This section is from the book "The Law Of Contracts", by William Herbert Page. Also available from Amazon: Commercial Contracts: A Practical Guide to Deals, Contracts, Agreements and Promises.
If the contract provides in express language that the certificate is not conclusive, and that it does not relieve the contractor from his liability to make good all defects, such certificate is of course not conclusive.1 A provision in a contract which gives to the architect power to reject materials,2 or to determine the amounts to be allowed for alterations,8 does not make his certificate conclusive so as to relieve the contractor from his express covenant to perform in accordance with the plans and specifications. If the contract provides that the certificate of the architect shall not relieve the contractor from his duty to perform in a good and workmanlike manner, the certificate is, of course, not conclusive as to such performance.4 A provision to the effect that the decision of the engineer is to be final as to the meaning of the plans and specifications, does not confer power upon the engineer to modify plain and unequivocal provisions of the contract,5 and his estimate or certificate which is given as a result of such unauthorized modification is not conclusive.6
11 Hennebique Construction Co. v. Boston Cold Storage & Terminal Co., 230 Mass. 456, L. R. A. 191SF, 374, 119 X. E. 948.
12 Mockler v. St. Vincent's Institution, 87 Mo. App. 473.
13 Kansas City, etc, Ry. v. Perkins, 88 Tex. 66 [sub nomine, Perkins v. Locke, 29 S. W 10481
14 Huckestein v. Kelly, etc, Co, 152 Pa. St. 631, 25 Atl. 747. And Bee Robinson v. Baird, 165 Pa. St. 505, 30 .VI. 1010.
1 Robins v. Goddard.  1 K. R. 294; Young v. Stein, 152 Mitch. 310. 17 L. R. A. (X.S.) 231, 116 N. W. 195.
2 Seaside v. Randies, - Or. -, 180 Pac. 319.
3 Hennebique Const. Co. v. Boston Cold Storage & Terminal Co., 230 Mass. 456, 119 X. E. 948.
A certificate of an architect or engineer will not be regarded as conclusive unless the plain and unequivocal language of the contract requires that such effect should be given to it.7
Whether a provision which gives to the architect the power to determine whether the contract has been performed or not, requires him to pass upon the work as it is performed, or whether he may reserve his decision until the contract is completed, is a question upon which there is a conflict of authority. In some jurisdictions it is said that the architect or engineer must decide the sufficiency of performance as the contract is being performed,8 and that his failure to object to defective work while it is being constructed prevents him from raising such question thereafter.9 In other jurisdictions it is said that the architect or engineer has no power to accept the work while it is being performed, so as to bind the owner unless the contract clearly gives him such power,10 and his failure to object to defective work as it is done does not prevent his principal from objecting thereto.11 If certificates are given while the work is progressing, such certificates are final as to all matters which were within the knowledge of the architect when such certificate was given,12 such as apparent defects. Such certificates are not conclusive as to defects which were not apparent when they were given, but which become apparent subsequently,13 especially if the contractor has promised to repair such defects.14 A mistake made by the engineer in issuing instalment certificates, may be corrected by him at the final estimate.15
4 Young v. Stein, 152 Mich. 310, 125 Am. St. Rep. 412, 17 L. R. A. (N.S.) 231, 116 N. W. 195.
5 Dyer v. Middle Kittitas Irrigation District, 40 Wash. 238, 82 Pac. 301.
6 Dyer v. Middle Kittitas Irrigation District, 40 Wash. 238, 82 Pac. 301.
7 England. Robins v. Goddard,  1 K. B. 294.
United States. Mercantile Trust Co. v. Hensey, 205 U. S. 298, 51 L. ed. 811; Central Trust Co. v. Louisville, etc., St. L. & T. R. R. Co., 70 Fed. 282.
Massachusetts. Hennebique Const. Co. v. Boston Cold Storage & Terminal Co., 230 Mass. 456, 119 N. E. 948.
Michigan. Young v. Stein, 152 Mich.
310, 125 Am. St. Rep. 412, 17 L. R. A. (N.S.) 231, 116 N. W. 195.
Oregon. Seaside v. Randies, - Or. -, 180 Pac. 319.
Washington. Dyer v. Middle Kittitas Irrigation District, 40 Wash. 238, 82 Pac. 301.
8 Kunze v. Jones, 200 Mich. 453, 166 N. W. 904; Central Union Stock Yards Co. v. Uvalde Asphalt Paving Co., 82 N. J. Eq. 246, 87 Atl. 235.
9 Kunze v. Jones, 200 Mich. 453, 166 N. W. 904.
10 Sterling v. Hurd, 44 Colo. 436, 98 Pac. 174.
11 Baumgartner v. Renton, 96 Wash. 588, 165 Pac. 484.
It may be provided specifically in the contract that a certificate given while work is in progress does not exempt the contractor from liability to replace work which is subsequently discovered to have been performed improperly.16 Under such a provision, a certificate which is given during the progress of the work does not prevent the owner from setting up a subsequent claim on account of defective material.17 If a contract provides that the certificate of an architect shall not be binding upon the property owner until the final acceptance of the building, final acceptance is impossible if the contractor abandons it before it is completed.18