on
behalf of any other security holder ("Proposed Registration"), other than
pursuant to a registration statement on Form S-4, S-8 or any successor thereto,
the Corporation shall promptly give written notice of such Proposed Registration
to the holder hereof and shall offer such holder hereof the right to request
inclusion of the shares issued or issuable upon the exercise of this Warrant in
the Proposed Registration.

     (2) The holder hereof shall have ten (10) days from the receipt of such
notice to deliver to the Corporation a written request specifying the number of
shares such holder hereof intends to sell and the holder's intended method of
disposition.

     (3) In the event that the Proposed Registration by the Corporation is, in
whole or in part, an underwritten public offering, the Corporation shall so
advise the holder hereof as part of the written notice given pursuant to Section
9(1), and any request under Section 9(2) must specify that the shares of Common
Stock be included in the underwriting on the same terms and conditions as the
shares of Common Stock, if any, otherwise being sold through underwriters under
such registration.

     (4) Upon receipt of a written request pursuant to Section 9(2), the
Corporation shall promptly use its commercially reasonable efforts to cause all
such shares of Common Stock held by the holder hereof to be registered under the
Securities Act (and included in any related qualifications under blue sky laws
or other compliance), to the extent required to permit the sale or disposition
as set forth in the Proposed Registration.

     (5) In the event that the offering is to be an underwritten offering, the
holder hereof proposing to distribute its shares of Common Stock through such
underwritten offering agrees to enter into an underwriting agreement with the
underwriter or underwriters selected for such underwriting by the Corporation.

     (6) Notwithstanding the foregoing, if in its good faith judgment, the
managing underwriter determines and advises in writing that the inclusion of all
shares of Common Stock issued or issuable with respect to this Warrant proposed
to be included in the underwritten public offering, together with any other
issued and outstanding shares of Common Stock proposed to be included therein by
holders other than the holder hereof, would interfere with the successful
marketing of such securities, then the number of such shares to be included in
such underwritten public offering shall be reduced, on a pro-rata basis (except
only those shares held by holders that have the benefit of any priority rights
previously granted such holders), based upon the number of shares held by each
holder, including the holder hereof; provided that, in connection with any such
public offering, if the managing underwriter reasonably and in good faith
recommends, which recommendation and supporting reasoning shall be delivered in
writing to the holders, that no amount of shares, including shares held by the
holder hereof, should be included in the underwritten public offering, then the
Corporation shall not be required to include any such shares in such public
offering.

10.  RECOGNITION OF REGISTERED OWNER. Prior to due presentment for registration
of transfer of this Warrant, the Corporation may treat the registered owner as
the person exclusively entitled to receive notices and otherwise to exercise
rights hereunder.
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Class "AM" Warrant No. ____
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11.  EFFECT OF STOCK SPLIT, ETC. If the Corporation, by stock dividend, split,
reverse split, reclassification of shares, or otherwise, changes as a whole the
outstanding Common Stock into a different number or class of shares, then:

     (1) the number and class of shares so changed shall, for the purposes of
this Warrant, replace the shares outstanding immediately prior to the change;
and

     (2) the Warrant purchase price in effect, and the number of shares
purchasable under this Warrant, immediately prior to the date upon which the
change becomes effective, shall be proportionately adjusted (the price to the
nearest cent).

12.  EFFECT OF MERGER, ETC. If the Corporation consolidates with or merges into
another corporation, the registered owner shall thereafter be entitled on
exercise to purchase, with respect to each share of Common Stock purchasable
hereunder immediately before the consolidation or merger becomes effective, the
securities or other consideration to which a holder of one share of Common Stock
is entitled in the consolidation or merger to assure that all the provisions of
this Warrant shall thereafter be applicable, as nearly as reasonable may be, to
any securities or other consideration so deliverable on exercise of this
Warrant. The Corporation shall not consolidate or merge unless, prior to
consummation, the successor corporation (if other than the Corporation) assumes
the obligations of this Section 12 by written instrument executed and mailed to
the registered owner at the address of the owner on the books of the
Corporation. A sale or lease of all or substantially all the assets of the
Corporation for consideration (apart from the assumption of obligations)
consisting primarily of securities is a consolidation or merger for the
foregoing purposes.

13.  NOTICE OF ADJUSTMENT. On the happening of an event requiring an adjustment
of the Warrant purchase price or shares purchasable hereunder, the Corporation
shall forthwith give written notice to the registered owner stating the adjusted
Warrant purchase price and the adjusted number and kind of securities or other
property purchasable hereunder resulting from the event and setting forth in
reasonable detail the method of calculation and the facts upon which the
calculation is based. The board of directors of the Corporation, acting in good
faith, shall determine the calculation. Irrespective of any adjustment or change
in the Warrant purchase price or the number of shares purchasable under this or
any other Warrant of like tenor, the Warrants theretofore and thereafter issued
may continue to express the Warrant purchase price per share and the number of
shares purchasable as were expressed in the Warrant when initially issued;
provided, however, the Corporation may, in its sole discretion, request the
holder of this Warrant to exchange this Warrant for a warrant of like tenor
reflecting such adjustment or change provided for in this Warrant, and the
holder of this Warrant agrees to cooperate with the Corporation to effect such
exchange.

14.  NOTICE AND EFFECT OF DISSOLUTION, ETC. In case a voluntary or involuntary
dissolution, liquidation, or winding up of the Corporation (other than in
connection with a consolidation or merger covered by Section 12 above) is at any
time proposed, the Corporation shall give at least 10 days written notice to the
registered owner prior to the record date as of which holders of Common Stock
will be entitled to receive distributions as a result of the proposed
transaction. Such notice shall contain: (1) the date on which the transaction is
to take place; (2) the record date as of which holders of Common Stock will be
entitled to receive distributions as a result of the transaction; (3) a brief
description of the transaction; (4) a brief description of the distributions to
be made to holders of Common Stock as a result of the transaction; and (5) an
estimate of the fair value of the distributions. On the date of the transaction,
if it actually occurs, this Warrant and all rights hereunder shall terminate.

15.  METHOD OF GIVING NOTICE; EXTENT REQUIRED. Notices shall be given by first
class mail, postage prepaid, addressed to the registered owner at the address of
the owner appearing in the records of the Corporation. No notice to warrant
holders is required except as specified in Sections 4, 9, 13 and 14.
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Class "AM" Warrant No. ____
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16.  ACCESS TO INFORMATION. The Corporation will provide an opportunity to any
registered owner of this Warrant to ask questions of management of the
Corporation and to obtain information to the extent the Corporation has made
such information publicly available prior to any exercise of the owner's rights
to purchase Common Stock under this Warrant. Requests for information and any
other questions concerning the business and affairs of the Corporation should be
directed to the Vice President of Investor Relations of the Corporation at its
main offices.

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Class "AM" Warrant No. ____
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         IN WITNESS WHEREOF, the Corporation has caused this Warrant to be
executed and delivered by its duly authorized officer as of the 13th day of
September, 2001.

                                         PROBEX CORP.

                                         By:      ____________________________
                                         Name:    Bruce A. Hall
                                         Title:   Senior Vice President
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Class "AM" Warrant No. ____
Page 7

                                  TRANSFER FORM

     For value received, the undersigned hereby sells, assigns, and transfers to

Name__________________________________

Address_________________________________


this Warrant and irrevocably appoints _______________________ attorney (with
full power of substitution) to transfer this Warrant on the books of the
Corporation.


Date:

__________________________________________

                                ________________________________________________
                                (Please sign exactly as name appears on Warrant)

                                Taxpayer ID No. ________________________

In the presence of              Signature guaranteed by

______________________________  _______________________________________________
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Class "AM" Warrant No. ____
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                                  EXERCISE FORM

         The undersigned hereby: (1) irrevocably subscribes for ________________
shares of your Common Stock pursuant to this Warrant, and encloses payment of
$____________________ therefor; (2) requests that a certificate for the shares
be issued in the name of the undersigned and delivered to the undersigned at the
address below; and (3) if such number of shares is not all of the shares
purchasable hereunder, that a new Warrant of like tenor for the balance of the
remaining shares purchasable hereunder be issued in the name of the undersigned
and delivered to the undersigned at the address below.

Date:

______________________________

                                 ______________________________________________
                                (Please sign exactly as name appears on Warrant)

                                       Address:_________________________________


                                               _________________________________


                                      Taxpayer ID No.___________________________